April 21, 2024

Conference on Quantum Information and Quantum Control III

Contributed Talks

Poster Abstracts

Bell Prize talk

Quantum Nonlocality: How does Nature perform the trick ?!?
Nicolas Gisin
Group of Applied Physics, University of Geneva, Switzerland

Nature is able to produce correlations that can’t be described by any theory using only local variables. Since the early intuitions of EPR and Schrödinger in 1935, and the first quantitative predictions by John Bell in 1964 followed by their experimental confirmation by Clauser, Aspect and co-workers in the1970’s and 1980’s, this absolutely astonishing fact has become routine in many labs around the world. Moreover, during the last 20 years it has been discovered that this sort of nonlocality is a useful cryptographic resource that has also been mastered outside the lab, over standard optical fiber telecom networks. Hence, one could think that quantum nonlocality has become banal.

But a clear understanding of how these nonlocal correlations happen remains a huge open problem. There is of course no question of a mechanical explanation of the type: one localized physical system “pushes” (interacts with) its direct neighbours, as we have in classical (Newtonian mechanics and general relatively). The impossibility of such a mechanical explanation is precisely the message of Bell’s inequalities.

But then: How come the correlations? How does Nature manage to produce random events that somehow manifest themselves at several locations? In brief: How does Nature perform the trick?

More or less everyone who tries to go beyond Hilbert space calculus uses pictures like: a first event influences a second one (e.g. Einstein’s famous spooky action at a distance). We present experiments that cast serious doubts on the viability of such pictures. Moreover, taking such pictures seriously raises deep concerns about the alleged “peaceful co-existence of quantum nonlocality and relativity”.

What are the alternatives to the picture sketches above? Some vague paths will be presented. I am afraid I have nothing better to offer. Importantly, if someone tells you he has a straightforward solution, beware! The problem is a serious one, central to today’s physics.

Abstracts - Invited Talks

How Much Quantum Noise is Detrimental to Entanglement
G.S. Agarwal
Oklahoma State University

I examine the effect of quantum noise on entanglement. The source of noise could be either an attenuator or even an amplifier which one would presumably use in quantum communication protocols. I present quantitative results on the survival of entanglement
as a result of various types of quantum noise. I consider entanglement for both continuous variables [1] and qubits [2].

1. G.S. Agarwal, S. Chaturvedi, arXiv:0906.2743
2. Sumanta Das, G. S. Agarwal arXiv:0901.2114


The Design and Function of Quantum Information Processors
David G. Cory

Quantum information theory provides a new framework for the development of sensors and actuators that rely on quantum dynamics to obtain efficiencies beyond their classical counterparts. Today we can build laboratory examples of small quantum devices from spin systems, optics, superconducting systems and even neutron beams. I will introduce some of the concepts underlying these devices. In particular showing simple schemes for improving device performance via quantum engineering. I will also explore some near term examples of practical quantum sensors.

Solid-state quantum memories for quantum repeaters
Nicolas Gisin, Hugo Zbinden, Mikael Afzelius, Hugues de Riedmatten,
Christoph Simon, Björn Lauritzen, Jirí Minár, Imam Usmani, Christoph Clausen
Group of Applied Physics, University of Geneva, Switzerland

Today’s quantum key servers must be connected by one or two uninterrupted optical fibers. The ultimate limit of such direct point to point quantum key distribution is around 300-500 km.

Future fiber-based quantum networks, able to connect many quantum key servers over arbitrary long distances, require both high-fidelity entanglement swapping and multi-mode quantum memories.

We first discuss the general vision of a global quantum network. Next, we present progress in:
Long distance point-to-point Quantum Key Distribution.
A semi-classical Quantum Key Distribution network.
Entanglement distribution using the Swisscom network between two villages 18 km apart with a continuous violation of Bell’s inequality during 24 hours.
A new protocol for an efficient multimode quantum memory based on atomic ensembles. The atomic ensemble, rare-earth ions, is “frozen” in a crystal inside a cryostat. The protocol is inspired from photon echoes, but avoids any control light pulse after the single-photon(s) is (are) stored in the medium, thus avoiding any noise due to fluorescence.
First demonstrations of the new protocol for multimode quantum memories. The coherence of the re-emitted photons is investigated in an interference experiment showing net visibilities above 95%.

In summary, many hundreds of km long quantum communication is a long term objective. Many of the necessary building blocks have been demonstrated, but usually in independent experiments and with insufficient specifications to meet the challenge. Nevertheless, today the roadmap is relatively clear and a lot of interesting physics shall be found along the journey.


Operational Computation with Quantum Stuff
Lucien Hardy
Perimeter Institute

We imagine that we have a supply of stuff that, we suppose, has quantum properties that are not well known. We consider a computational scheme in which we attach inputs and outputs to this stuff to assist a computation (which can be classical). We consider how we might go about operationally characterizing the stuff to enable effective quantum computation.


