SCIENTIFIC PROGRAMS AND ACTIVITIES

April 23, 2014

August 8-12, 2011
the Fields Institute, 222 College Street, Toronto

Abstracts

Invited Speakers
Marco Bellini Single-photon-level light manipulation and amplification
Charles Bennett Information is Quantum---how physics has helped us understand what information is and what can be done with it

Robin Blume-Kohout Tomography for fault-tolerance: confidence regions for quantum hardware
Ben Buchler Gas-phase quantum memory
Vladimir Buzek  
Jianshu Cao Optimization of coherent energy transfer in light-harvesting systems
Man-Duen Choi My Personal Adventure in Quantum Wonderland
John Howell Weak Values and Precision Measurements
Vladimir Korepin Correlation Functions of 1 D anyons
David Kribs A family of norms with applications in quantum information
Gershon Kurizki Quantum thermodynamics via measurements on non-Markovian time scales
Adrian Lupascu Quantum gates for superconducting qubits with fixed coupling
Alexander Lvovsky Remote preparation of arbitrary states of an atomic collective
Tomas Mancal Photoinduced dynamics in photosynthetic complexes under incoherent light excitation
Alexandra Olaya-Castro Quantum coherence in biology: facts, fiction and challenges
Sandu Popescu Virtual qubits, virtual temperatures, and the foundations of thermodynamics
Christine Silberhorn Pulsed quantum states of light for multi-mode quantum systems
Shigeki Takeuchi Photonic quantum circuits and their application
Tzu-Chieh Wei Affleck-Kennedy-Lieb-Tasaki states as a resource for universal quantum computation


Single-photon-level light manipulation and amplification
by
Marco Bellini

Istituto Nazionale di Ottica - CNR
Coauthors: A. Zavatta, C. Polycarpou

The addition and subtraction of single photons to/from arbitrary light fields have been recently demonstrated, and sequences or coherent superpositions of these operations have been used to test fundamental quantum rules. Now, an interesting new application of these tools has been proven: the noiseless linear amplification of quantum light states. We have experimentally shown that a non-deterministic noiseless amplifier based on a sequence of photon addition and subtraction can greatly outperform other approaches and might be used to distill and concentrate entanglement, form part of a quantum repeater, improve the performance of phase-estimation schemes, and enable high-fidelity probabilistic cloning and discrimination of coherent states.
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Information is Quantum---how physics has helped us understand what information is and what can be done with it
by
Charles Bennett
, IBM

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Tomography for fault-tolerance: confidence regions for quantum hardware
by
Robin Blume-Kohout

Los Alamos National Laboratory

Fault tolerant quantum computation will depend upon highly reliable characterization of individual components - "states and gates". This requires region (a.k.a. interval) estimators, rather than the point estimators provided by current tomographic schemes. In this talk, I introduce likelihood-ratio (LR) confidence regions for quantum states and processes. I prove that LR regions are optimally powerful (i.e., have minimum expected volume). I also derive the distribution of the LR statistic for tomographic data (a necessary step in constructing LR regions), and show how it differs from a canonical c2 distribution. Finally, I demonstrate LR regions in action and confirm that they work.

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Gas-phase quantum memory
by
Ben Buchler

The Australian National University
Coauthors: Mahdi Hosseini, Ben Sparkes, Geoff Campbell and Ping Koy Lam

A practical optical quantum memory must be able to store and recall quantum states on demand with high efficiency and low noise. Ideally, the platform for the memory would also be simple and inexpensive. In this talk we present tomographic reconstruction of quantum states that have been stored in an off-the-shelf rubidium vapour cell operating at around 80 degrees Celsius. Our analysis demonstrates an optical memory with quantum fidelity as high as 98% and recall efficiency up to 87%. In order to unambiguously verify that our memory beats the quantum no-cloning limit we employ state independent verification using conditional variance and signal transfer coefficients.

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Optimization of coherent energy transfer in light-harvesting systems
by
Jianshu Cao
, Massachusetts Institute of Technology

Energy transfer in photosynthesis is the initial step in the conversion of solar energy into chemical energy for human consumption. This talk will discuss the optimal conditions under which photosynthetic light-harvesting systems can achieve maximal energy transfer efficiency, i.e., the maximal exciton mobility. A simple scaling theory is developed to explain the optimal energy transfer efficiency, as a function of temperature, noise level, and solvent relaxation time-scale, and the dependence on the initial state preparation due to photo-excitation. A perturbation technique is then developed on the basis of NIBA (non-interaction blip approximation) to systematically map a quantum network to a kinetic network, where the leading order is hopping and higher order corrections are non-local quantum effects. . In addition, the influence of intrinsic symmetry of the exciton system on the efficient and robust light-harvesting energy transfer is demonstrated for LH2 B850 rings. These results provide useful insights to the natural selection in light-harvesting systems and optimal design principles of artificial energy devices.

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My Personal Adventure in Quantum Wonderland
by
Man-Duen Choi,
Department of Mathematics, University of Toronto,

Suddenly, there arises the new era of REAL quantum computers.As the time runs backwards in an alternating world through the looking
glass, I have to come back to the same old scene to release myself from all sorts of quantum entanglements.

Now, I am ready to give an expository talk, about my personal adventure, in the Quantum Wonderland. In particular, I shall give a MODERN report
on my 1975 paper (Completely positive linear maps on complex matrices, Linear Algebra Appl. 10 (1975), pp. 285-290) which has been cited in 650 research articles (as shown in Google Scholars of August 1, 2011).

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Weak Values and Precision Measurements
by
John Howell

University of Rochester
Coauthors: David Starling, Benjamin Dixon, and Andrew Jordan

Weak values were originally introduced by Aharanov, Albert and Vaidman for understanding the arrow of time in quantum mechanics. Among other things, weak values have recently proved useful in amplifying very small effects. I will introduce the ideas of weak values and discuss some of our recent experimental results in which we were able to observe a deflection of a laser beam of less than 1 picoradian (equivalent to measuring a deflection of the width of a hair at the distance of the moon). Perhaps more interestingly, the noise properties lead to a suppression of technical noise and an amplification of the signal to noise ratio to the optimal value for coherent states for standard beam deflection techniques. I will also discuss the use of weak values and precision deflection measurements for precision spectral and phase measurements.

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Correlation Functions of 1 D anyons
by
Vladimir Korepin

Yang Institute for Theoretical Physics
Coauthors: D. Averin

One dimensional anyons describes edge state of fqhe. The edge state is used to perform gates in topological quantum computation. I will consider a model of 1D anyons with local interaction and show how correlation functions depend on statistics.

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A family of norms with applications in quantum information
by
David Kribs

University of Guelph
Coauthors: Nathaniel Johnston

In this talk I will discuss recent work with Nathaniel Johnston in which we consider a family of operator norms that quantify the degree of entanglement in quantum states. The norms are defined by the Schmidt decomposition theorem for quantum states, and they can be used to tackle two fundamental problems in quantum information: the classification problem for k-positive linear maps and entanglement witnesses, and the existence problem for non-positive partial transpose bound entangled states. I’ll begin by giving an overview, then discuss some properties of the norms and their applications.

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Quantum thermodynamics via measurements on non-Markovian time scales
by
Gershon Kurizki

Weizmann Institute of Sciences, WIS

The anomalies of work, heating or cooling induced by frequent perturbations of open quantum systems are intimately related to the little-explored quantum correlations (entanglement or discord) that arise between the system (e.g., a qubit) and a thermal bath because of their coupling. Such correlations, which have previously eluded attention, have been shown by us both theoretically [1-4] and experimentally [5] to profoundly change the dynamics of the bath and the system once we perturb the system within the bath-memory (non-Markovian) time scales.The required perturbations can either be effected by frequent projective measurements of the qubit energy, or by its frequent modulation (ultrafast driving), giving rise to novel , anomalous regimes:

a) Quantum heat engines (QHE): We present a hitherto unexplored QHE design, based on anomalies that arise from frequent quantum nondemolition (QND) measurements or phase flips of a qubit in contact with a non-Markovian bath [1,4]. Either operation results in a non-equilibrium state that starts evolving and can close a cycle via qubit-modulation by a piston, e.g., a coherently-driven oscillator mode. An intriguing anticipated consequence of such QND operations is the ability to extract net work (from the qubit to the piston) using a single bath, although such operations do not acquire information that can be converted into work, as opposed to Maxwell's demon. This anomaly may appear to contradict the second law, but in fact it does not, once the measurement or phase-flip cost in energy and entropy is accounted for.

b) Entanglement-based QHE: Two or more qubits coupled to the same bath mode have recently been predicted by us to be inevitably entangled via the bath[6]. This entanglement is expected to principally affect the QHE performance.

c) Non-Markovian quantum refrigerator (QR):Ultrafast cooling (purification) of qubits, may be attained at non-Markovian time-scales by frequent quantum measurements or phase shifts [3,5].It allows us to put forward a novel, highly-compact, QR design which consists of a single qubit simultaneously coupled to hot and cold non-Markovian baths. Phase flips of the qubit at high rates are shown to cause refrigeration: Heat may then flow from the cold to the hot bath via the qubit. The third law is upheld: under no circumstances can the bath refrigeration attain absolute zero.

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Quantum gates for superconducting qubits with fixed coupling
by
Adrian Lupascu

Institute for Quantum Computing and Department of Physics and Astronomy, University of Waterloo
Coauthors: Nature Physics , Letter Selective darkening of degenerate transitions demonstrated with two superconducting quantum bits P. C. de Groot, J. Lisenfeld, R. N. Schouten, S. Ashhab, A. Lupascu, C. J. P. M. Harmans, J. E. Mooij, Jean-Luc Orgiazzi

At low temperature, nanoscale electrical circuits based on superconductors behave as artificial atoms. Their dynamics is described by a small number of degrees of freedom, and their properties can be tailored by circuit design.

Superconducting artificial atoms have potential applications in quantum information processing and provide a testbed for the study of light-matter interaction in a new regime of ultra-strong interactions.

I will discuss a new type of two-qubit gate applicable to qubits with fixed coupling. This type of gate requires the same resources as needed for single-qubit control (ie independent driving pulses on two qubits). I will discuss the experimental implementation using flux-type superconducting qubits. I will also present experimental work in progress on the application of this method to a system formed by a few qubits.


