SCIENTIFIC PROGRAMS AND ACTIVITIES

October 22, 2014
THE FIELDS INSTITUTE FOR RESEARCH IN MATHEMATICAL SCIENCES
August 12-16, 2013

 

Contributed Poster Index
(all posters are to be max. 36" wide by 42"high)
Posters displays
Tuesday August 13
Posters displays
Wednesday August 14
  • Koji Azuma
    Classical analog of "quantum nonlocality without entanglement"
  • James Bateman
    Improving the efficiency of N-photon super-resolution using an optical centroid measurement

  • Salil Bedkihal
    Flux dependent effects in degenerate and symmetric quantum double-dot Aharonov-Bohm interferometer
  • Aharon Brodutch
    Restricted, distributed quantum gate
    s
  • Zhu Cao
    Efficient Synchronization Scheme for Quantum Communication
  • Aurelia Chenu
    Sunlight can not be viewed as a series of random ultra-fast pulses
  • Greg Dmochowski
    Increasing The Giant Kerr Effect By Narrowing The EIT Window Beyond The Signal Bandwidth
  • Amir Feizpour
    Weak-value Amplification of Low-light-level Cross Phase Modulation
  • Kent Fisher
    Quantum computing on encrypted data
    Roohollah (Farid) Ghobadi Creating and detecting micro-macro photon-number entanglement
  • Roohollah (Farid) Ghobadi
    Creating and detecting micro-macro photon-number entanglement
  • Gilad Gour
    Universal Uncertainty Relations
  • Horacio Grinberg
    Nonclassical effects in highly nonlinear two-level spin models
  • Timur Grinev
    Coherent control and incoherent excitation dynamics of pyrazine
  • Andres Estrada Guerra
    Non-Markovian effects in the dynamics of entanglement in high temperature limit
  • Matin Hallaji
    Quantum control of population transfer between vibrational states in an optical lattice
  • Wolfram Helwig
    Absolutely Maximal Entanglement and Quantum Secret Sharing
  • Nathaniel Johnston
    On the Minimum Size of Unextendible Product Bases
  • Dongpeng Kang
    Bragg reflection waveguides: The platform for monolithic quantum optics in semiconductors
  • Eric Kopp
    New Control Frontiers in Noiseless Subspace
  • Hoi Kwan Lau
    Rapid laser-free ion cooling by controlled collision
  • Juan David Botero
    On the Dynamics of Spin-1/2 particles: A Phase-space Path-integral Approach
  • Rolf Horn
    On chip generation of polarization entanglement in a monolithic semiconductor waveguide
  • Hoi Kwan Lau
    Quantum secret sharing with continuous variable cluster states
  • Xiongfeng Ma
    Experimental realization of measurement-device-independent quantum key distribution
  • Dylan Mahler
    Adaptive quantum state tomography improves accuracy quadratically
  • Sebastian Duque Mesa
    Relativistic Dynamical Quantum Non-locality
  • Leonardo A Pachon
    Coherent Phase Control in Closed and Open Quantum Systems
  • Alexandru Paler
    Resource Optimization in Topological Quantum Computation: Verification.
  • Kyungdeock Park (Daniel)
    Heat Bath Algorithmic Cooling and Multiple Rounds Quantum Error Correction Using Nuclear and Electron Spins
  • Nicolas Quesada
    Self-calibrating tomography for non-unitary processes
  • Christoph Reinhardt
    Design of a Strong Optomechanical Trap

  • Katja Ried
    Quantum process tomography with initial correlations

  • Lee Rozema
    Experimental Demonstration of Quantum Data Compression
  • Lena Simine
    Numerical simulations of molecular conducting junction: transport and stability
  • Cathal Smyth and Dr. Gregory D. Scholes
    A Method of Developing Analytical Multipartite Measures for mixed W-like States
  • Xin Song
    Enhanced probing of fermionic interaction using weak-value amplification
  • Zhiyuan Tang
    Experimental demonstration of polarization encoding measurement-device-independent quantum key distribution
  • Johan F. Triana
    The Quantum Limit at Thermal Equilibrium
  • Timur Tscherbul
    Quantum coherent dynamics of Rydberg atoms driven by cold blackbody radiation
  • Tian Wang
    Demonstrating macroscopic entanglement based on Kerr non-linearities requires extreme phase resolution
  • X. Xing
    Multidimensional quantum information based on temporal photon modulation
  • Feihu Xu
    Measurement Device Independent Quantum Key Distribution in a Practical Setting
  • Zhen Zhang
    Decoy-state quantum key distribution with biased basis choice
  • Eric Zhu
    Broadband Polarization Entanglement Generation in a Poled Fiber

WITHDRAWN
  • Clement Ampadu
    Decoherence Matrix of the Gudder-Sorkin Type for Quantum Walks on Z^2 and the Konno-Segawa Conjecture
  • Agata M Branczyk
    Optimised shaping of optical nonlinearities in poled crystals
  • Sinan Bugu
    Fusing Several Polarization Based Entangled Photonic W States
  • Peter Cameron
    Quantum Impedances, Entanglement, State Reduction, and the Black Hole Information Paradox

 

  • Ray-Kuang Lee
    Spin-flip for a Parity-Time symmetric Hamiltonian on the Bloch sphere
  • Andreia Mendonça Saguia
    One-norm geometric quantum discord under decoherence
  • Angelo Lucia
    Stability of local quantum dissipative systems
  • Vaibhav Madhok
    Information gain in tomography - A quantum signature of chaos
  • Shengjun Wu
    State and process tomography via weak measurements

Koji Azuma NTT Basic Research Laboratories
Classical analog of "quantum nonlocality without entanglement"
Coauthors: Masato Koashi, Shinya Nakamura, and Nobuyuki Imoto

Quantum separable operations are defined as those that cannot produce entanglement from separable states from scratch, and it is known that they strictly surpass local operations and classical communication (LOCC) in a number of tasks, which is sometimes referred to as "quantum nonlocality without entanglement." Here we consider a task with such a gap regarding the trade-off between state discrimination and preservation of entanglement. We show that this task has a complete classical analog, in which distant parties attempt to preserve secrecy of given bits as much as possible while they also try to discriminate whether their bits are the same or not. This purely classical scenario is shown to have the same amount of the gap as seen in the quantum case. This fact suggests that the public communication (corresponding to LOCC in the quantum case) is less powerful than "classical" separable operations that cannot produce secret key from scratch. As a result, contrary to a common belief that may be inferred from previous known examples in quantum information theory, quantum properties, such as nonorthogonality, measurement backaction, and entanglement, are not essential in the existence of a nonzero gap between the separable operations and LOCC.
This presentation is based on the paper in arXiv:1303.1269.