Theory of coherent resonance energy transfer for coherent initial condition
Seogjoo Jang
Department of Chemistry and Biochemistry, Queens College of the City University of New York, 65-30 Kissena Blvd., Flushing, NY 11367

A theory of coherent resonance energy transfer developed recently [Journal of Chemical Physics 129, 101104 (2008)] is extended for coherent initial condition. For the general situation where the initial excitation can be arbitrary linear combination of donor and acceptor excitations, a second order time local and polaron transformed quantum master equation is derived. The inhomogeneous terms in the resulting equation have contributions not only from the initial donor and acceptor populations but also from their coherence terms. Numerical tests are performed for general super Ohmic spectral densities where the bath coupled to the donor excitation can be correlated with that coupled to the acceptor excitation. The results show the sensitivity of the early nonstationary population dynamics on the relative phase of initial donor and acceptor excitations. It is also shown that the contribution of inhomogeneous terms is more significant for coherent initial excitations than for initial excitation localized in the donor only. The overall numerical results demonstrate the importance of including all the competing effects such as nonequilibrium, nonMarkovian, and quantum coherence for quantitative modeling of population dynamics of resonance energy transfer.

Quantum Communication and Information Processing with Photons - Experiments and Outlook
Thomas Jennewein
Institute for Quantum Computing, University of Waterloo

Photons are good candidates for quantum information processing, and they have been widely used in implementations of quantum communication and quantum information processing. I will outline recent experimental results on long distance quantum communication with entangled photons, and the ongoing activities towards performing satellite based quantum networks. Furthermore, I will present the results of theoretical studies on the actual requirements for photon sources and detectors in order to perform photonic quantum computing outside the coincidence basis, which will be an important next experimental step towards scalable quantum information processing with photons.


Complementarity and security of quantum key distribution
Masato Koashi
Osaka University

In the seminal paper by Einstein, Podolsky and Rosen, one finds a pair of complementary tasks that can never be completed at the same time, since otherwise it would violate the uncertainty relation. By suitably defining such a pair of tasks, we are able to establish simple quantitative relations between the feasibility of the complementary tasks and distillable entanglement or distillable key. This approach is also useful for proving the security of practical quantum key distribution protocols, since the security is established through an operationally defined quantity, namely, the success probability of the complementary task. In some occasions, this probability can be estimated even when little is known about the apparatuses used in the protocol.


Synthesizing arbitrary photon states in a superconducting resonator: The quantum digital to analog converter
John Martinis
University of California Santa Barbara

Two-level systems, or qubits, can be prepared in arbitrary quantum states with exquisite control, just using classical electrical signals. Achieving the same degree of control over harmonic resonators has remained elusive, due to their infinite number of equally spaced energy levels. Here we exploit the good control over a superconducting phase qubit by using it to pump photons into a high-Q coplanar wave guide resonator and, subsequently, to read out the resonator state. This scheme has previously allowed us to prepare and detect photon number states (Fock states) in the resonator and to measure their decay. Using a generalization of this scheme by Law and Eberly, we can now create arbitrary quantum states of the photon field with up to approximately 10 photons. We analyze the prepared states by directly mapping out the corresponding Wigner function, which is the phase-space equivalent to the density matrix and provides a complete description of the quantum state.


Electronic excited states in optically active biomolecules: functional quantum systems with a tuneable environment interaction
Ross McKenzie
University of Queensland, Brisbane, Australia

Optically active molecules (chromophores) are crucial to the function of wide range of biomolecules. Examples, include the green flourescent protein, porphyrins associated with photosynthesis, and retinal associated with vision. The electronic states of the chromophores can be viewed as discrete quantum systems which are interacting with an environment composed of the surrounding protein and water. The interaction of the chromophore with its environment may be modelled quantum mechanically by an independent boson model which describes a two-level quantum system interacting with a bath of harmonic oscillators. Femtosecond laser spectroscopy experiments give a parametrisation of the spectral density describing the system-environment interaction for a wide range of chromophores and proteins. This spectral density completely determines the quantum dynamics and decoherence of electronic excited states. We have recently proposed and analysed several continuum dielectric models of the environment[1]. Our results provide a framework to understand experimental measurements and molecular dynamics simulations, including the relative importance of the contributions of the protein, the water bound to the surface of the protein, and the bulk water to decoherence. Our results show that because biomolecules function in a ``hot and wet'' environment, quantum coherence will generally not be significant for processes occuring slower than a picosend, the timescale for the dielectric relaxation of water. The ``collapse'' of the quantum state of the chromophore due to continuous measurement of its state by the environment occurs on the timescale of 10's femtoseconds.