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Remote preparation of arbitrary states of an atomic collective
by
Alexander Lvovsky

University of Calgary
Coauthors: A. MacRae and A. I. Lvovsky

In the Duan-Lukin-Cirac-Zoller protocol, single collective excitations of atomic ensembles and, subsequently, heralded single photons, can be prepared by conditional photon number measurements on Raman scattered light. More complex measurements, such as projections onto displaced Fock states, permit preparation of these collective excitations in arbitrary quantum states. These states can be characterized by converting them into the optical form and applying homodyne tomography.
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Photoinduced dynamics in photosynthetic complexes under incoherent light excitation
by
Tomas Mancal

Faculty of Mathematics and Physics, Charles University in Prague

Photosynthetic antennae collect Sun light, and direct the excitation energy to so-called reaction centers, where chemical part of the photosynthetic process starts. In this contribution, we will consider theoretically the dynamics of a model antenna subjected to external pumping by a light source. Within a completely quantum
mechanical treatment, we derive a general formula, which enables us to assess the effects of different light properties on the photo-induced dynamics of excitations in a molecular system. We show that, once the properties of light are known in terms of a certain two-point correlation function, the only information needed to reconstruct the system dynamics is the reduced evolution superoperator. The latter quantity is, in principle, accessible through ultrafast nonlinear spectroscopy. Considering a direct excitation of a small molecular antenna by incoherent light, we find that excitation of coherences is possible due to the overlap of homogeneous line shapes associated with different excitonic states. In Markov and secular approximations, the amount of coherence is
significant only under fast relaxation, and both the populations and coherences between exciton states become static at long times. We will discuss the implications of these findings for the current debate about the relevance of electronic coherences detected in some photosynthetic antennae by ultrafast spectroscopy.

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Quantum coherence in biology:facts, fiction and challenges
by
Alexandra Olaya-Castro

Department of Physics and Astronomy, University College London

The idea that quantum superpositions can survive in molecular componenets of living organisms, in conditions of biological relevance and for long enough to be exploited in life processes is fascinating -at least for lovers of the quantum world. Light-initiated reactions in biological systems are some of the extraordinary phenomena that have for long been suspected to benefit from coherent quantum effects. In this talk I would like to discuss facts supporting these ideas, the importance (or not) of such quantum phenomena in biology, the fictional stories that these ideas have inspired but most importantly the scienfic opportunities that are being opened.

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Virtual qubits, virtual temperatures, and the foundations of thermodynamics
by
Sandu Popescu

University of Bristol
Coauthors: Nicolas Brunner, Noah Linden and Paul Skrzypczyk

In my talk I will present a new view on the foundations of thermodynamics.

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Pulsed quantum states of light for multi-mode quantum systems
by
Christine Silberhorn

University of Paderborn, Warburgerstr 100, 33098 Paderborn, Germany
Coauthors: B. Brecht, K.N. Cassemiro, A. Christ, A. Eckstein, K. Laiho, A. Schreiber, C. Söller

Optical networks, which comprise multiple optical modes as well as highly non-classical states of light, have been investigated intensively over the last two decades in various theoretical proposals. They can serve as an ideal test-bed for different application in quantum information science. Most recently the role of coherence and quantum properties in quantum walk architectures has attracted attention. However, the implementation of experi¬mental setups with increasing complexity in terms of number of modes and input states with distinct quantum char¬acteristics implicates several challenges. These are related to the controlled interaction amongst different channels, the detection of a large number of modes, their stabilization as well as the synchronization with interferometric precision. For pulsed light the preparation of pure photonic quantum states in combination with appropriate state characterization constitutes an additional challenge for the implementation of practical systems.

Using pulsed light in combination with time multiplexing offers an attractive approach to implement quantum systems with multiple spectral-temporal modes and increased complexity. We present our recent progress for generation, characterization and controlled manipulation of pulsed quantum states of light in multi-mode architectures.

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Photonic quantum circuits and their application
by
Shigeki Takeuchi

Hokkaido Univ., Osaka Univ.
Coauthors: Ryo Okamoto (Hokkaido Univ., Osaka Univ.) Masazumi Fujiwara (Hokkaido Univ., Osaka Univ.) Hong-Quan Zhao (Hokkaido Univ., Osaka Univ.) Holger F. Hofmann (Hiroshima Univ.) Jeremy L. O’Brien (Univ. Bristol)

In this talk, we report our recent effort for making photonic quantum circuits and discuss their possible applications.

The first example is we `an entanglement filter’[1]. The ability to filter quantum states is a key capability in quantum information science and technology, where one-qubit filters, or polarizers, have found wide application. Filtering on the basis of entanglement requires extension to multi-qubit filters with qubit-qubit interactions. We demonstrate an optical entanglement filter that passes a pair of photons if they have the desired correlations of their polarization. Such a device has been proposed for photonic qubits[2], however, the technical requirements to build such a device, an optical circuit with two ancillary photons and multiple quantum gates, requiring both quantum interference and classical interference in several nested interferomters, have been lacking. We demonstrate an entanglement filter by combining two key recent technological approaches---a displaced-Sagnac architecture[3] and partially polarizing beam splitters[4]. The entangling capability of the filter was verified, distinguishing it from classical ones.

The second example is the optical quantum circuit of a Knill-Laflamme-Milburn (KLM) CNOT gate[5]. This photonic quantum circuit combines two efficient `artificial’ nonlinear elements. We developed a stable architecture to realize the required four-photon network of nested multiple interferometers, and found that the average gate fidelity of our experimental quantum CNOT gate is 0.82 ± 0.01[6]. This result confirms the first step in the KLM `recipe' for all-optical quantum computation, and should be useful for on-demand entanglement generation and purification.

We will also briefly introduce our recent activities on the realization of solid state quantum using diamond nano-crystals coupled to tapered optical fibers and microsphere resonators [7-9].

This work was supported in part by Grant-in-Aid from JSPS, Quantum Cybernetics project, JST-CREST project, FIRST Program of JSPS, Special Coordination Funds for Promoting Science and Technology, and the GCOE program, and Research Foundation for Opto-Science and Technology.

References
1 R. Okamoto, J. O’Brien, H. F. Hofmann, T. Nagata, K. Sasaki and S. Takeuchi, Science 323, 483, (2009)
2. H. F. Hofmann and S. Takeuchi, Phys. Rev. Lett. 88, 147901 (2009).
3. T. Nagata, R. Okamoto, J. O’Brien, K. Sasaki and S. Takeuchi, Science 316 726 (2007).
4. R. Okamoto, H. F. Hofmann, S. Takeuchi and K. Sasaki, Phys. Rev. Lett. 95, 210506 (2005).
5. E. Knill, R. Laflamme, and G. J. Milburn, Nature 409, 46–52 (2001).
6. R. Okamoto, J. L. O’Brien, H. F. Hofmann and S. Takeuchi, PNAS, published online before print June 6, 2011, doi: 10.1073/pnas.1018839108
7. H. Takashima, T. Asai, K. Toubaru, M. Fujiwara, K. Sasaki, and S. Takeuchi, Opt. Exp. 18, 15169 (2010).
8. A. Tanaka, T. Asai, K. Toubaru, H. Takashima, M. Fujiwara, R. Okamoto, and S. Takeuchi, Opt. Exp. 19, 422 (2011).
9. M. Fujiwara, K. Toubaru and S. Takeuchi, Opt. Exp., 19, 8596 (2011).

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Affleck-Kennedy-Lieb-Tasaki states as a resource for universal quantum computation
by
Tzu-Chieh Wei

University of British Columbia

The study of quantum spin systems dates back to the early twentieth century and has been an active research field. Quantum computation, on the other hand, is a relatively new research field, of less than three decades of age. Here, we investigate the question of whether quantum computational resource states can arise as ground states of two-body interacting Hamiltonians in spin systems. It is known that unfortunately cluster states, the first known resource state, cannot be the unique ground states of such Hamiltonians. We shall investigate a few examples that are constructed from Affleck-Kennedy-Lieb-Tasaki (AKLT) models of quantum antiferromagnets and show an intriguing connection between these AKLT-type states and cluster states. In particular the spin-3/2 two-dimensional AKLT state on the hexagonal lattice, as well as the Cai-Miyake-Dur-Briegel AKLT-like state on the decorated hexagonal lattice, can be locally converted to a cluster state and hence is a universal resource for measurement-based quantum computation.

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Contributed Talks
Clement Ampadu On the Ambainis-Bach-Nayak-Vishwanath-Watrous Conjecture
Mohammad Ansari Critical current noise and junction resonators in Josephson junction from interacting trap states
Charles Bamber Direct Measurement of the Quantum Wavefunction
Shohini Ghose Analysis of multiqubit entanglement and nonlocality for optimal quantum communication
Brendon Higgins Multiple-copy state discrimination: Thinking globally, acting locally
A.B. Klimov Isotropic and squeezed fluctuations in n-qubit system
Jordan Kyriakidis Optimal Trajectories for Quantum Adiabatic Processing
   
Ahsan Nazir A variational master equation approach to dissipative energy transfer dynamics
Marco Piani All non-classical correlations can be activated into distillable entanglement
Alberto Politi Integrated Quantum Photonics
Peter Repcan Scavenging quantum information: Multiple observations of quantum systems
Changliang Ren Quantum tomography of photonic time -energy entanglement by photon bunching with short-time reference pulses
Dylan Saunders The Power of Many Settings or Many Outcomes in Experimental Demonstrations of EPR-Steering
Torsten Scholak On the efficiency of excitonic energy transport
Marcus P. da Silva Practical characterization of quantum devices without tomography
Neil Sinclair Broadband waveguide quantum memory for entangled photons
Ben Sparkes Spectral Manipulation of Optical Pulses Using the Gradient Echo Memory Scheme
Ioannis Thanopulos Coherent Control of Intramolecular Energy Transfer in 24-mode Pyrazine
Peter Turner The curious nonexistence of Gaussian 2-designs
Christian Weedbrook Quantum Cryptography Approaching the Classical Limit
Lian-ao Wu Nondeterminstic fast ground state cooling of a mechanical resonator
Shengjun Wu A general framework of weak measurement and its application to optical non-linearlity


On the Ambainis-Bach-Nayak-Vishwanath-Watrous Conjecture
by
Clement Ampadu

Coauthors: Not Applicable

We show the flaw in "N. Konno, T. Namiki, T. Soshi and A. Sudbury, Absorption problems for quantum walks in one dimension, J. Phys. A: Math. Gen. 36 (2003), 241-253" and provide the necessary correction in the case of the finite Hadamard Walk and use it to show that conjecture 11 of Ambainis-Bach-Nayak- Vishwanath-Watrous in "A. Ambainis, E. Bach, A. Nayak, A. Vishwanath and A. Watrous, One-dimensional Quantum Walks, Proceedings of the 33rd Annual ACM Symposium on the Theory of Computing (2001), 37-49." is false.

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Critical current noise and junction resonators in Josephson junction from interacting trap states
by
Mohammad Ansari

IQC, University if Waterloo
Coauthors: Frank Wilhelm

We analyze the impact of trap states in the oxide layer of superconducting tunnel junctions on the fluctuation of the Josephson current and thus on coherence in superconducting qubits. We are extending previous studies of noninteracting traps to the case where the traps have on-site electron repulsion. We use second order perturbation theory which allows to obtain analytical results limited to small and intermediate repulsion. Remarkably, it still reproduces the main features of the model as identified from the Numerical Renormalization group. We present analytical formulations for the subgap bound state energies, the singlet-doublet phase boundary, and the spectral weights, which are in agreement with recent numerical renormalization group analysis. We show that interactions can reverse the supercurrent across the trap. We finally work out the spectrum of junction resonators for qubits in the presence of on-site repulsive electrons and analyze its dependence on microscopic parameters that may be controlled by fabrication.