James Bateman, University of Toronto
Improving the efficiency of N-photon super-resolution using an optical centroid measurement
Coauthors: Lee A. Rozema, Amir Feizpour, Dylan H. Mahler, Aephraim M.
Steinberg

Precise measurements using light and the precise manipulation of light are essential to many modern technologies. The resolution of the smallest possible features is required in diverse applications ranging from technical fields such as lithography to basic sciences and biomedical imaging. However, these measurements face fundamental limits. For instance, the resolution of spatial features is limited by the diffraction of light. There has been much work towards surpassing these limits using novel quantum states. The so-called N00N is known to exhibit super-resolution, displaying an N-photon interference pattern which is N times narrower than that of classical light.
However, observing such a spatial interference pattern is very inefficient. The probability of all N photons arriving at the same point in space decreases exponentially with N. Here, we experimentally overcome this hurdle by utilizing an optical centroid measurement. By using an array of 11 single photon detectors, and measuring N-photon correlations among all 11 detectors we observe the spatial N-fold super-resolution without the exponential loss. We will present experimental results for N=2 and progress towards N=3.

Salil Bedkihal University of Toronto, Chemical Physics Theroy Group, Department of Chemistry
Flux dependent effects in degenerate and symmetric quantum double-dot Aharonov-Bohm interferometer
Coauthors: Malay Bandyopadhyay, Department of Physics, Indian Institute of Technology, Bhubaneshwar India, Dvira Segal, University of Toronto, Chemical Physics Theory Group, Department of Chemistry

We study the steady-state characteristics and the transient behaviour of the non equilibrium double-dot Aharonov-Bohm interferometer using analytical tools and numerically exact influence functional path integrals. Our simple setup includes non interacting degenerate quantum dots that are coupled to two biased metallic leads at the same strength. A magnetic flux pierces the interferometer perpendicularly. As we tune the degenerate dot energies away from the symmetric point we observe four non-trivial magnetic flux control effects: (i) flux dependency of the occupation of the dots, (ii) magnetic flux induced occupation difference between the dots, at degeneracy, (iii) the effect of phase-localization of the dots coherence holds only at the symmetric point, while in general both real and imaginary parts of the coherence are non-zero, and (iv) coherent evolution survives even when the dephasing strength, introduced via Büttiker probes, is large and comparable to the dots energies and the bias voltage. In fact, finite elastic dephasing can actually introduce new types of coherent oscillations into the systems dynamics. These four phenomena take place when the dots energies are gated, to be positioned away from the symmetric point, demonstrating that the combination of bias voltage, magnetic flux and gating field, can provide delicate control over the occupation of each of the quantum dots, and their coherence.

Juan David Botero Instituto de Física, Universidad de Antioquia, Medellín, Colombia
On the Dynamics of Spin-1/2 particles: A Phase-space Path-integral Approach
Coauthors: Leonardo A. Pachón

The two-level quantum system is the most fundamental element in quantum-information-processing theory (QIPT) and one of its more natural physical implementations comprises a spin-1/2-system. Entangling these systems and their subsequence manipulation, base on the non-local character of quantum correlations, are the most fundamental protocols in QIPT. The non-locality that is exploited in those protocols is a non-locality between quantum systems; however, in order to get a complete picture of the quantum correlations, one has to analyze the influence of the non-local character of the quantum dynamics itself (dynamical non-locality).

We use the proposal given by Bjork et al [1] to construct the Wigner function in a discrete phase space, then with the aim to analize the dynamics of the of the spin-1/2 particles, we develop a formula for the discrete Wigner propagator and calculate it by means of a direct method based on the path integral formalism for discrete systems[2]. Having already the explicit form for the Wigner propagator, we can see explicitly the non-local behavior of the quantum dynamics for the discrete systems.
[1] G. Björk, A. Klimov, L. Sánchez-Soto. Progress in Optics, 51, 496 (2008)
[2] L.S. Schulman. Techniques and applications of path integration. Wiley-interscience publication. 1Ed (1996)

Aharon Brodutch IQC, University of Waterloo
Restricted, distributed quantum gates
Coauthors:

The role of entanglement in quantum algorithms is somewhat challenged by the existence of mixed state algorithms that generate very little entanglement [1]. Moreover it not clear if any other property of quantum states can account for the source behind the quantum advantage [2]. A different candidate for this source is quantum gates or more generally quantum operations. In this case entanglement can be brought into the picture by considering distributed implementations. To minimize resources it is useful to take into account only the relevant set of input states and simplify the gate's distributed implantation [3,4]. Using this approach we can identify the need for entanglement resources as a function of the input/output sets. This lets us make meaningful statements about the 'quantumness' of various mixed state algorithms.
[1] Laflamme, R., D. G. Cory, C. Negrevergne, and L. Viola, 2002, Quantum Inf. Comput. 2, 166
[2] Vedral, V., 2010, Found. Phys. 40, 1141.
[3] Brodutch, A., and D. R. Terno, 2011, Phys. Rev. A 83, 010301.
[4] Brodutch, A., arXiv:1207.5105

 

Zhu Cao Center for Quantum Information, Institute for Interdisciplinary Information Sciences, Tsinghua University, Beijing 100084 China
Efficient Synchronization Scheme for Quantum Communication
Coauthors: Hai-Lin Yong, Yan-Lin Tang, Wei-Yue Liu, Dong-Dong Li, Cheng-Zhi Peng

Quantum key distribution (QKD) is the first practical application in quantum information science. Time synchronization technology plays an important role in QKD implementations. In this field, the synchronization precision is required to be in the sub-nanosecond regime, while the accuracy of the current GPS system is in the order of a few nanoseconds. To fill this gap, we propose an effective synchronization algorithm, where we calibrate the frequency difference and the offset between two remote clocks using internal correlation between quantum signals. More specifically, the frequency difference is derived by the ratio between the transmitter's and receiver's internal clock time lengths within a large number of GPS signals. Then the offset is figured by fitting an optimal offset to maximize the raw key rate. With the frequency difference and the offset calibrated, we achieve a sub-nanosecond-precision synchronization.

With our synchronization algorithm, we complete two QKD field tests. In our free-space 32 km decoy-state QKD experiment test, we manage to substantially improve the synchronization precision, comparing to the conventional hardware-based synchronization scheme. As a result, we are able to decrease the error rate and increase the raw key rate. Results from another test, where the transmitter is set in a moving vehicle, show that our synchronization scheme is robust under harsh conditions. These two tests demonstrate that our scheme can be useful for future global high-speed QKD with a LEO satellite. Finally we remark that our scheme may also be valuable for other quantum communication applications, such as teleportation.