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Direct Measurement of the Quantum Wavefunction
by
Charles Bamber

National Research Council
Coauthors: Jeff S. Lundeen, Brandon Sutherland, Aabid Patel, Corey Stewart

Central to quantum theory, the wavefunction is a complex distribution associated with a quantum system. Despite its fundamental role, it is typically introduced as an abstract element of the theory with no explicit definition. Rather, physicists come to a working understanding of it through its use to calculate measurement outcome probabilities through the Born Rule. Tomographic methods can reconstruct the wavefunction from measured probabilities. In contrast, we present a method to directly measure the wavefunction so that its real and imaginary components appear on our measurement apparatus. We will describe an experimental example by directly measuring the transverse spatial wavefunction of a single photon. This method gives the wavefunction a plain and general meaning in terms of a specific set of operations in the lab.

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Analysis of multiqubit entanglement and nonlocality for optimal quantum communication
by
Shohini Ghose

Wilfrid Laurier University
Coauthors: Atul Kumar, Angele Hamel and Alexei Kaltchenko

Quantum entanglement can be used as a resource for efficient information transfer between different parties via protocols such as dense coding or teleportation. For two-party communication, the maximally entangled Bell states serve as a resource for efficient and secure communication. In networks of three or more parties, multiqubit entanglement allows the possibility of new protocols and flexible communication between different members of a network. Our goal is to analyze important classes of multiqubit entangled states and develop novel schemes for quantum communication in multiparty networks. We have explored the nonlocal properties of different N-qubit partially entangled states and generalized the well-known relationship between 2-qubit entanglement and violation of the Bell-CHSH inequality to the case of N-qubit entanglement and violation of an N-qubit Bell inequality. Based on these results, we discuss the use of partially entangled states for optimal communication in a network. We also discuss new protocols for quantum voting and password sharing.

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Multiple-copy state discrimination: Thinking globally, acting locally
by
Brendon Higgins

Institute for Quantum Computing, University of Waterloo
Coauthors: Andrew C. Doherty, Steven D. Bartlett, Geoff J. Pryde, Howard M. Wiseman

The degree to which pairs of nonorthogonal qubit states can be discriminated is an important topic for gaining insight into practical and fundamental questions about quantum measurement. In this context, it is known that, when a sequence of multiple copies of a state are available, discrimination schemes that employ adaptive local measurements that are locally optimized (i.e. optimized for each copy measured) can exhibit higher error rates than schemes in which the measurement bases are fixed. In this case, a scheme employing optimization over the entire set of measurements (global optimization) is necessary to attain the least error with conclusive outcomes.

Here we theoretically investigate the different schemes one obtains by applying local and global optimization techniques to the task of discriminating qubit states under depolarizing mixture. We consider both fixed and adaptive local-measurement approaches, and compare their performance (in terms of error probability) to that of an optimal entangling measurement on all the available copies. For moderate numbers of copies, we find global optimization produces complicated patterns of measurement settings, compared to those produced by local optimization. As the number of copies grows large, we determine asymptotic scalings of discrimination schemes, verifying that adaptivity gains no advantage over fixed schemes in this regime. Furthermore, we show that here the most naive local measurement scheme, employing fixed locally-optimal local measurements on each copy, exhibits scaling of error probability as good as any other local measurement scheme, except for qubit states with less than about 2% mixture.

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Isotropic and squeezed fluctuations in n-qubit system
by
A.B. Klimov

Dept. de Física, Universidad de Guadalajara, Guadalajara, Mexico
Coauthors: C. Munoz

We show that 2n coherent states of an n-qubit system, generated by application of the discrete displacement operators to a symmetric fiducial state have isotropic fluctuations, with á DS2 ñ = n, in a specific tangent plane, which in general is not orthogonal to the mean spin direction. This allows to use them as reference states to define a discrete squeezing for non-symmetric n -qubit states. Examples of states with reduced fluctuations, obtained after application of XOR gates to correlate (partially entangle) qubits are analyzed. We also extend the idea of the isotropic fluctuation plane to n-qubit states with different types of symmetry, which allows us to characterize quantum correlations (in particular squeezing) in terms of specific second-order moments.

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Optimal Trajectories for Quantum Adiabatic Processing
by
Jordan Kyriakidis

Dalhousie University
Coauthors: William Macready (D-Wave Systems, inc.), Robert Archibald (Dalhousie University), Navid Yousefabadi (Dalhousie Unversity)

We show how any classical logic circuit (eg, multiplication) can be expressed as an optimisation problem. The resulting circuit is effectively omnidirectional; for example, output states can be fixed and the corresponding input states computed; the multiplication circuit can thereby be used for factoring, with the optimisation cast as a problem in adiabatic quantum computing. We have developed general heuristics to find explicit trajectories from initial to final model Hamiltonians that minimise the overall computation time. Particularly for NP-type problems, where high-accuracy is not essential, trajectories have been found whose efficacy vastly exceeds that of the usual linear trajectory, and may even change the scaling behaviour of the algorithm. Explicit examples will be given, including the factoring of up to 6-bit integers.

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A variational master equation approach to dissipative energy transfer dynamics
by
Ahsan Nazir

Imperial College London
Coauthors: Dara McCutcheon

Recent experiments demonstrating signatures of quantum coherence in the energy transfer dynamics of a variety of light-harvesting systems [1] have sparked renewed interest in the theoretical modelling of energy transfer processes. A major challenge remains the development of techniques which allow one to probe the diverse parameter regimes relevant to such systems. Master equation methods provide useful tools with which to efficiently analyse energy transfer dynamics in the presence of an external environment. However, they are often valid only in rather restrictive parameter regimes, limiting their applicability in the present context.

Here, I shall present a versatile variational master equation approach to the non-equilibrium dynamics of dissipative quantum systems, that allows for the exploration of a wide range of parameter regimes within a single formalism. Derived through the combination of a variationally-optimised unitary transformation [2] and the time-local projection operator technique, the master equation can be applied to a range of bath spectral densities, and can account for both non-Markovian and non-equilibrium environmental effects. Applying the formalism in the case of excitation energy transfer, I shall show that while it correctly reproduces Redfield [3], polaron [4], and Foerster [5] dynamics in the appropriate limits, it can also be used in intermediate regimes where none of these theories may be applicable.

Variational master equations thus represent a promising avenue for the exploration of dissipative dynamics in a variety of physical systems.

[1] See, for example, H. Lee, Y.-C. Cheng, and G. R. Fleming, Science 316, 1462 (2007); G. S. Engel et al., Nature 446, 782 (2007); E. Collini and G. D. Scholes, Science 323, 369 (2009); E. Collini et al. Nature 463, 644 (2010); G. Panitchayangkoon et al., Proc. Natl. Acad. Sci. 107, 12766 (2010)

[2] R. Silbey and R. A. Harris, J. Chem. Phys. 80, 2615 (1984)

[3] A. G. Redfield, Adv. Magn. Reson. 1, 1 (1965)

[4] S. Jang et al., J. Chem. Phys. 129, 101104 (2008); A. Nazir, Phys. Rev. Lett. 103, 146404 (2009)

[5] Th. Foerster, Discuss. Faraday Soc. 27, 7 (1959)

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All non-classical correlations can be activated into distillable entanglement
by
Marco Piani

Institute for Quantum Computing and Department of Physics and Astronomy, University of Waterloo
Coauthors: Sevag Gharibian, Gerardo Adesso, John Calsamiglia, Pawel Horodecki, Andreas Winter

We devise a protocol in which general non-classical multipartite correlations produce a physically relevant effect, leading to the creation of bipartite entanglement. In particular, we show that the relative entropy of quantumness, which measures all non-classical correlations among subsystems of a quantum system, is equivalent to and can be operationally interpreted as the minimum distillable entanglement generated between the system and local ancillae in our protocol. We emphasize the key role of state mixedness in maximizing non-classicality: Mixed entangled states can be arbitrarily more non-classical than separable and pure entangled states.

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Integrated Quantum Photonics
by
Alberto Politi

University of Bristol
Coauthors: Jonathan C. F. Matthews, Anthony Laing, Alberto Peruzzo, Konstantinos Poulios, Jasmin Meinecke, Daniel Fry, Damien Bonneau, Pete Shadbolt, Pruet Kalasuwan, Xiao-Qi Zhou, Mirko Lobino, Mark G. Thompson and Jeremy L. O'Brien

Quantum information science aims to harness uniquely quantum mechanical properties to enhance measurement and information technologies, and to explore fundamental aspects of quantum physics. Encoding quantum information in photons is appealing thanks to their low-noise properties and ease of manipulation at the single qubit level, and their promise in the fields of quantum communication, metrology and other quantum technologies. We have developed an integrated waveguide approach to photonic quantum circuits for high performance, miniaturisation and scalability. Here we report high-fidelity silica-on-silicon integrated optical realisations of key quantum photonic circuits, including two-photon quantum interference and a controlled-NOT logic gate. We have demonstrated controlled manipulation of up to four photons on-chip, including high-fidelity single qubit operations, using a lithographically patterned resistive phase shifter. We have used this architecture to implement a small-scale compiled version of Shor’s quantum factoring algorithm and demonstrated heralded generation of tunable four photon entangled states from a six photon input. We have combined waveguide photonic circuits with superconducting single photon detectors. Finally, we describe complex quantum interference behaviour in multi-mode interference devices with up to eight inputs and outputs, and quantum walks of correlated particles in arrays of coupled waveguides.

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Scavenging quantum information: Multiple observations of quantum systems
by
Peter Rapcan,
Slovak Academy of Sciences

Given an unknown state of a qudit that has already been measured optimally, can one still extract any information about the original
unknown state? Clearly, after a maximally informative measurement, the state of the system `collapses' into a post-measurement state from which the same observer cannot obtain further information about the original state of the system. However, the system still encodes a significant amount of information about the original preparation for a second observer who is unaware of the actions of the first one. We study how a series of independentobservers can obtain, or scavenge, information about the unknown state of a system (quantified by the fidelity) when they sequentiallymeasure it. We give closed-form expressions for the estimation fidelity, when one or several qudits are available to carry information about the single-qudit state, and study the `classical' limit when an arbitrarily large number of observers can obtain (nearly) complete information on the system. In addition to the case where all observers perform most informative measurements we study the scenario where a finite number of observers estimate the state with equal fidelity,regardless of their position in the measurement sequence; and the scenario where all observers use identical measurement apparata (up to a mutually unknown orientation) chosen so that a particular observer's estimation fidelity is maximized.

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Quantum tomography of photonic time -energy entanglement by photon bunching with short-time reference pulses
by
Changliang Ren

Graduate School of Advanced Sciences of Matter, Hiroshima University
Coauthors: Holger F. Hofmann (Hiroshima University)

Characterizing time-energy entanglement of photons is particularly challenging because of the difficulty of realizing time-resolved quantum measurements. Hence, we analyse the probability of obtaining the full quantum states of photons in their time-energy degree of freedom by bunching with short-time reference pulses. We show that the complete quantum coherence in time can be obtained using reference pulses in a superposition of two short time pulses. The application to entanglement shows that the method allows an efficient detection of temporal entanglement using entanglement witness criterion obtainable with only a minimal number of measurement settings.

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The Power of Many Settings or Many Outcomes in Experimental Demonstrations of EPR-Steering
by Dylan Saunders

Centre for Quantum Dynamics, Griffith University
Coauthors: Geoff Pryde, Dylan Saunders, Matt Palsson, David Evans, Steve Jones

Einstein Podolsky and Rosen (EPR) first highlighted the fact that ``as a consequence of two different measurements performed upon the first system, the second system may be left in states with two different [kinds of] wave functions'' [1]. In the same year, Schrödinger introduced the term steering to describe the EPR paradox, and discussed the possibility of using more than two kinds of measurements [2]. Surprisingly, it is only very recently that general EPR-steering inequalities, allowing for measurements of an arbitrary number of different observables by the two parties, have been developed [3], following the first formal definition of EPR-steering [4]. This proved that demonstrating EPR-steering is strictly easier than demonstrating Bell-nonlocality, but strictly harder than demonstrating entanglement (that is, nonseparability).

I will describe two recent experimental demonstrations of this hierarchy. In [5] we implemented more than two settings so as to be able to show, for the first time, that EPR-steering occurs for mixed entangled states that cannot possibly demonstrate Bell-nonlocality. Increasing the number of measurement settings beyond two – we use up to six – dramatically increases the robustness of EPR-steering to noise. In [6] we implemented the maximally parsimonious demonstrations of the three types of nonlocality, involving 16, 12, and 9 different possible joint outcomes in the cases of Bell-nonlocality, EPR-steering, and entanglement respectively. In the latter two cases, this involved using a non-projective 3-outcome measurement (the “trine”).

REFERENCES
[1] A. Einstein, B. Podolsky and N. Rosen, Phys. Rev. 47, 777 (1935).
[2] E. Schrödinger, Proc. Camb. Phil. Soc. 31, 555 (1935).
[3] E. G. Cavalcanti, S. J. Jones, H. M. Wiseman, & M. D. Reid, Phys. Rev. A.80, 032112 (2009).
[4] H. M. Wiseman, S. J. Jones and A. C. Doherty, Phys. Rev. Lett. 98, 149492 (2007).
[5] D. J. Saunders, S. J. Jones, H. M. Wiseman, and G. J. Pryde, Nature Physics 6, 845-9 (2010).
[6] D. J. Saunders et al., http://arxiv.org/abs/1103.0306

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On the efficiency of excitonic energy transport
by
Torsten Scholak

Physikalisches Institut, Albert-Ludwigs-Universitat Freiburg, Hermann-Herder-Strass
Coauthors: Thomas Wellens, Andreas Buchleitner

What is the role of quantum coherence for the mechanisms underlying efficient energy transport though photosynthetic light-harvesting complexes? To explore this question, we conduct a large-scale statistical survey of excitation transport in ensembles of spatially disordered, finitely sized molecular networks with dipolar interactions in the presence of tunable dephasing noise, and we compare the efficiency of noise-assisted transport with that achievable by means of constructive quantum interference. In contrast to the common presumption that coherent effects generally lead to localization and thus to suppression of transport, we prove the existence of certain rare optimal molecular configurations that mediate highly efficient coherent excitation transport. Although dephasing noise---which gradually destroys interference and thereby gives rise to essentially classical transport---enhances the efficiency of most configurations in our statistical ensemble, the detected optimal configurations yield systematically higher transport efficiencies and attain the maximum efficiency in the absence of noise. These insights---combined with recent experimental demonstrations [1] of long-lived coherence in certain light-harvesting structures---provide a strong hint that nature takes advantage of quantum mechanical coherent dynamics in order to enhance the efficiency of principal tasks.

[1] G. S. Engel et al., Nature, 446, 782 (2007)

[2] T. Scholak et al., arXiv:1103.2944v1

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Practical characterization of quantum devices without tomography
by
Marcus P. da Silva

Raytheon BBN Technologies
Coauthors: Marcus P. da Silva (Raytheon BBN Technologies), Olivier Landon-Cardinal (U. de Sherbrooke), David Poulin (U. de Sherbrooke)

The complexity of quantum tomography experiments presents a major obstacle for the characterization of even moderately large quantum information devices. Part of the problem is that tomography generates much more data than is actually sought. We describe how a more targeted approach allows for (i) the verification of the fidelity of an experiment to a theoretical state or process, and for (ii) the estimation of which state or process from a reduced subset best matches the experimental data. Both these cases lead to a significant reduction in quantum and classical resources when compared against tomography - in general this is a quadratic reduction, but for some cases of practical interest we obtain an exponential reduction. In particular, we show that for fidelity estimation a constant number of different local experimental settings always suffices, and that this number of settings is always smaller than what is required for tomography.

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Broadband waveguide quantum memory for entangled photons
by
Neil Sinclair

Institute for Quantum Information Science and Department of Physics and Astronomy, University of Calgary
Coauthors: E. Saglamyurek (University of Calgary) J. Jin (University of Calgary) J. A. Slater (University of Calgary) D. Oblak (University of Calgary) F. Bussières (University of Geneva) M. George (University of Paderborn) R. Ricken (University of Paderborn) W. Sohler (University of Paderborn) W. Tittel (University of Calgary)

Reversible mapping of quantum states, particularly entangled states, between light and matter is important for advanced applications of quantum information science. This mapping, i.e. operation of a quantum memory [1], is imperative for realizing quantum repeaters [2] and quantum networks [3]. Here we report the reversible transfer of photon–photon entanglement into entanglement between a photon and a collective atomic excitation in a solid-state device [4] (see also [5]). Specifically, we generate time-bin enangled pairs of photons [6] at the low-loss 795 nm (in free-space) and 1532 nm (in fibre) wavelengths. The 795 nm photons are sent into a thulium-doped lithium niobate waveguide cooled to 3K, absorbed by the Tm ions, and retrieved after 7 ns by means of a photon-echo quantum memory protocol employing an atomic frequency comb [7]. The acceptance bandwidth of the memory has been expanded to 5 GHz, more than one order of magnitude larger than the previous state-of-the-art [8], to match the spectral width of the filtered 795 nm photons. The entanglement-preserving nature of our storage device is assessed through quantum state tomography before and after storage. Within statistical error, we find a perfect mapping process. Furthermore, by violating the CHSH inequality [9], we directly verify the nonlocal nature of the generated and stored entangled photons.

[1] A. Lvovsky, B. C. Sanders, and W. Tittel, Optical quantum memory, Nature Photonics 3, 706-71 (2009).

[2] N. Sangouard et al., Quantum repeaters based on atomic ensembles and linear optics, Rev. Mod. Phys. 83, 33-80 (2011).

[3] H. J. Kimble, The quantum internet, Nature 453, 1023-1030 (2008).

[4] E. Saglamyurek et al., Broadband waveguide quantum memory for entangled photons, Nature 469, 512-515 (2011).

[5] C. Clausen et al., Quantum storage of photonic entanglement in a crystal, Nature 469, 508-511 (2011).

[6] I. Marcikic et al., Distribution of time-bin entangled qubits over 50 km of optical fiber, Phys. Rev. Lett. 93, 180502 (2004).

[7] M. Afzelius et al., Multimode quantum memory based on atomic frequency combs, Phys. Rev. A 79, 052329 (2009).

[8] I. Usmani et al., Mapping multiple photonic qubits into and out of one solid-state atomic ensemble, Nat. Comm. 1 (12), 1-7 (2010).

[9] J. F. Clauser et al., Proposed experiment to test local hidden-variable theories, Phys. Rev. Lett. 23, 880-884 (1969).

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Spectral Manipulation of Optical Pulses Using the Gradient Echo Memory Scheme
by
Ben Sparkes

Centre for Quantum Computation and Communication Technology, The Australian National University
Coauthors: M. Hosseini, P. K. Lam, and B. C. Buchler

The burgeoning fields of quantum computing and quantum key distribution have created a demand for a quantum memory. The gradient echo memory (GEM) is one such scheme that can boast efficiencies approaching unity. Here we investigate the ability of GEM to spectrally manipulate light pulses stored in the memory. Spectral manipulation is important for pulse compression sideband extraction, and matching of pulse spectra to resonant and spectroscopic systems, as well as the potential to increase qubit rates in quantum communications networks. We present both theoretical and experimental results demonstrating the ability to shift the frequency, as well as spectrally compress or expand a pulse. Also the ability of GEM to recall different frequency components of a pulse at different times, and interfere two initially time separated pulses that are stored in the memory, are shown.

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Coherent Control of Intramolecular Energy Transfer in 24-mode Pyrazine
by
Ioannis Thanopulos

Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation
Coauthors: Xuan Li, Paul Brumer, and Moshe Shapir zine Xuan Li, Paul Brumer, Moshe Shapiro

We study the intramolecular energy transfer from the S2 excited electronic state of pyrazine into the energetically lower-lying first singlet state S1 due to internal conversion, during and after femtosecond laser irradiation. The dynamics is studied within an highly efficient methodology for computing quantum dynamics for radiationless transitions in multi-dimensional configurational spaces. We further investigate the use of bichromatic laser pulses with simple analytical pulse profiles in order to control the population transfer from S0 to the S2 and S1 excited electronic states. We find that 80% of the total initial population on S0 can be transferred to the S1 state within 100 fs by such pulses. We also find that the population in S2 can be 60% of the total initial population of S0 within 60 fs, and about 30% at the end of a 100 fs pulse.

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The curious nonexistence of Gaussian 2-designs
by
Peter Turner

University of Tokyo
Coauthors: Robin Blume-Kohout

Quantum t-designs –-- ensembles of quantum pure states whose t-th (and lower) moments mimic those of the uniform distribution of states in Hilbert space -–- have found a variety of applications in quantum information science and the foundations of quantum theory. They have primarily been studied in finite-dimensional Hilbert spaces, although some continuous-variable 1-designs (such as the coherent states) have a long and illustrious history. While 1-designs are nice, 2-designs seem to be the most useful and interesting. They are far superior to 1-designs –-- often optimal --– for a variety of tasks, including quantum state and process tomography.