Aurelia Chenu Department of Chemistry and Center for Quantum Information and Quantum Control, 80 Saint George Street, University of Toronto, Toronto, Ontario, M5S 3H6 Canada
Sunlight can not be viewed as a series of random ultra-fast pulses
Coauthors: Agata M. Branczyk1, Greg D. Scholes1, and John Sipe2 1 Department of Chemistry and Center for Quantum Information and Quantum Control, 80 Saint George Street,University of Toronto, Toronto, Ontario, M5S 3H6 Canada 2 Department of Physics, 60 Saint George Street, University of Toronto, Toronto, Ontario, M5R 3C3 Canada

Dynamics of energy transfer in photosynthetic complexes occurs on a femtosecond (fs) time scale. It can be resolved with ultrafast non-linear spectroscopy [1], where coherent fs pulses are being used to excite the molecular systems. This is in contrast with natural excitation conditions given by almost continuous and fully incoherent light from the sun. In an attempt to make connections between the experimental results using 2D electronic spectroscopy and the biological processes occurring in photosynthetic organisms under natural conditions, it has been suggested that sunlight can be viewed as a series of random ultra-fast pulses, with a duration as short as the bandwidth allows [2].
To investigate this proposal, we construct a quantum state of light composed of an incoherent mixture of multi-mode coherent states. In the attempt to fit the properties of thermal light, we show that the radiation spectrum and the photon statistics can be well represented by a mixture of pulses, as long as their spectral bandwidth is narrow enough (>ps pulses). However, no physical solution can be found for fs pulses, for which the bandwidth is comparable to that of thermal light. Going to the second-order correlation function and the simultaneous excitation of two atoms, any mixture of pulses is expected to fail in representing excitation by thermal light in general.
References:
[1] E. Collini et al., Nature, 463, 644 (2010).
[2] Y.-C. Cheng & G.R. Fleming, Annu. Rev. Phys. Chem., 60, 241 (2009).

Greg Dmochowski University of Toronto, Department of Physics
Increasing The Giant Kerr Effect By Narrowing The EIT Window Beyond The Signal Bandwidth
Coauthors: Amir Feizpour, Matin Hallaji, Chao Zhuang, Alex Hayat, Aephraim Steinberg

We experimentally show that EIT-based Kerr nonlinearities continue to benefit from narrowing the EIT window even as the signal bandwidth comes to exceed this transparency width. While previous studies have shown that narrow transparency windows yield slow step-response times, thereby suggesting a limitation of EIT-enhanced nonlinearities, our results show that many practical applications of such nonlinearities, which rely on pulsed fields, are not hindered by these effects. In fact, these slow dynamics are at the root of the enhancement offered by EIT in the regime of most interest, namely, narrow EIT windows combined with high intensity, broadband signal pulses. For applications such as quantum non-demolition measurements and nonlinear optical gates where the goal is simply to detect an observable single-shot phase shift, we see that EIT can be used to increase the signal size even for broadband signal pulses.

Amir Feizpour University of Toronto
Weak-value Amplification of Low-light-level Cross Phase Modulation
Coauthors: Greg Dmochowski, Matin Hallaji, Chao Zhuang, Alex Hayat, Aephraim M. Steinberg

We report on our experimental progress towards observing weak-value amplification of low-light-level cross-phase modulation. This will be the first observation of a weak measurement relying on true entanglement between distinct systems which has no classical interpretations, unlike previous weak measurement experiments. In this scheme, classical pulses at single-photon level are sent to an interferometer one arm of which is interacting with a probe pulse through a cross-Kerr effect. Post-selecting on having m photons in the dark port of the interferometer results in an amplified m photon cross phase shift.

Kent Fisher Institute for Quantum Computing, University of Waterloo
Quantum computing on encrypted data
Coauthors: Anne Broadbent, L. Krister Shalm, Zhizhong Yan, Jonathan Lavoie, Robert Prevedel, Thomas Jennewein, Kevin Resch

Performing computations on encrypted data is of strong significance for protecting privacy over public networks. Such capabilities would allow a client with a weak computer to send sensitive data to a more powerful,but untrusted, server to be processed. Recent works, called fully homomorphic encryption schemes, have produced a long sought-after solution to the problem of carrying out classical computations on encrypted data. Here we present an efficient solution to the quantum analogue of this problem, allowing arbitrary quantum computations to be carried out on encrypted quantum data. We prove that an untrusted server can carry out a universal set of quantum gates on encrypted qubits without learning any information about the inputs while the client, who knows the decryption key, can easily obtain the computed results. We experimentally demonstrate, using single photons and linear optics, the encryption and decryption scheme for each quantum gate in a set sufficient for arbitrary quantum computations. Our protocol can be easily incorporated into the design of quantum servers with few extra resources. This result paves the way for delegated quantum computing to take place, ensuring the privacy and security of future quantum networks.

Roohollah (Farid) Ghobadi Institute for Quantum Science and Technology, University of Calgary
Creating and detecting micro-macro photon-number entanglement
Coauthors: Alexander Lvovsky and Christoph Simon

We propose a scheme for the observation of micro-macro entanglement in photon number based on amplifying and de-amplifying a single-photon entangled state in combination with homodyne quantum state tomography. The created micro-macro entangled state, which exists between the amplification and de-amplification steps, is a superposition of two components with mean photon numbers that differ by approximately a factor of three. We show that for reasonable values of photon loss it should be possible to detect micro-macro photon-number entanglement where the macro system has a mean number of one hundred photons or more.

Gilad Gour Department of Mathematics and Statistics, IQST, University of Calgary
Universal Uncertainty Relations
Coauthors: Shmuel Friedland, Vlad Gheorghiu

Uncertainty relations are a distinctive characteristic of quantum theory that imposes intrinsic limitations on the precision with which physical properties can be simultaneously determined. The modern work on uncertainty relations employs entropic measures to quantify the lack of knowledge associated with measuring non-commuting observables. However, I will show here that there is no fundamental reason for using entropies as quantifiers; in fact, any functional relation that characterizes the uncertainty of the measurement outcomes can be used to define an uncertainty relation. Starting from a simple assumption that any measure of uncertainty is non-decreasing under mere relabeling of the measurement outcomes, I will show that Schur-concave functions are the most general uncertainty quantifiers. I will then introduce a novel fine-grained uncertainty relation written in terms of a majorization relation, which generates an infinite family of distinct scalar uncertainty relations via the application of arbitrary measures of uncertainty. This infinite family of uncertainty relations includes all the known entropic uncertainty relations, but is not limited to them. In this sense, the relation is universally valid and captures the essence of the uncertainty principle in quantum theory.

Horacio Grinberg Department of Physics, FCEyN, University of Buenos Aires, and IFIBA, Argentina
Nonclassical effects in highly nonlinear two-level spin models
Coauthors:

The nonclassical squeezing effect emerging from a nonlienar coupling model (generalized Jaynes-Cummings model) of a two-level atom interacting with a bimodal cavity field via two-photon transitions is investigated in the rotating wave approximation. Various Bloch coherent initial states (rotated states) for the atomic sysem are assumed. Initially the atomic system and the field are in disentangled state, where the field modes are in Glauber coherent states via Poisson distribution. The model is numerically tested against simulations of time evolution of the based Heisenberg uncertainty variance and Shannon information entropy squeezng factors. The quantum state purity is computed and used as a criterion to get information about the entanglement of the components of the system. Analytical expression of the total density operator matrix elements at t > 0 shows in fact, the present nonlinear model to be strongly entangled, where each of the definite initial Bloch coherent states are reduced to statistical mixtures. Thus, the present model does not preserve the modulus of the Bloch vector.

Timur Grinev Chemical Physics Theory Group, Department of Chemistry, and Center for Quantum Information and Quantum Control, University of Toronto, Toronto, Ontario M5S 3H6, Canada
Coherent control and incoherent excitation dynamics of pyrazine
Coauthors: Paul Brumer

First, we present coherent control of internal conversion (IC) between the S_1 and S_2 singlet excited electronic states in pyrazine. S_2 state is populated from S_0 singlet state in the process of weak field excitation. Coherent control with respect to certain control objective is performed by shaping the exciting laser. Excitation and IC are considered simultaneously. Successful control is demonstrated by optimizing both the amplitude and phase profiles of the laser, and its dependence on the properties of S_2 resonances is established.

Second, we present the S_0 -> S_2/S_1 photoexcitation dynamics of pyrazine due to weak incoherent CW light excitation after sudden turn-on of the light. Dynamical evolution of S_2 and S_1 populations, as well as the purity of the excited mixed state, is studied. It is shown, that the S_1 to S_2 populations ratio becomes constant in the long time regime, thus being an evidence of the spatially distributed nature of the resulting excited mixed state. At the same time, the excited mixed state purity decreases monotonically, but non-uniformly, to a small asymptotic value (which is still restricted in value by the purity of the maximally mixed state).

Andres Estrada Guerra Universidad de Antioquia
Non-Markovian effects in the dynamics of entanglement in high temperature limit
Coauthors: Leonardo Pachon Contreras

In the past years, some quantum phenomena have been observed at macroscopic scales. In particular, superconductivity, coherent superpositions of Bose-Einstein condensates and interference patterns in fullerenes have been detected. This fact has made that the border between the quantum and classical realms become more diffuse and intricate, although, more interesting, than before.

However, in order to observe these quantum features, one needs to reach the low temperature regime, E/(kBT), where E denotes a characteristic system energy-scale and kBT the thermal energy. Therefore, some delicate and elaborate cooling processes have been developed.

Our work aims to show that, even in the the high temperature regime, some quantum features such entanglement can be present, if the system is placed out from equilibrium. In particular, we study the non-Markovian dynamic of two different harmonic oscillators coupled to different baths at different temperatures and with different coupling-to-the-bath-strengths. We found that, despite the absence of symmetries in the parameters space, entanglement between the oscillators can be created and maintained in the long-time regime. We also discuss the implementation of our setup for studying the influence of the non-Markovian dynamics in the optimal sideband cooling of nano-mechanical resonators.

Matin Hallaji Physics Department, University of Toronto
Quantum control of population transfer between vibrational states in an optical lattice
Coauthors: Chao Zhuang, Alex Hayat, and Aephraim M. Steinberg

We experiment on two quantum control techniques, Adiabatic Rapid Passage (ARP) and Gradient Ascent Pulse Engineering (GRAPE), to realize population transfer between vibrational states of atoms trapped in an optical lattice. The ARP pulse gives the highest population transfer among all the techniques we have tested so far: 38.9±0.2 of the initial ground state population is transferred into the first excited state, which exceeds the 1/e boundary of coupling the ground and the first excited vibrational states in a harmonic oscillator potential. The ARP pulse also gives the highest normalized population inversion among all the techniques we have tested so far: the highest ratio of the difference between the ground state and the first excited state population to the sum of the ground state and the first excited state population is 0.21±0.02. For the GRAPE technique, we use the GRAPE algorithm to engineer a pulse involving both the displacement of the optical lattice and modulation of the lattice depth, while the fidelity between the state after the pulse is applied and the first excited state is taken as the figure of merit. The GRAPE pulse gives as high population transfer as the ARP pulse does: 39 ± 2 of the initial ground state population is transferred into the first excited state. The GRAPE pulse outperforms the ARP pulse if the leakage is concerned. Because the GRAPE pulse gives almost no leakage compared to the 18.7 ± 0.3 leakage for the ARP pulse, when the highest population transfer occurs.

Wolfram Helwig University of Toronto
Absolutely Maximal Entanglement and Quantum Secret Sharing
Coauthors: Wei Cui, José Ignacio Latorre, Arnau Riera, Hoi-Kwong Lo

We study the existence of absolutely maximally entangled (AME) states in quantum mechanics and its applications to quantum information. AME states are characterized by being maximally entangled for all bipartitions of the system and exhibit genuine multipartite entanglement. We show that these states exist for any number of parties if the system dimension is chosen appropriately, and that they can be conveniently described within the graph states formalism for qudits.

With such states, we present a novel parallel teleportation protocol which teleports multiple quantum states between groups of senders and receivers. The notable features of this protocol are that (i) the partition into senders and receivers can be chosen after the state has been distributed, and (ii) one group has to perform joint quantum operations while the parties of the other group only have to act locally on their system. We also prove the equivalence between pure state quantum secret sharing schemes and AME states with an even number of parties.

Rolf Horn University of Waterloo, Institute for Quantum Computing
On chip generation of polarization entanglement in a monolithic semiconductor waveguide
Coauthors: Piotr Kolenderski, Dongpeng Kang, Payam Abolghasem, Carmelo Scarcella, Adriano Della Frera, Alberto Tosi, Lukas G. Helt, Sergei V. Zhukovsky, John E. Sipe, Gregor Weihs, Amr S. Helmy, Thomas Jennewein

From unraveling the mysteries of the quantum world, to solving really hard problems, a quest of those in the quantum information community is to discover a technology that will facilitate large scale implementations of quantum processes. In photonics, the quest starts with finding a stable and scalable source of single and entangled photons -- the building blocks of a photonic quantum computer. Here we present the Bragg Reflection Waveguide (BRW), a tiny, stable and scalable semiconductor waveguide, capable of directly producing polarization entangled photons. It's design is perhaps the most truly monolithic of any photon source available today; -- the architecture on which it is built promises electrical self pumping, and in contrast to many other non-linear optics type sources, nothing is required to create entanglement but the device itself. To demonstrate this, we examine the photon pairs produced via Spontaneous Parametric Down Conversion in a 2.2mm long, 3.8 micron wide BRW. We perform quantum state tomography on the photon pairs, splitting them immediately after they emerge from the chip, and show their significant departure from classical behaviour. Solidified via the observation of their spectra, we calculate a concurrence of approximately 0.5, demonstrate polarization entanglement visibilities from 64% to 96% in various basis, and determine the fidelity with a maximally entangled state to be 0.83. Combined with the BRW's truly monolithic architecture these results signify the BRW chip architecture as a serious contender on which to build large scale implementations of optical quantum processes.

Nathaniel Johnston Institute for Quantum Computing, University of Waterloo
On the Minimum Size of Unextendible Product Bases
Coauthors: Jianxin Chen

A long-standing open question asks for the minimum number of vectors needed to form an unextendible product basis in a given bipartite or multipartite Hilbert space. A solution to this problem has applications to the construction of bound entangled states and Bell inequalities with no quantum violation. A partial solution was found by Alon and Lovasz in 2001, but since then only a few other cases have been solved. We solve all remaining bipartite cases (i.e., where there are only 2 subsystems), all remaining qubit cases (i.e., where each local dimension is 2), as well as many other multipartite cases.