Finite-dimensional 2-designs include mutually unbiased bases (MUBs) and symmetric informationally complete POVMs (SICPOVMs), and are known to have an intimate relationship with what is sometimes called the generalized Pauli, or Heisenberg-Weyl, group of transformations. The natural extension of this to infinite dimensional Hilbert space is the group of displacements and squeezings in phase space, the symplectic group --- indeed, the concept of discrete phase space was motivated in part by the study of 2-designs. The symplectic group is similarly intimately related with the set of Gaussian states on phase space. It is therefore natural to think that the subset of uniformly distributed Gaussian states form a 2-design for infinite dimensions.

However, we find that 2-designs for continuous variables cannot be constructed out of Gaussian states, and furthermore that it is not even possible to get close using them. We will discuss how this surprising situation arises and repercussions for continuous variable quantum systems.

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Quantum Cryptography Approaching the Classical Limit
by
Christian Weedbrook

University of Toronto
Coauthors: Stefano Pirandola, Seth Lloyd, and Timothy C. Ralph

We consider the security of continuous-variable quantum cryptography as we approach the classical limit, i.e., when the unknown preparation noise at the sender’s station becomes significantly noisy or thermal (even by as much as 10, 000 times greater than the variance of the vacuum mode). We show that, provided the channel transmission losses do not exceed 50on the channel transmission, and is therefore incredibly robust against significant amounts of excess preparation noise. We extend these results to consider for the first time quantum cryptography at wavelengths considerably longer than optical and find that regions of security still exist all the way down to the microwave.

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Nondeterministic fast ground state cooling of a mechanical resonator
by
Lian-ao wu

department of theoretical physics, university of the Basque Country
Coauthors: Yong Li, Lian-Ao Wu, Ying-Dan Wang, Li-Ping Yang

We present an ultrafast feasible scheme for ground state cooling of a mechanical resonator via repeated random time-interval measurements on an auxiliary flux qubit. We find that the ground state cooling can be achieved with several such measurements. The cooling efficiency hardly depends on the time-intervals between any two consecutive measurements. The scheme is also robust against environmental noises

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A general framework of weak measurement and its application to optical non-linearlity
by
Shengjun Wu

Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China
Coauthors: Yang Li

We extend the original idea of weak measurement to the case of a general preselection (mixed state) and a general postselection (a projection onto a subspace), we provide a complete treatment for both the regime when the preselection and the postselection (PP) are almost orthogonal and the regime when they are exactly orthogonal. We surprisingly find that for a fixed interaction strength, there may exist a maximum signal amplification and a corresponding optimum choice of PP to achieve it. We also find interesting quantities, the orthogonal weak values, which play the role of weak values for the case when the PP are exactly orthogonal. We also study how weak measurement can amplify the nonlinearity effect in a nonlinear crystal.

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Poster Contributors
L M Arevalo Aguilar Properties of the three qubits entangled state generated by the application of the Relative Phase Gate
Victor V. Albert Two-Photon Parity and Analytical Approximations to the Two-Photon Rabi Hamiltonian
Viacheslav Burenkov Security of high speed quantum key distribution with finite detector dead time
John Calsamiglia Attainability of Chernoff bound by LOCC
Eric Chitambar The Number of Measurement and Broadcast Rounds Needed to Perform Certain LOCC Operations
Artur Czerwinski Mutually unbiased bases in Majorana representation
Ardavan Darabi Violation of Heisenberg’s Precision Limit by Weak Measurements Using a Composite Circuit One-Way model of Quantum Computing
Hossein Tavakoli Dinani Qutrit squeezing via semiclassical evolution
Greg Dmochowki Towards single-photon cross-phase modulation in cold atoms
Francesca Fassioli Quantum coherence and optimal electronic energy transfer in light-harvesting antenna proteins PE545 and PC645 of cryptophyte algae
Amir Feizpour On the Possibility of Amplifying Single-Photon Nonlinearity Using Weak Measurement
Chi-Hang Fred Fung Universal Squash Model For Optical Communications Using Linear Optics And Threshold Detectors
Savannah Garmon Analysis of exceptional points in open quantum systems and QPT analogy for the appearance of the resonant state
Timur Grinev Coherent Quantum Control in a System of Overlapping Resonances: Simultaneous Excitation and Decay to the Continuum
Alex Hayat Multidimensional Quantum Communication by Temporal Phase Manipulation
Hoda Hossein-Nejad Delocalization-Enhanced Long-Range Energy Transfer between Cryptophyte Algae PE545 Antenna Proteins
Piotr Kolenderski Playing the Aharon-Vaidman quantum game with a photonic qutrit.
Dawei Lu Simulation of chemical isomerization reaction dynamics on an NMR quantum simulator
Dylan Mahler Finding Decoherence Free Subspaces Without Quantum Process Tomography
Iman Marvian A generalization of Noether's theorem and the information-theoretic approach to the study of symmetric dynamics
Leonardo A Pachon Thermalization of Open Quantum Systems
Nicolás Quesada Entanglement dynamics in coupled harmonic oscillators
Roya Radgohar Decoherence in quantum walks on one-dimensional regular networks
Rebecca Ronke Knitting distributed cluster states with spin chains
Lee A. Rozema On the Choice of Input States for Process Tomography
Alexandr Sergeevich Adaptive qubit Hamiltonian parameter estimation in presence of dephasing applied to double quantum dots.
Astha Sethi Control in classical limit: Robustness against decoherence in an optical lattice
Cathal Smyth Measures and Implications of Electronic Coherence in Photosynthetic Light-Harvesting
Yasaman Soudagar A Photonic Loop-Graph State for One-way Quantum Computing
Nathan Wiebe Improved Accuracy for Adiabatic Quantum State Transfer
Mark M. Wilde Quantum trade-off coding for bosonic communication
Xingxing Xing Slowing single photons with cold Rb atoms
Feihu Xu High speed quantum random number generation with quantum phase noise
Claire X. Yu Mechanisms of quantum energy transfer in spin chains

Properties of the three qubits entangled state generated by the application of the Relative Phase Gate
by
L M Arevalo Aguilar

Benemerita Universidad Autonoma de Puebla
Coauthors: P. C. Garcia Quijas (Instituto Tecnologico de Leon)

In this contribution we study the properties of a new entangled state. This state is generated by the application of the Relative Phase Gate (with an arbitrary phase #966;) on three qubits. The Relative Phase gate was recently defined (Quan. Inf. Compt. 10, 190 (2010)) for two and three qubits and it represents a slightly different conditional quantum evolution to that which appears in the application of the usual quantum gates. We calculate some entangled measures, like the residual tangle, defined for three qubits to measure the degree of entanglement of this states.

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Two-Photon Parity and Analytical Approximations to the Two-Photon Rabi Hamiltonian
by
Victor V. Albert

Chemical Physics Theory, Department of Chemistry
Coauthors: Gregory D. Scholes and Paul Brumer

We study a close relative of the well-known spin-boson/Rabi Hamiltonian, the two-photon Rabi Hamiltonian (TPRH). This Hamiltonian describes a two-level system interacting with a quantum harmonic oscillator via quadratic coupling. As opposed to a displacement in position in the case of the Rabi Hamiltonian, the coupling in the two-photon Rabi Hamiltonian is through frequency displacement or “squeezing.” This Hamiltonian arose from describing two-photon processes in quantum optics and can potentially model any two-level system for which the two levels are at different frequencies.
We introduce the bosonic two-photon parity operator, which allows us to separate the TPRH into four parity subspaces, providing insight into the mathematical structure of its spectrum, significantly simplifying the numerics, and revealing some interesting dynamical properties. As an example of the power of this technique, we obtain a highly accurate analytical approximation to the energies of this Hamiltonian.

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Security of high speed quantum key distribution with finite detector dead time
by
Viacheslav Burenkov

University of Toronto
Coauthors: Bing Qi, Ben Fortescue, Hoi-Kwong Lo

The security of a high speed quantum key distribution system with finite detector dead time t is analyzed. When the transmission rate becomes higher than the maximum count rate of the individual detectors (1/t), security issues affect the algorithm for sifting bits. Analytical calculations and numerical simulations of the Bennett-Brassard BB84 protocol are performed. We study Rogers et al.'s protocol (introduced in "Detector dead-time effects and paralyzability in high-speed quantum key distribution, " New J. Phys. 9 (2007) 319) in the presence of an active eavesdropper Eve who has the power to perform an intercept-resend attack. It is shown that Rogers et al.'s protocol is no longer secure. More specifically, Eve can induce a basis-dependent detection efficiency at the receiver's end. Modified key sifting schemes that are secure in the presence of dead time and an active eavesdropper are then introduced. We analyze and compare these secure sifting schemes for this active Eve scenario, and calculate and simulate their key generation rate. It is shown that the maximum key generation rate is 1/(2t) for passive basis selection, and 1/t for active basis selection. The security analysis for finite detector dead time is also extended to the decoy state BB84 protocol.

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Attainability of Chernoff bound by LOCC
by
John Calsamiglia

Universitat Autònoma de Barcelona, Spain
Coauthors: J. I. de Vicente, R. Munoz-Tapia and E. Bagan

Hypothesis testing is a fundamental problem in statistical inference and has been a crucial element in the development of information sciences. The Chernoff bound gives the minimal average probability of error when discriminating two hypothesis given a large number of i.i.d. observations. We have addressed the quantum counterpart of this problem, i.e., discriminate between two known states of a system given a large number of copies. The Quantum Chernoff bound gives an (asymptotically attainable) upper-bound for the error probability of discriminating many copies of two possible states using the most general collective measurement. We showed that in general this bound cannot be reached by repeating the same fixed measurements on every copy, but it remained an open question whether adaptive measurements schemes, which do use classical communication, can saturate the bound. We have shown how to efficiently compute bounds on the LOCC discrimination between two mixed states. In contrast with the pure--state case, these experimentally feasible protocols perform strictly worse than the general collective ones. We find that in order for LOCC and collective protocols to achieve the same accuracy, the former can require up to twice the number of copies than the latter. This gap in the error rates takes its largest value in the region of nearly pure, but strictly mixed, states. Excluding this region, there are no significant differences in performance between the simplest (repeated) and optimal LOCC strategies. This shows that while the performance of collective pure-state discrimination is not very much affected by the presence of noise, the optimal LOCC protocols change drastically if the states become slightly mixed. Similar approaches can be used to bound the power of separable strategies in other similar settings, which is still one of the most elusive questions in quantum communication.

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The Number of Measurement and Broadcast Rounds Needed to Perform Certain LOCC Operations
by
Eric Chitambar

The University of Toronto

Despite its importance to quantum information, the class of Local Operations with Classical Communication (LOCC) is still not satisfactorily understood. For instance, very little is known about what new operational possibilities become available using LOCC as more rounds of measurement and communication are performed. In this talk, I will present surprising new results concerning the round dependence of certain LOCC tasks.

In particular, we will see how the class of LOCC operations becomes strictly more powerful as additional rounds of classical communication are permitted. More precisely, for every n, there always exists an n round protocol that is impossible to implement in n-2 rounds. Furthermore, certain tasks become possible if and only if the protocol uses an infinite (unbounded) number of rounds.

The LOCC process examined is the conversion of the state |W> = v{1/3}(|100>+|010>+|001>) into bipartite pure entanglement shared between any two of three parties. Such a task is known as random distillation, having been first studied by Fortescue and Lo [Phys. Rev. Lett. 98, 260501 (2007)]. We will additionally find that, for the random distillation of |W> to succeed with probability one, the required number of rounds discontinuously jumps from four to an unbounded number when the amount of distilled entanglement gets too large.

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Mutually unbiased bases in Majorana representation
by
Artur Czerwinski

Nicolaus Copernicus University in Torun (Poland)
Coauthors: Piotr Kolenderski (Institute for Quantum Computing)

The Majorana representation, which was firstly shown in 1932, still remains an interesting problem and many of its aspects are so far unknown. The representation allows to present pure state of spin-J system as 2J points on Bloch-Riemann sphere . Moreover, when the action of unitary operator on the state is reflected in rotation of the corresponding points as rigid solid.
Recently one of us showed the generalization of Majorana representation for N spin-J systems [Open Sys. Inf. Dyn., 12, 107], which features the elegant behaviour when unitary transformations are considered. Moreover this approach proved to be particularly useful in analysis of decoherence free subspaces.
This contributions aims in further development of Majorana representation and analysis of its application in quantum information science. In particular, we investigate the geometrical structure of orthogonal states within both Majorana representation and its generalization. The importance of mutually unbiased bases in quantum information is apparent. It turns out that the Majorana representation provides a helpful approach for deeper understanding of the structure of the basis states.

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Violation of Heisenberg’s Precision Limit by Weak Measurements Using a Composite Circuit One-Way model of Quantum Computing
by
Ardavan Darabi

University of Toronto
Coauthors: Lee Rozema(University of Toronto) Dylan Mahler(University of Toronto) Yasaman Soudagar(University of Toronto) Alex Hayat(University of Toronto) Aephraim Steinberg(University of Toronto)

Along with the uncertainty principle that relates simultaneous statistics of non-commuting observables for a quantum state, Heisenberg postulated another set of relations which set a lower limit on the disturbance to an observable caused by a second measurement of another possibly non-commuting observable[1]. These relations, though previously accepted, were shown to be inaccurate [2] shedding doubt on various widely-accepted limitations of high-precision microscopy, spectroscopy and other metrology concepts and offering new insights into foundations of quantum physics.

A theoretical scheme for testing the precision-disturbance relation of Ozawa based on quantum information concepts was proposed in [3]. In this proposal the hurdle of destructive measurements, which previously impeded such tests, is addressed by the weak value approach of [4]. This scheme is based on a 3-qubit quantum circuit that requires two controlled-NOT gates of variable strength with a common control qubit.

Here, we present an experimental realization of Heisenberg’s precision limit violation based on weak value measurements. We implement a one-way quantum computing technique, using entanglement as the substrate for quantum gates. A pair of polarization entangled photons is used to carry out two consecutive CNOT operations on one qubit, where the outcome of the first CNOT is teleported to the second photon of the pair.

[1] Heisenberg W 1983 Quantum Theory and Measurement ed J A Wheeler and W H Zurek (Princeton, NJ:Princeton University Press) pp 62–84 (originally published in 1927 Z. Phys. 43 172) [2] Ozawa M 2004 Ann. Phys. NY 311 350 [3] Lund A P and Wiseman H 2010 New J. Phys. 12 093011 [4] Aharonov Y, Albert D Z and Vaidman L 1988 Phys. Rev. Lett. 60 1351

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Qutrit squeezing via semiclassical evolution
by
Hossein Tavakoli Dinani

Department of Physics, Lakehead University, Thunder Bay, Ontario P7B 5E1, Canada
Coauthors: Andrei B. Klimov, Hubert de Guise

Recently quantum systems more complex than the qubit have been studied in the context of possible applications to quantum information processes. In particular, the qutrit-like systems with symmetry group SU(3) naturally appear in three-level system. In this talk we will discuss how a particular type of squeezing, derived by comparing the minimum fluctuation of an observable and the fluctuation of this observable in a coherent state, can be understood using an approximate form of SU(3) Wigner functions. The short time dynamics required to generate this squeezing is obtained from the classical evolution generated by a simple Hamiltonian quadratic in the generators of su(3). Some numerical comparison between the dynamics for the fully quantum, exact Wigner functions and its approximation will be presented.

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Towards single-photon cross-phase modulation in cold atoms
by
Greg Dmochowki

University of Toronto
Coauthors: X. Xing, A. Feizpour, C. Zhuang, M. Hallaji, A.M. Steinberg

Single photons have long been thought of as ideal quantum information carriers but due to their very weak interactions with one another, have encountered obstacles as an architecture for quantum computation itself. The inability to efficiently and deterministically perform universal quantum logic gates using photons has prevented their usage in such contexts.
Attempts to get around this have made use of a non-linear optical medium, which can act as a mediator between two light fields. A Chi-3 non-linear medium, for example, can produce an effective light-light interaction by way of cross-phase modulation, whereby the presence of one light field in the medium produces a phase shift on the second light field. Making use of this interaction with a single photon is very challenging as the resulting phase shift is exceedingly small. However, there exist tricks such as electromagnetically-induced transparency, which utilize atomic coherence effects to enhance the non-linear phase shift. This has allowed for the measurement of a cross-phase shift at the few hundred photon level (Chen et al. 2010). Moreover, recent insights into weak measurements have proposed that judicious post-selection can further improve the signal to noise ratio in such experiments (Feizpour et al. 2011).
Given these tools, a highly sensitive technique to measure small phase shifts is still necessary. The ability to detect such a cross-phase shift would allow us to perform quantum non-demolition measurements of single photons, which then pave the way for the implementation of universal quantum logic gates. We report on our progress in utilizing beat-note interferometry to measure the phase shift produced by a single photon on a classical field.

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Quantum coherence and optimal electronic energy transfer in light-harvesting antenna proteins PE545 and PC645 of cryptophyte algae
by
Francesca Fassioli

University of Toronto
Coauthors: Alexandra Olaya-Castro and Gregory D. Scholes

We study electronic energy transfer (EET) in the light-harvesting antenna proteins PE545 and PC645 isolated from marine cryptophyte algae, where long-lived quantum coherence has been reported at room temperature [1]. To do so, we apply a recently developed non-perturbative method, based on the reduced hierarchical equation approach [2-4]. This method is capable of interpolating between the regimes of weak and strong system-bath coupling, thus allowing an analysis of EET under a wide range of environmental parameters. The optimal conditions for fast and efficient EET, as well as the role of quantum coherence are investigated in this work.

[1] E. Collini, C.Y. Wong, K.E. Wilk, P.M.G. Curmi, P. Brumer and G.D. Scholes, Nature 463 (2010) 644.
[2] Y. Tanimura and R. Kubo, J. Phys. Soc. Jpn. 58 (1989) 101.
[3] A. Ishizaki and G. R. Fleming, J. Chem. Phys. 130 (2009) 234111.
[4] Q. Shi, L. Chen, G. Nan,1 R-X. Xu and Y. Yan, J. Chem. Phys. 130 (2009) 084105.

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On the Possibility of Amplifying Single-Photon Nonlinearity Using Weak Measurement
by
Amir Feizpour

University of Toronto, Physics Department
Coauthors: Xingxing Xing, Aephraim Steinberg

One of the most important challenges of optical quantum information processing has been to create and detect optical non-linearities at few-photon level. There has been many new proposals for engineering larger non-linear effects, eg by using atomic coherence effects such as electromagnetically induced transparency. However a great deal of conceptual and technical progress is needed before obtaining the required non-linearities.
On the other hand, there have recently been several demonstrations of how weak measurements can be used to amplify very small physical quantities in systems dominated by technical noise. This "weak value amplification" is known to lead to large effects on a "probe", but at the expense of discarding a large fraction of the data; the ultimate effect on signal-to-noise ratio given this tradeoff remains a subtle question.
We show theoretically that weak measurement can be used to “amplify” optical nonlinearities at the single photon level, such that the effect of one properly post-selected photon on a classical beam may be as large as that of many un-post-selected photons. We find that “weak-value amplification” offers a marked improvement in signal-to-noise ratio in the presence of technical noise with long correlation times. Unlike previous weak measurement experiments, our proposed scheme has no classical equivalent.

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Universal Squash Model For Optical Communications Using Linear Optics And Threshold Detectors
by
Chi-Hang Fred Fung

University of Hong Kong
Coauthors: H. F. Chau and Hoi-Kwong Lo

Quantum communications often rely on single photons as information carriers in order to exploit their quantum mechanical properties. However, practical detectors are often threshold detectors that are incapable of resolving the number of photons received. This apparently subtle issue has surprisingly immense implication to many quantum communications protocols. In fact, it has been shown that this issue leads to many problems including fake violation of Bell's inequality, insecurity of quantum key distribution, and false entanglement verification. The source of these problems is the theoretical consideration of the incoming signals for detection being single-photon signals; but in practice they may be multi-photon signals. We report a universal solution that is protocol-independent to bridge this gap between theory and experiments.

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Analysis of exceptional points in open quantum systems and QPT analogy for the appearance of the resonant state
by
Savannah Garmon

University of Toronto, CQIQC
Coauthors: Dvira Segal (University of Toronto), Naomichi Hatano (University of Tokyo), Ingrid Rotter (MPI for the Physics of Complex Systems)

We propose an analysis technique for the exceptional points (EPs) occurring in the discrete spectrum of open quantum systems, relying on a semi-infinite chain coupled to an endpoint impurity as a prototype model. We outline our method to locate the EPs in such systems and carry this out for our prototype, further obtaining an eigenvalue expansion in the vicinity of the EPs that gives rise to characteristic exponents. Finally, we offer a heuristic QPT analogy for the emergence of the resonance (giving rise to broken time symmetry via exponential decay) in which the decay width plays the role of the order parameter; the associated critical exponent is determined by the eigenvalue expansion in the vicinity of the EP.

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Coherent Quantum Control in a System of Overlapping Resonances: Simultaneous Excitation and Decay to the Continuum
by
Timur Grinev

Chemical Physics Theory Group, Department of Chemistry, and CQIQC, University of Toronto
Coauthors: Moshe Shapiro, Paul Brumer

The coherent control of the simultaneous (weak field) preparation and decay of a system of overlapping resonances coupled to a continuum of states has been investigated. The current approach is a generalization of a previous theory for the post-preparation coherent control of internal conversion in the presence of overlapping resonances. The relation between the previous and present theories has been exposed. Two control objectives with different constraints have been investigated numerically in a simple one-dimensional iodine bromide model involving two or more overlapping resonances.