Dongpeng Kang Department of Electrical & Computer Engineering, University of Toronto
Bragg reflection waveguides: The platform for monolithic quantum optics in semiconductors
Coauthors: Amr S. Helmy

Photon pairs are one of the most important and widely used nonclassical states of light in quantum optics. They are indispensible sources in various applications in domains such as quantum key distribution, optical quantum computing, amongst others. One of the more popular methods to generate photon pairs is via spontaneous parametric down-conversion, which requires a laser source pumping a nonlinear crystal in a specific set of configurations. The system is generally bulky, vulnerable, and sensitive to the external environment, therefore it’s useful in specially equipped labs. On the other hand, a mobile and commercially viable quantum information processing system, such as an optical quantum computer, requires chip-scale, portable, robust sources of photon pairs operating at room temperature. Although significant progress has been made using different techniques, electrically pumped, room-temperature photon pairs are still unavailable. To this end, Bragg reflection waveguides (BRWs) made of III-V semiconductors such as Aluminum Gallium Arsenide have been shown as the most promising platform to realize this class of sources. Efficient photon pair generation as well as polarization entanglement have been demonstrated in BRWs.

In this work, we first review BRWs as a platform for phase matching in isotropic and highly dispersive semiconductors. We will show dispersion and birefringence engineering can be employed to tailor the properties of the photon pairs, for example, to generate polarization entangled photons on-chip without any off-chip compensation or interferometry. Our results, combined with its truly monolithic nature, show that BRWs could lead to fully integrated nonclassical photon sources.

Eric Kopp University of Toronto
New Control Frontiers in Noiseless Subspaces
Coauthors:

Quantum control is largely divided into two independent problems: control for the purpose of protecting information and preventing decoherence, and control for the purpose of manipulating states to accomplish computational goals. Achieving both control objectives simultaneously is a formidable task and has typically only been addressed for specific low-dimensional systems. Our research examines control strategies for realizing a universal set of operations (gates) while confining states to noiseless subspaces in systems of 4 qubits and greater. These strategies are applicable to a broad class of models and control inputs. Aspects of the research focus on recasting a specific class of noiseless subspace into a classical problem in geometric control, and addressing the computational challenges in working with extremely large, extremely sparse tensor operator representations in an efficient way. Preliminary results will also be shown for a 4-qubit 'representative' trapped ion model with a realistic experimental setup.

Hoi Kwan Lau University of Toronto
Rapid laser-free ion cooling by controlled collision
Coauthors:

I propose a method to transfer the axial motional excitation of a hot ion to a coolant ion with possibly different mass by precisely controlling the ion separation and the local trapping potentials during ion collision. The whole cooling process can be conducted diabatically, involving only a few oscillation periods of the harmonic trap. With sufficient coolant ions pre-prepared, this method can rapidly re-cool ion qubits in quantum information processing without applying lengthy laser cooling.

Hoi Kwan Lau University of Toronto
Quantum secret sharing with continuous variable cluster states
Coauthors: Christian Weedbrook

We extend the idea of cluster state quantum secret sharing to the continuous variable regime. Both classical and quantum information can be shared by distributing finitely squeezed continuous variable cluster states through either secure or insecure channels. We show that the security key rate of the classical information sharing can be obtained by standard continuous variable quantum key distribution techniques. We analyse the performance of quantum state sharing by computing the shared entanglement of between the authorised parties and the dealer. Our techniques can be applied to analyse the security of general continuous variable quantum secret sharing.

Xiongfeng Ma Tsinghua University
Experimental realization of measurement-device-independent quantum key distribution
Coauthors: Yang Liu, Teng-Yun Chen, Liu-Jun Wang, Hao Liang, Guo-Liang Shentu, Jian Wang, Ke Cui, Hua-Lei Yin, Nai-Le Liu, Li Li, Jason S. Pelc, M. M. Fejer, Cheng-Zhi Peng, Qiang Zhang, and Jian-Wei Pan

Throughout history, every advance in encryption has been defeated by advances in hacking, often with severe consequences. Quantum cryptography [1] holds the promise to end this battle by offering unconditional security when ideal single-photon sources and detectors are employed. Unfortunately, ideal devices never exist in practice and device imperfections have become the targets of various attacks. By developing up-conversion single-photon detectors with high efficiency and low noise, we faithfully demonstrate the measurement-device-independent quantum key distribution (MDI-QKD) protocol [2], which is immune to all hacking strategies on detection. Meanwhile, we employ the decoy-state method [3] to defend attacks on non-ideal source. By assuming a trusted source scenario, our practical system, which generates more than 25 kbits secure key over a 50 km fiber link, serves as a step stone in the quest for unconditionally secure communications with realistic devices.
The gap between ideal devices and realistic setups has been the root of various security loopholes [4], which have become the targets of many attacks [5,6]. Tremendous efforts have been made towards loophole-free QKD with practical devices [7,8]. However, the question of whether security loopholes will ever be exhausted and closed still remains. Here, we report a QKD experiment that closes the loopholes in detection and hence can achieve secure communication in a trusted source scenario. Firstly, ideal single-photon sources are replaced with weak coherent states by varying mean photon intensities, a technique called decoy-state method [3]. Secondly, by implementing the recently developed MDI-QKD protocol [2], all the detection side channels are removed from our system.
[1]. C. H. Bennett and G. Brassard, in Proceedings of the IEEE International Conference on Computers, Systems and Signal Processing (IEEE Press, New York, 1984) pp. 175-179.
[2]. H.-K. Lo, M. Curty, and B. Qi, Phys. Rev. Lett. 108, 130503 (2012).
[3]. H.-K. Lo, X. Ma, and K. Chen, Phys. Rev. Lett. 94, 230504 (2005).
[4]. D. Gottesman, H.-K. Lo, N. Lutkenhaus, and J. Preskill, Quantum Inf. Comput. 4, 325 (2004).
[5]. V. Makarov, A. Anisimov, and J. Skaar,Phys. Rev. A 74, 022313 (2006).
[6]. B. Qi, C.-H. F. Fung, H.-K. Lo, and X. Ma, Quantum Inf. Comput. 7, 073 (2007).
[7]. D. Mayers and A. Yao, in FOCS, 39th Annual Symposium on Foundations of Computer Science (IEEE, Computer Society Press, Los Alamitos, 1998), p. 503.
[8]. A. Acin, N. Gisin, and L. Masanes, Phys. Rev. Lett. 97, 120405 (2006).