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Multidimensional Quantum Communication by Temporal Phase Manipulation
by
Alex Hayat

Centre for Quantum Information and Quantum Control, and the Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7, Canada.
Coauthors: Xingxing Xing, Amir Feizpour, and Aephraim M. Steinberg

In contrast to photon internal polarization degree of freedom limited only to two dimensions, external degrees of freedom related to space and time can be used to construct infinite dimensional Hilbert spaces. Spatial degree of freedom has been recently employed to encode multidimensional quantum information using photon orbital angular momentum, however this approach is not suitable for the single-mode fiber optical communication. The temporal degree of freedom has been employed for both quantum communication and entanglement. However the in these Franson-type interferometer approaches, the Hilbert space is limited to only a few discrete time bins. Recently, single-photon sates generated in cavity parametric downconversion (PDC) with extremely long coherence time have become available, allowing the full advantage of the infinite-dimensional temporal degree of freedom.
Here we propose a multidimensional quantum communication scheme based on temporal phase modulation of long-coherence single photons. In this scheme, the multidimensional Hilbert space basis are comprised of an infinite set of orthonormal temporal phase profiles. Ultralong coherence times enable using electronics for electro optical modulation (EOM) of the heralded single photon wavepacket from PDC in the transmitter in order to determine the quantum state to be transmitted, while the multidimensional projection in the receiver is realized by splitting the input into several branches and writing a phase of the state to be projected onto in every branch, followed by a narrowband optical filter.
For characterization of time-entanglement, not limited to a small number of time bins, we show that temporal phase modulation can be employed for quantum state tomography in the infinite time Hilbert space. In contrast to time bin approach where only two or three times interfere, in our scheme many different times interfere in order to yield a projection in the detector. The time resolution in this scheme is limited only by the speed of the EOM relative to the coherence length of the photon, and therefore in principle this time-entanglement characterization can be continuous.

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Delocalization-Enhanced Long-Range Energy Transfer between Cryptophyte Algae PE545 Antenna Proteins
by
Hoda Hossein-Nejad

University of Toronto
Coauthors: Carles Curutchet, Greg Scholes

We study the dynamics of interprotein energy transfer in a cluster, consisting of four units of phycoerythrin 545 (PE545) antenna proteins via a hybrid quantum-classical approach. Our results indicate that persistent exciton delocalization is an important implication of the quantum nature of energy transfer on a multiprotein length scale, and that a hybrid quantum-classical approach is a viable starting point in studies of long-range energy transfer in condensed phase biological systems.

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Playing the Aharon-Vaidman quantum game with a photonic qutrit.
by
Piotr Kolenderski

Instytut Fizyki UMK
Coauthors: Li Youning, Tong Zhao, Mathew Volpini, Urbashi Shinha, Thomas Jennewein

We present a simple experimental scheme, which allows to encode and measure the quantum state of a qutrit and simulate the Aharon-Vaidman quantum game. The three level system is encoded in a spatial mode of a single photon passing through a system of slits. Within this scenario one can prepare the class of qutrit states by controlling the direction of a photon propagation and the number of slits that are open. The rank 7 POVM was implemented by placing a single photon detector in the respective positions related to "near and far field". This allowed us to topographically reconstruction of a pure state and play the quantum game.

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Simulation of chemical isomerization reaction dynamics on an NMR quantum simulator
by
Dawei Lu

University of Science and Technology, China

Quantum simulation can beat current classical computers with minimally a few tens of qubits. We report an experimental demonstration that a small nuclear-magnetic-resonance (NMR) quantum simulator is already able to simulate the dynamics of a prototype laser-driven isomerization reaction using engineered quantum control pulses. The experimental results agree well with classical simulations. We conclude that the quantum simulation of chemical reaction dynamics not computable on current classical computers is feasible in the near future.

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Finding Decoherence Free Subspaces Without Quantum Process Tomography
by
Dylan Mahler

Dept. of Physics, 60 St. George St., University of Toronto, Toronto, Ontario, Canada M5S 1A7
Coauthors: L. Rozema A. Darabi A.M. Steinberg

It is well known that one of the greatest challenges facing quantum computation and communication today is quantum decoherence. Quantum decoherence destroys information contained in quantum superposition states, and effectively creates classical mixtures. There have been several strategies for minimizing quantum deoherence, or even circumventing it, but these methods do not get rid of decoherence. Rather, they construct the system in such a way that decoherence can be corrected for. Another method for circumventing decoherence exists that allows decoherence to be minimized at the cost of information: decoherence free subspaces. In order to characterize a process completely, quantum process tomography must be performed. Quantum process tomography is exponentially expensive, in that it scales as 24n, where n is the number of qubits in the system. This task becomes rapidly infeasible as the size of the system increases. As a result, it is essential that we ask whether we can learn something about the process without doing full process tomography. In this poster, we will show that by only making a maximum of O(23n) measurements, we can identify all the decoherence free subspaces for a given process. If the process possesses certain properties, the number of measurements needed can be as few as O(22n) – on the order of state tomography. We also present an experiment in which a 2-qubit process containing a decoherence free subspace is characterized in only 32 measurements – instead of the 256 measurements required for full process tomography.

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A generalization of Noether's theorem and the information-theoretic approach to the study of symmetric dynamics
by
Iman Marvian

Perimeter Institute, IQC
Coauthors: Robert W. Spekkens

Information theory provides a novel approach to study of the consequences of symmetry of dynamics which goes far beyond the traditional conservation laws and Noether's theorem. The conservation laws are not applicable to the dissipative and open systems. In fact, as we will show, even in the case of closed system dynamics if the state of system is not pure the conservation laws do not capture all the consequences of symmetry. Using information theoretic approach to this problem we introduce new quantities called asymmetry monotones, that if the system is closed they are constant of motion and otherwise, if the system is open, they are always non-increasing. We also explain how different results in quantum information theory can have non-trivial consequences about the symmetric dynamics of quantum systems.

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Thermalization of Open Quantum Systems
by
Leonardo A Pachon

Department of Chemistry, University of Toronto
Coauthors: Paul Brumer

In generic isolated systems, non-equilibrium dynamics is expected to result in thermalization: a relaxation to states in which the values of macroscopic quantities are stationary, universal with respect to widely differing initial conditions, and predictable using statistical mechanics. From a classical point of view, the thermalization of a system S coupled to a thermal bath TB is an ergodic process. However, it not obvious, at the quantum level, to what extend this process can be considered as an incoherent situation. The situation is more involving when the system S+TB, already thermalized, is in contact with a second thermal bath BB. Here we show that second scenario, the thermalization of an open quantum system, is accompanied by the presence of quantum features.

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Entanglement dynamics in coupled harmonic oscillators
by
Nicolás Quesada

University of Toronto

I study the dynamics of the entanglement of two harmonic oscillators linearly coupled and undergoing decoherence by interacting with memoryless independent reservoirs. A Markovian master equation of the form [1]:

dr/dt=i [r, g(a1 a2f+a1fa2)]+ 2
å
i=1
{gj/2 ( 2 aj rajf-ajfajr-rajfaj )+Pj/2 ( 2ajf raj-aj ajfr-raj ajf ) }

for the reduced density operator r of the two bosonic modes with creation (annihilation) operators af, bf (a, b) is solved to obtain the time dynamics of the covariance matrix of the density operator. Because the underlying system plus reservoir dynamics that gives rise to the above master equation is linear, a state that has a Gaussian Wigner characteristic function (i.e. a Gaussian state) will evolve to another state with the same type of characteristic function [2]. To determine if the state of the system is entangled I use the Simon function [3] which gives a necessary and sufficient condition for entanglement between the two modes of a Gaussian state. I show that the steady state of the system is separable for all values of the parameters and analyze the dynamics of two mode squeezed states as well as those of a state that is initially the tensor product of the vacuum in one mode and a single mode squeezed state in the other.

[1] F. P. Laussy, E. del Valle, and C. Tejedor, “Luminescence spectra of quantum dots in microcavities. i. bosons, ” Phys. Rev. B, vol. 79, no. 23, p. 235325, 2009.
[2] A. Rivas, A. D. K. Plato, S. F. Huelga, and M. B. Plenio, “Markovian master equations: a critical study, ” New J. Phys., vol. 12, no. 11, p. 113032, 2010.
[3] R. Simon, “Peres-horodecki separability criterion for continuous variable systems, ” Phys. Rev. Lett., vol. 84, no. 12, pp. 2726–2729, 2000.

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Decoherence in quantum walks on one-dimensional regular networks
by
Roya Radgohar

University of Kurdistan, Sanandaj, Iran
Coauthors: Shahriar Salimi(university of Kurdistan)

We study the effect of decoherence in continuous-time quantum walks on one-dimensional regular networks. One-dimensional regular network with distance parameter l = 2, can be constructed by a ring lattice of N nodes in which each node is connected to its 2l nearest neighbors (l on either side). We assume that every node is represented by a quantum dot continuously monitored by an individual point contact. This measuring process induces decoherence. For small rates of decoherence, we use the perturbation theory and find that mixing time is independent of the distance parameter and is inversely proportional to the rate of decoherence. Therefore, small decoherence can make short the mixing time in continuous-time quantum walks. In appearance of large decoherence, our analytical results show that mixing time is inversely proportional to squared parameter l, but is linearly proportional to the decoherence rate. Thus, while adding links decreases mixing time, large decoherence deteriorates continuous-time quantum walks.

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Knitting distributed cluster states with spin chains
by
Rebecca Ronke

University of York
Coauthors: Irene D'Amico Tim P. Spiller

Cluster states are one of the fundamental resources of quantum computing and crucial for a vast number of quantum protocols. In this contribution, we present a protocol for producing distributed cluster state ladders of arbitrary length using only a single spin chain. Spin chains have recently been the subject of many studies and are a promising candidate for quantum information transfer. The proposed protocol makes use of a spin chain set up for perfect state transfer and requires access for injection and extraction of excitations at the two end spins only. An outline of potential sources of errors and their effect on the fabrication of cluster states will also be given.

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On the Choice of Input States for Process Tomography
by
Lee A. Rozema

University of Toronto
Coauthors: A. Darabi, D.H. Mahler, R. Blume-Kohout, and A. M. Steinberg

One of the roadblocks to quantum computation and cryptography is decoherence. In order bypass the problem of decoherence it must first be characterized. By performing quantum process tomography (QPT) the decohering effects of the system can be discovered. In standard QPT one sends a complete set of states through the process and characterizes each state at the output. From this information the process and the decoherence can be reconstructed. In order to best characterize the decoherence the set of input states must be sensitive to it.

In 2008, Lobino et. al. performed QPT using a set of coherent states as the input states. Coherent states have the advantage that they are relatively easy to prepare in the laboratory but if the goal is to characterize decoherence they leave much to be desired. Since states with smaller phase space structure are more sensitive to decoherence we study the advantages of characterizing a decohering process using different sets of input states.

Our systems are states of N identical photons distributed between two polarization modes, the resultant polarization state can be represented in phase space as a quasi probability distribution on the Poincare sphere. We generate sets of 2-photon and 3-photon input states and characterize their sensitivity to detecting a decohering process.

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Adaptive qubit Hamiltonian parameter estimation in presence of dephasing applied to double quantum dots.
by
Alexandr Sergeevich

University of Sydney, Australia
Coauthors: Stephen Bartlett (University of Sydney)

We present an optimized two-electron double-quantum-dot qubit Hamiltonian parameter estimation algorithms based on Bayesian reasoning. Qubit evolution in such a system is driven by magnetic field difference between the electrons in each part of the dot. This field is in part due to the nuclei of surrounding atoms, and because of its instability it is important to estimate its value quickly. We use a procedure consisting of repeated measurements in same fixed basis after different time intervals, which is motivated by experimental limitations on preparing and measuring qubits in a general basis (as required by most parameter estimation algorithms). Based on the set of outcomes one can make Bayesian inferences about the unknown qubit evolution frequency. The precision of estimation depends on the number of measurements performed and intervals between these measurements. Another important issue is the measurement procedure itself which gives higher fidelity for longer measurement times which are in turn much longer than the characteristic evolution times for solid state systems. Thus, in order to optimize the estimation it is crucial to minimize the number of measurement steps keeping in mind the trade-off between the measurement fidelity and measurement time as well. We present the optimal adaptive (each following evolution time interval depends on the outcomes of the previous measurements) and non-adaptive algorithms which can be used to estimate the qubit evolution frequency and show how the effects of measurement fidelity and dephasing will affect the results.

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Control in classical limit: Robustness against decoherence in an optical lattice
by
Astha Sethi

80 St George Street Toronto M3S 5H6
Coauthors: Prof. Paul Brumer

Recently it has been shown that control, measured as non-zero average momentum in an 1D optical lattice, survives in classical limit and is robust against moderate decoherence. In addition to the earlier work, the same system is studied here to understand the influence of the degree and type of decoherence on the control in the classical and quantum regime. The survival of transport in the presence of strong decoherence induced via spontaneous emission is observed. We demonstrate that even large number of jumps do not destroy the transport significantly as distinct from spatial jitter which does alter the dynamics and destroy transport. The underlying theory for such observations will be discussed in the poster.

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Measures and Implications of Electronic Coherence in Photosynthetic Light-Harvesting
by
Cathal Smyth

Physics Department, University of Toronto
Coauthors: Dr. Francesca Fassioli, Dr. Gregory D. Scholes

We present the various different methods employed in measuring delocalization in light harvesting complexes, and focus on deriving direct relations between traditional inverse participation ratios and entanglement measures. The B850 ring from the LH2 complex in Rhodopseudomonas acidophila acts as our model system. By analysing the behaviour of these metrics under Electronic Energy Transfer (EET) dynamics in the B850 ring, we conclude that measures of entanglement are far more robust (in terms of timescale, temperature and level of decoherence) than inverse participation ratios, and are therefore more appropriate for the purpose of studying the time evolution of coherence in a system.

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A Photonic Loop-Graph State for One-way Quantum Computing
by
Yasaman Soudagar

Laboratoire des fibres optiques,Centre d'optique, photonique et laser, École Polytechnique de Montréal
Coauthors: Xingxing Xing, Elham Kashefi, Nicolas Godbout, Aephraim M. Steinberg

We report on the experimental realization of a 4-qubit loop-graph state that uses polarization and path degrees of freedom of photons to implement the logical qubits. The loop in this graph allows one to use the so called ‘generalized flow’, which is shown to optimize the complexity depth of the computation and possibly reduce the required number of qubits compared to a cluster. In addition, this graph corresponds to a circuit with a time-like loop, hence it provides an operational method for better understanding such loops.

For the experiment we start with a symmetric maximally entangled polarization state. The path qubits are then added to each photon using 50:50 beam splitters. We use a novel method that takes advantage of the symmetry in polarization states to do a controlled-Z operation between the polarization of one photon and the path of the other only by using a half-wave plate. This is the key step that enables us to realize this loop graph that has never been implemented before.

Using 4-qubit state tomography we completely characterize the loop graph state. To show the equivalency of this graph state to the circuit, that is found using the generalized flow, we carry out various computations with this state and contrast the outputs to what we expect to get from an equivalent circuit.

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Improved Accuracy for Adiabatic Quantum State Transfer
by
Nathan Wiebe

IQC, University of Waterloo
Coauthors: Nathan Babcock

We present a technique that dramatically improves the accuracy of adiabatic state transfer for a broad class of realistic Hamiltonians. For some systems, the total error scaling can be quadratically reduced at a fixed maximum transfer rate and these improvements rely only on the judicious choice of the total evolution time. Our technique may be immediately applicable to existing experiments utilizing adiabatic passage. We give two examples as proofs-of-principle, showing quadratic error reductions for an adiabatic search algorithm and a tunable two-qubit quantum logic gate.

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Quantum trade-off coding for bosonic communication
by
Mark M. Wilde

McGill University
Coauthors: Patrick Hayden and Saikat Guha

Recent work has precisely characterized the achievable trade-offs between three key information processing tasks---classical communication (generation or consumption), quantum communication (generation or consumption), and shared entanglement (distribution or consumption), measured in bits, qubits, and ebits per channel use respectively. Slices and corner points of this three-dimensional region reduce to several well-known quantum communication protocols over noisy channels. A single trade-off coding technique can attain any point in the region and can outperform time-sharing between the best known protocols for accomplishing each information processing task alone. Previously, the benefits of trade-off coding that had been found were too small to be of much practical value (for the dephasing and the universal cloning machine channels, for instance). In this article, we demonstrate to the contrary that the associated performance gains are remarkably high for several physically relevant bosonic channels that model free-space / fiber-optic links, thermalizing channels, or amplifiers (or even relativistic communication). We show that significant performance gains from trade-off coding also apply when trading photon-number resources between transmitting public and private classical information simultaneously over secret-key-assisted bosonic channels.

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Slowing single photons with cold Rb atoms
by
Xingxing Xing

University of Toronto
Coauthors: Rockson Chang, Alex Hayat, Amir Feizpour, Aephraim Steinberg

Spontaneous Parametric Down-Conversion (SPDC) has been widely used to generate single photons and entanglement in many quantum information applications. Using a far-below-threshold Optical Parametric Oscillator (OPO), we sucessfully modified the output spectrum of the SPDC, which allows us to generate long-coherence-time single photons whose bandwidth is matched with the Rb natrual linewidth.

We demonstrate the slowing down of these single photons using a cold Rb cloud in a Magneto-Optical Trap (MOT). Due to the extreme narrow linewidth of the Rb transition compared to a conventional medium such as glass, the group velocity of the single photon is slowed dramatically down to ~104 m/s. Using a high resolution time-to-digital conversion unit, we are able to directly measure such a delay. Our measurement also serves as an important step towards the realization of many important quantum information proposals which involves using a large optical nonlinearities by cold atoms.

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High speed quantum random number generation with quantum phase noise
by
Feihu Xu

Center for Quantum Information and Quantum Control, Department of Physics and Department of Electrical & Computer Engineering, University of Toronto, Toronto, Ontario, Canada
Coauthors: Bing Qi, Xiongfeng Ma, He Xu, Haoxuan Zheng, and Hoi-Kwong Lo

Random number generators(RNG) are important in many felds of science and technology [1-3]. Currently, fast random numbers can easily be generated from either computer algorithms or the chaotic behavior of complex systems [4]. However, both of the two schemes are deterministic in nature and, thus cannot generate true random numbers with information-theoretically provable randomness. Quantum RNG (QRNG), on the other hand, is able to generate perfect random numbers from the truly probabilistic nature of fundamental quantum processes [5]. Up to now, QRNGs are limited by system implementation complexities and generation rates [5-7] owing to the strict requirements on measuring the quantum properties. The highest generation rate for the commercial QRNGs is only 16 Mbits/s [8].

Recently, our group [9] and Guo et al [10] presented a QRNG based on the laser's phase noise, and the generation rates were 500Mbits/s and 20 Mbits/s respectively. Here, we experimentally demonstrate a simple and ultrahigh speed QRNG by measuring the quantum phase noise of a single mode diode laser operating at a low intensity level. We employ the intrinsic randomness of the spontaneous emission phase noise of laser as the high entropy source and a phase stabilized delayed self-heterodyning system to measure the phase noise. One common problem existing in various QRNG schemes is how to deal with the contribution from classical noise sources added by a practical detection system, which in principle cannot be used to generate true random numbers. We solve this problem by quantifying and separating quantum entropy from classical entropy in both theoretical analysis and experimental verification. Combined with sophisticated post processing, the final generation rate is up to 6.25 Gbits/s, which is verified by stringent randomness test suits.

[1] N. Meteopolis and S. Ulam, Journal of the American Statistical Association 44, 335 (1949).
[2] B. Schneier and P. Sutherland, Applied cryptography: protocols, algorithms, and source code in C (John Wiley & Sons, Inc. New York, NY, USA, 1995).
[3] V. Jacques, E. Wu, F. Grosshans, F. Treussart, P. Grangier, A. Aspect, and J. Roch, Science 315, 966 (2007).
[4] I. Kanter, Y. Aviad, I. Reidler, E. Cohen, and M. Rosenbluh, Nature Photonics 4, 58 (2009).
[5] T. Jennewein, U. Achleitner, G.Weihs, H.Weinfurter, and A. Zeilinger, Review of Scientific Instruments 71, 1675 (2000).
[6] M. Wayne and P. Kwiat, Opt. Express 18, 9351 (2010).
[7] C. Gabriel, C. Wittmann, D. Sych, R. Dong, W. Mauerer, U. Andersen, C. Marquardt, and G. Leuchs, Nature Photonics 4, 711 (2010).
[8] http://www.idquantique.com/true-random-number-generator/products-overview.html
[9] B. Qi, Y.-M. Chi, H.-K. Lo, and L. Qian, Opt. Lett. 35, 312 (2010).
[10] H. Guo, W. Tang, Y. Liu, and W. Wei, Physical Review E 81, 051137 (2010).

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Mechanisms of quantum energy transfer in spin chains
by
Claire Yu, University of Toronto


In this study we investigated the mechanisms of energy transfer between two thermal baths through a chain of spin-1/2 particles, under the on- and off-resonance limits. The methods developed for analytic study are completely general and do not rely on the details of reservoirs, spin chain or their interaction. On-resonance refers to the scenario where the chain excitation frequency is included in the bath spectra, and master equation approach is used to solve for system dynamics. Off-resonance refers to when the spin-chain frequency is much higher than the bath cut-off frequencies, or when bath temperatures are low. Energy Transfer Born Oppenheimer (ETBO) formalism is developed for this case. It is found that energy transfer is ballistic under the on-resonance limit while it occurs by superexchange under the off-resonance limit. Crossover was also studied for specific models.
arXiv:1107.4334v1 [cond-mat.mes-hall]

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