Dylan Mahler University of Toronto,
Adaptive quantum state tomography improves accuracy quadratically
Coauthors: Lee A. Rozema, Ardavan Darabi, Chris Ferrie, Robin Blume-Kohout, and A.M. Steinberg

In quantum state tomography, an informationally complete set of measurements is made on N identically prepared quantum systems and from these measurements the quantum state can be determined. In the limit as N → ∞, the estimate of the state converges on the true state. The rate at which this convergence occurs depends on both the state and the measurements used to probe the state. On the one hand, since nothing is known a priori about the state being probed, a set of maximally unbiased measurements should be made. On the other hand, if something was known about the state being measured a set of biased measurements would yield a more accurate estimate. It has been shown[1, 2] that by adaptively choosing measurements, optimal accuracy in the state estimate can be obtained regardless of the state being measured. Here we present an experimental demonstration of one- and two-qubit adaptive tomography that achieves a rate of convergence of approximately 1-O([1/N]) in the quantum state fidelity with only a single adaptive step and local measurements, as compared to 1-O([1/(√(N))]) for standard tomography. [1] Phys. Rev. Lett. 97, 130501 (2006) [2] Phys. Rev. A 85, 052120 (2012)

Sebastian Duque Mesa
Relativistic Dynamical Quantum Non-locality
Coauthors: Leonardo A. Pachon

In nonrelativistic quantum mechanics, quantum correlations are largely thought to be absolute. However, when they are studied in the framework of relativistic quantum mechanics they could depend on the reference frame [1]. In particular, two particles could be entangled in one reference frame but unentangled in another one, thus quantum non-locality depends upon the reference frame.
Here, the non-locality of quantum dynamics was tracked, by working to the Weyl’s representation of quantum mechanics, to the superposition principle. This is a kind of single particle non-locality, of different nature as the discussed above [2]. We extend this work to the relativistic framework of quantum mechanics. To do so, we review the basics of the relativistic Weyl’s formalism and discuss the construction of the path-integral representation of the Wigner function, as well as the influence of the reference frame on this dynamical quantum non-locality.
References:
[1] Robert M. Gingrich and Christoph Adami. Quantum entanglement of moving bodies. Phys. Rev. Lett., 89:270402, Dec 2002.
[2] S. Popescu. Dynamical quantum non-locality. Nature Phys., 6:151, 2010.

Leonardo A. Pachon Department of Chemistry, University of Toronto
Coherent Phase Control in Closed and Open Quantum Systems
Coauthors: Paul Brumer

The underlying mechanisms for one photon phase control are revealed through a master equation approach and based on the path integral approach in the energy basis representation. Specifically, two mechanisms are identified, one operating on the laser time scale and the other on the time scale of the system-bath interaction. The effect of the secular and non-secular Markovian approximations are carefully examined. We discuss the possibility of enhancing this environment-assisted effect when a description based on sub-Ohmic spectral densities applies.

Kyungdeock Park (Daniel) Institute for Quantum Computing
Heat Bath Algorithmic Cooling and Multiple Rounds Quantum Error Correction Using Nuclear and Electron Spins
Coauthors: Robabeh Darabad, Ben Criger, Jonathan Baugh and Raymond Laflamme

Nuclear Magnetic Resonance (NMR)-based devices have been excellent test beds for Quantum Information Processing (QIP). However, the spin polarization bias in a typical experimental setup is very small at thermal equilibrium, giving a highly mixed qubit, and the polarization decreases exponentially in the number of qubits. Thus it is very difficult to have close-to-pure ancilla qubits which are essential in the implementation of quantum error correction (QEC). Moreover, for practically stable systems against noise, QEC should be performed multiple rounds. This requires ancilla qubits to be refreshed at the initial stage of each round to very high polarization. In order to accomplish this in NMR QEC experiment, we seek to implement Heat Bath Algorithmic Cooling (HBAC) with cold electron spin bath. HBAC is an implementation independent cooling method that combines reversible entropy compression and interaction with the cold external bath. It is capable of cooling a qubit of interest far beyond the bath polarization. Electron spins possess higher polarization and faster relaxation rate than nuclear spins under similar experimental conditions, and thus can be used as the heat bath while nuclear spins encode system qubits. In this talk, I will present our progress towards achieving high polarization of nuclear spin qubits using an electron spin and HBAC. In addition, I will show how this will be used in future for the experimental realization of multiple rounds of three-qubit QEC.

Alexandru Paler University of Passau
Resource Optimization in Topological Quantum Computation: Verification.
Coauthors: Simon Devitt*, Kae Nemoto*, Ilia Polian+; * National Institute of Informatics, Tokyo, Japan; + University of Passau, Passau, Germany;

Recent advances in large scale quantum architecture design has focused on utilizing topological codes to perform necessary error correction protocols. These codes use a geometric description to specify quantum circuits in terms of topological braiding. Recent results have introduced several techniques to optimize topological circuits by compressing the overall 3-dimensional volume of the circuit description which acts to minimize the total number of physical qubits and the total amount of computational time needed to realize a given circuit [1].
These compression techniques have as yet only been implemented manually, and on small topological circuits. Therefore, it is reasonably straightforward to check that no mistakes are made. Future classical programs and game based efforts [2] that are used to compile and optimized topological circuits will be automated and used to compress extremely large topological structures. As with classical circuit designs, the output of these automated protocols must be verified before accepted.
In this presentation we will outline the steps required to verify topological quantum circuits. We will illustrate several algorithmic steps that are required in order to accurately check the function of optimized circuits without having to directly simulate topological computation.
[1] A.G. Fowler and S.J. Devitt, arXiv:1209.0510
[2] www.qubit-game.com

Nicolas Quesada University of Toronto
Self-calibrating tomography for non-unitary processes
Coauthors: Agata M. Branczyk and Daniel F.V. James

Characterizing quantum states and processes is a key step for many quantum information and quantum computing protocols [1]. We extend upon the idea of using an incompletely characterized process to perform quantum state tomography---known as self-calibrating tomography [2,3]---by including the possibility that the process itself is not unitary. We study a two level atom, with an unknown dipole moment, that undergoes spontaneous emission and is irradiated by a laser whose phase and intensity can be controlled at will. We show that by using five different parameter settings of the electric field of the laser it is possible to reconstruct the state as well as obtain the unknown spontaneous emission rate and dipole moment of the atom---simultaneously performing quantum state and quantum process tomography.
References:
[1] M. A. Nielsen and I. L. Chuang, Quantum computation and quantum information (Cambridge university press, 2010).
[2] A. Branczyk, D. H. Mahler, L. A. Rozema, A. Darabi, A. M. Steinberg, and D. F. James, “Self-calibrating quantum state tomography,” New Journal of Physics 14, 085003 (2012).
[3] N. Quesada, A. M. Branczyk, and D. F. James, “Self-calibrating tomography for multi-dimensional systems,” arXiv preprint arXiv:1212.0556 (2012).

Katja Ried Perimeter Institute for Theoretical Physics
Quantum process tomography with initial correlations
Coauthors: Robert W. Spekkens

When preparing input states for quantum process tomography (QPT), one may face undesired correlations between the system and environment degrees of freedom. In this case the results obtained by the standard QPT scheme may not characterize the process in question accurately. Instead, the data may reflect properties of the joint initial state of system and environment, as one would expect in quantum state tomography (QST). We present a unified framework for QPT and QST that can handle this scenario and report on progress in distinguishing the “process-type” from the “state-type” contributions in data from this tomography.

Christoph Reinhardt McGill University
Design of a Strong Optomechanical Trap
Coauthors: Simon Bernard, Jack Sankey

We report progress toward an optomechanical setup in which a partially-reflective micromechanical element is positioned within an optical cavity formed by two rigidly-fixed mirrors. This three-mirror system provides a highly versatile platform for studying new optomechanical effects; in particular, it is possible to generate a nonlinear coupling in which the cavity resonance varies quadratically as a function of mechanical displacement, enabling (among other things) a strong cavity optical trap. We fabricate our mechanical elements by patterning free-standing silicon nitride membranes into lightweight, weakly-tethered “trampolines” so that a strong optical trap can completely dominate over the forces exerted by the supporting material. Such systems are predicted to achieve extraordinarily high mechanical quality factors, and our ultimate goal is to use them to sense incredibly small forces, such as those exerted by quantum systems prepared in superposition states.

Lee Rozema University of Toronto
Experimental Demonstration of Quantum Data Compression
Coauthors: Dylan H. Mahler, Alex Hayat, Peter S. Turner, and Aephraim M. Steinberg

In quantum state tomography N identically prepared copies of a quantum state are measured to reconstruct a density matrix describing the single particle state. One purpose of reconstructing a density matrix is to allow the prediction of measurements that could have been made on the initial state. On the other hand, if only one measurement is of interest then performing that measurement on the each of the N copies of the state will yield the most accurate estimate. However, if the measurement choice is unknown the quantum states must be stored in a quantum memory until a later time. The question then becomes: how much memory is required?

Hilbert space grows exponentially in the number of qubits. The dimensionality of an N qubit system is 2N, but if all of the qubits are identical the initial N qubit state can be described by the symmetric subspace, which has dimension N+1. Physically, the information of the initial state can be mapped onto the first log2(N+1) qubits using the Quantum Schur-Weyl transform (QSWT), leading to an exponential savings in space.

Here, we present an experiment compressing three qubits into two. In our experiment the three qubits are encoded in two photons. The first photon encodes a path and a polarization qubit, while the second photon encodes a single polarization qubit. We use the QSWT to map all of the information from the 3 qubits onto the path and polarization qubits encoded in the first photon, allowing us to discard the second photon.

Lena Simine Chemical Physics Theory Group, Dept. of Chemistry, University of Toronto
Numerical simulations of molecular conducting junction: transport and stability
Coauthors: Dvira Segal

We present a computational study of a minimalistic molecular conducting junction using a numerically exact path integral method. The effects of bias induced vibrational instability and mode equilibration with secondary phonon modes are investigated. We also address the competition between direct tunneling and phonon assisted transport, and look into thermoelectric regime.The comparison of exact numerical simulations to perturbative master equation results indicate on the importance of high order electron-phonon scattering processes.

Xin Song University of Toronto
Enhanced probing of fermionic interaction using weak-value amplification
Coauthors: Amir Feizpour, Yao Tian, Alex Hayat, Aephraim Steinberg

We demonstrate a scheme for enhanced probing of an interaction between two single fermions by probing the spin-dependent energy splitting of an excitonic system in semiconductor quantum dots based on weak-value amplification. Since both spin and energy of the anisotropic electron-hole exchange interaction in quantum dots can be mapped to emitted photons, we can use the polarization of these emitted photons to initialize and post-select the system. By preparing and post-selecting the emitted photons into two quasi-orthogonal quantum polarization state |i> and |f>, which satisfies <i|f> << 1, we are able to obtain an enhanced outcome of the weak value <A>=<f|A|i>/<f|i>, which is proportional to the energy splitting of the excitonic system. Weak-value amplification provides an effective technique for enhanced-precision measurement of fermion system when considering the limitation due to slow noise.

Zhiyuan Tang University of Toronto
Experimental demonstration of polarization encoding measurement-device-independent quantum key distribution
Coauthors: Zhongfa Liao, Feihu Xu, Bing Qi, Li Qian, Hoi-Kwong Lo

Measurement-device-independent quantum key distribution (MDI-QKD) has been proposed to close all the potential security loopholes due to imperfections in the detectors without compromising the performance of a standard QKD system [1]. Various experimental attempts on MDI-QKD have been reported in both time-bin [2, 3] and polarization encoding [4]. We remark that in [2, 4] only Bell state measurements with different combinations of BB84 states and photon levels are conducted, and thus no real MDI-QKD (which requires Alice and Bob randomly switch their qubits’ states and intensity levels) is implemented. A complete time-bin encoding MDI-QKD experiment has been reported in [3]. However, phase randomization, a crucial assumption in the security of QKD, is neglected in their experiment, which leaves the system vulnerable to attacks on the imperfect weak coherent sources [5].

Here we report the first complete demonstration of polarization encoding MDI-QKD over 10 km optical fiber. Decoy state technique is employed to estimate gain and error rate of single photon signals. Photon levels and probability distributions for the signal and decoy states are chosen numerically to optimize the key rate. Active phase randomization is implemented for the first time in MDI-QKD to protect against attacks on the imperfect sources. A 1600-bit secure key is generated in our experiment. Our experiment verifies the feasibility to implement MDI-QKD with polarization encoding.

[1] H. –K. Lo, M. Curty , and B. Qi, “Measurement-Device-Independent Quantum Key Distribution,” Phys. Rev. Lett. 108, 130503 (2012).

[2] A. Rubenok, et al., “A Quantum Key Distribution Immune to Detector Attacks,” arXiv: 1204.0738.

[3] Y. Liu, et al., “Experimental Measurement-Device-Independent Quantum Key Distribution,” arXiv:1209.6178.

[4] T. Ferreira da Silva, et al., “Proof-of-Principe Demonstration of Measurement Device Independent QKD Using Polarization Qubits,” arXiv: 1207.6345.

[5] Y. Tang, et al., “Source Attack of Decoy State Quantum Key Distribution Using Phase Information,” arXiv: 1304.2541.

Johan F. Triana Instituo de Física, Universidad de Antioquia
The Quantum Limit at Thermal Equilibrium
Coauthors: Leonardo A. Pachón (Instituo de Física, Universidad de Antioquia)

The aim of constructing and designing machines working at the nanometre-length scale, such as atomic motors, photocells, gyrators or heat engines, has boosted the developing of a quantum version of thermodynamics. One of the foundational conundra in this emerging field is, to what extent nanomachines can display quantum features and how this quantum behaviour could be used to improve their efficiency. Intuitively, one can suggest that if the energy of the thermal fluctuations is much smaller than the typical energy scale of the nanosystem, then there is room for the nanosystem to revels its quantum nature. However, as it has been discussed recently in almost all fields related to quantum mechanics (e.g quantum information science, quantum biophysics, nanotechnology, quantum chemsitry or condensed matter physics), the border between the quantum/classical operating regime is far from being trivial. We predict here, at thermodynamical equilibrium, the existence of a regime where, e.g., nanoelectromechanical structures or optomechanical systems can be found in an entangled state at high temperature assisted by the non-Markovian interactions. Complementarily, we report the existence of a second regime, characterized by Markovian interactions at low temperature, where quantum nanodevices do not thermalize into the canonical Boltzmann distribution, and therefore all their thermodynamical properties are expected to deviate, even, from current quantum thermodynamics. Our findings not only provides a solid ground for understanding the presence of quantum features in most of current investigations in bio and handmade systems, but also points out the direction to follow in protecting and isolating of quantum systems.

Timur Tscherbul Department of Chemistry and Centre for Quantum Information and Quantum Control, University of Toronto
Quantum coherent dynamics of Rydberg atoms driven by cold blackbody radiation
Coauthors: Paul Brumer

The interaction of incoherent blackbody radiation with atoms and molecules is usually considered in the framework of Markovian rate equations parametrized by the Einstein coefficients, leading to a linear increase of excited-state populations with time. While the validity of the rate equations is justified by the extremely short coherence time of hot blackbody radiation (2 ps at a temperature of 4100 K), deviations from the linear behavior are expected on shorter time scales. By solving perturbative equations of motion for the density matrix of an atom interacting with a cold thermal reservoir of radiation field modes, we obtain the dynamics of eigenstate populations and coherences without invoking the Markovian approximation. The theory is applied to examine the coherent effects in highly excited Rydberg atoms subject to a cosmic microwave background radiation.

 

X. Xing University of Toronto
Multidimensional quantum information based on temporal photon modulation
Coauthors: A. Hayat, A. Feizpour, and A. M. Steinberg.

Multidimensional quantum information processing has been shown to open a wide range of possibilities. The spatial degree of freedom has been recently employed to encode multidimensional quantum information using photon orbital angular momentum. This approach, however, is not suitable for the single-mode fiber-optical communication infrastructure. We demonstrate experimentally a multidimensional quantum information encoding approach based on temporal modulation of single photons, where the Hilbert space can be spanned by an in-principle infinite set of orthonormal temporal profiles. We implement the temporal encoding using a scheme where the projection onto temporal modes is implemented by an electro-optical modulator and a narrow-band optical filter. The demonstrated temporal multidimensional quantum encoding allows quantum communication over existing fiber optical infrastructure, as well as probing multidimensional time entanglement approaching the limit of continuous-time measurements.

Zhen Zhang Tsinghua University
Decoy-state quantum key distribution with biased basis choice
Coauthors: Zhengchao Wei,Weilong Wang, Xiongfeng Ma

  Quantum key distribution (QKD) plays an important role in the field of Quantum Information. The most well-known QKD scheme is BB84 protocol [1], where a single photon source is assumed. In reality, a perfect single photon source does not exist. Instead, highly attenuated lasers are widely used for QKD. The multi-photon component in a laser source leads to a security threat (e.g., photon number splitting attack [2]). The decoy-state method is proposed to address this issue by using more than one intensities and its security has been proven by Lo, Ma, and Chen [3].
Meanwhile, in the original BB84, Alice encodes the key information randomly into the X and Z bases with the same probability and Bob measures the received qubits in two bases randomly with equal probabilities. We denote the basis-sift factor to be the ratio between the lengths of the sifted key and the raw key. The basis-sift factor of the original BB84 protocol is 1/2. The efficient BB84 protocol proposed by Lo, Chau and Ardehali [4], in which Alice and Bob put a bias between the probabilities of choosing the Z and X bases, can improve the basis-sift factor up to 100%.
In this work, we propose a QKD protocol that combines the decoy-state method with the efficient BB84 protocol. In this scheme, Alice sends all signal states in the Z basis. We optimize the probabilities of Alice sends decoy states, signal states and vacuum states, the probabilities of the X and Z bases in decoy state, the probabilities of the X and Z bases Bob chooses for measure, and the intensity of the decoy state. From the simulation result, after taking into account of statistical fluctuations, our protocol can improve the key rate by at least 45% comparing to the original decoy-state protocol.

[1] C. H. Bennett and G. Brassard, in Proceedings of the IEEE International Conference on Computers, Systems and Signal Processing (IEEE Press, New York, 1984) pp. 175-179.

[2] G. Brassard, N. Lutkenhaus, T. Mor, and B. C. Sanders, Phys. Rev. Lett. , 85, 1330 (2000).

[3] H.-K. Lo, X. Ma, and K. Chen, Phys. Rev. Lett. 94, 230504 (2005).

[4] H.-K. Lo, H. F. Chau , and M. Ardehali, Journal of Cryptology 18, 133 (2005).

Tian Wang Institute for Quantum Science and Technology, University of Calgary
Demonstrating macroscopic entanglement based on Kerr non-linearities requires extreme phase resolution
Coauthors: Roohollah Ghobadi, Sadegh Raeisi, Christoph Simon

 

Entangled coherent states, which can in principle be created using strong Kerr non-linearities, allow the violation of Bell inequalities for very coarse-grained measurements. This seems to contradict a recent conjecture that observing quantum effects in macroscopic systems generally requires very precise measurements. However, here we show that both the creation of the required states and the required measurements rely on being able to control the phase of the necessary Kerr-nonlinearity based unitary operations with extreme precision. And the requirement for phase control increases dramatically with increasing size of the cat state. This lends support to the idea that there is a general principle that makes macroscopic quantum effects difficult to observe, even in the absence of decoherence.

 

Feihu Xu University of Toronto
Measurement Device Independent Quantum Key Distribution in a Practical Setting
Coauthors: Marcos Curty, Bing Qi, Wei Cui, Charles Ci Wen Lim, Kiyoshi Tamaki, and Hoi-Kwong Lo

A ground-breaking scheme – measurement device independent QKD (MDI-QKD) [Phys. Rev. Lett. 108, 130503, 2012] – was proposed to solve the “quantum hacking” problem. More precisely, MDI-QKD removes all attacks in the detection system, the most important loophole of QKD implementations. It is highly practical and can be implemented with standard optical components. Very recently, MDI-QKD has been demonstrated by a number of research groups, but before it is applicable in real life, it is important to resolve a number of practical issues.

In this paper, we solve the practical issues in the real-life implementations of MDI-QKD. Firstly, we study the physical origins of the quantum bit error rate in real-life MDI-QKD by proposing general models for various practical errors. Secondly, we present a rigorous method to study both the finite-decoy protocol and the finite-key analysis. In the finite-key analysis, we use the Chernoff bound to estimate the statistical fluctuations and consider the smooth min-entropy formalism to analyze the finite-key effect. Finally, we offer a general framework to evaluate the optimal choice of intensities of signal and decoy states. Our result is of particular interest both to researchers hoping to demonstrate MDI-QKD and to others performing non-QKD experiments involving quantum interference.

Eric Zhu Department of Electrical & Computer Engineering, University of Toronto
Broadband Polarization Entanglement Generation in a Poled Fiber
Coauthors: Z. Tang, L. Qian, L.G. Helt, M. Liscidini, J.E. Sipe, C. Corbari, P.G. Kazansky

In this paper, we will give an overview of our recent work in poled twin-hole fiber, fiber that has a non-zero second-order nonlinearity. Along with the intrinsic form birefringence of the fiber, we have been able to exploit the type-II phase-matched parametric downconversion process to generate high-fidelity polarization-entangled photon pairs. We emphasize that this generation of polarization entanglement is direct, without the need for interferometric means or walkoff compensation.
The quality of our source is examined through a number of characterization techniques, including two-photon interference, Hong-Ou-Mandel interference, and quantum state tomography. Furthermore, the unique dispersion properties of the poled fiber allow for broadband polarization-entanglement over 100 nm centered at 1550 nm, opening up our source to many potential applications from high-resolution quantum optical coherence tomography, to wavelength-division-multiplexed schemes of distributing entangled photons to multiple bi-parties for quantum cryptography and other exciting quantum technologies.

 

 

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