[1] J. Gilmore and R.H. McKenzie, J. Phys. Chem. A 112, 2162 (2008).


Quantum Networks with Ions, Phonons, and Photons
Christopher Monroe
Joint Quantum Institute, University of Maryland and NIST

Trapped atomic ions are among the most promising candidates for quantum information processing. All of the fundamental quantum operations have been demonstrated on this system, and the central challenge now is how to scale the system to larger numbers of qubits. By entangling atomic qubits through both deterministic phonon and probabilistic photon interfaces, the trapped ion system can be scaled in various ways for applications in quantum communication, quantum computing, and quantum simulations. I will discuss several options and issues for such atomic quantum networks, along with state-of-the-art experimental progress.


Spectroscopy of biological molecules using coherent control
Marcus Motzkus
Philipps-Universität Marburg

Coherent control as a field of current research has expanded significantly in recent years. In chemistry the core competence remains the steering of photo-induced processes into a desired channel while suppressing unwanted pathways. Also in biology it has been experimentally demonstrated in natural occurring complexes [1,2] as well as in artificial dyads [3] that the ratio between reaction pathways can be influenced by phase and amplitude shaped laser pulses.
In addition to this pure optimization process, the concept of coherent control offers also a novel approach for the study of general light-mater interaction and in particular for the application of optical spectroscopy. Not only new insight into the complex dynamics in large molecules is obtained by comparison of shaped and unshaped laser pulses but also robust and simplified implementations of powerful nonlinear optical techniques are easily realized. This idea forms the conceptual basis of Quantum Control Spectroscopy (QCS) and has already yielded important results on molecular vibrational dynamics and biological function unattainable by conventional spectroscopic techniques.[1,3-5]
In this contribution we employ QCS to unravel the ultrafast dynamics near a conical intersection between the two electronicly excited states S2 and S1 in ß-carotene[5-7] and new developments will be presented which cover different directions of coherent control from medical applications to the development of multiphoton microscopy.[8]

[1] J. L. Herek et al., Nature 417, 533 (2002).
[2] V.I. Prokhorenko et al., Science 313, 1257 (2006)
[3] J. Savolainen et al., PNAS 105, 7641 (2008).
[4] T. Buckup et al., Journal of Photochemistry and Photobiology A 180, 314 (2006).
[5] J. Hauer et al., Chem Phys 350, 220 (2008)
[6] J. Hauer et al., Journal of Physical Chemistry A 111, 10517 (2007).
[7] J. Hauer et al., Chem. Phys. Lett. 421, 523 (2006)
[8] B. von Vacano and M. Motzkus, PCCP 10, 186 (2008).


A photonic cluster state machine gun
Terry G. Rudolph
Imperial College

Cluster states are multi-qubit entangled states which have the remarkable property that, once prepared, they can be used to perform quantum computation by making only single qubit measurements.
The problem of constructing a quantum computer therefore reduces to that of preparing these states. Over the last few years one of the more promising architectures for doing so has been single photon optics. However the resource requirements are still prohibitive. In this talk I will discuss a way of turning a single photon source - in particular one built from a self-assembled quantum dot - into a device capable of firing out long strings of entangled cluster state. Remarkably this device can be fired for times much longer than the typical decoherence times of the electron, because any errors on the spin become localized on the emitted photons instead. Such a device would reduce the resource requirements for optical quantum computing by many orders of magnitude.


Going beyond Gaussian limits on continuous variable processing and measurement
Masahide Sasaki
National Institute of Information and Communications Technology

The amplitude and phase quadratures of optical field, the so called continuous variables (CVs), play major roles both in photonic- and quantum-information and communications technology (P- and Q-ICT, respectively). Gaussian states, typically coherent states, and Gaussian operations on CVs serve as a complete basis for P-ICT. They are, however, only a part of the full potential of optical fields. Q-ICT provides a new paradigm to overcome the limits of P-ICT and to realize the quantum-limited measurement and the ultimate capacity of optical channels. Recent theories revealed that higher order operations beyond a fully Gaussian setting, namely non-Gaussian operations on CVs, are essentially required to realize the Q-ICT paradigm. We present recent advances of non-Gaussian operations and measurements to overcome the Gaussian limits. We first present latest results on non-Gaussian state generation and manipulation using photon counting and squeezed states, and then present an implementation of quantum receiver with a photon number resolving detector.

Coherently wired light-harvesting in a photosynthetic marine alga at ambient temperature
Greg Scholes
Institute for Optical Sciences and Centre for Quantum Information and Quantum Control, University of Toronto

The photosynthetic machinery of plants, algae, and bacteria has diversified and evolved over billions of years. The initial reactions involve absorption of light by molecules in specialized light-harvesting antenna proteins followed by remarkably efficient funneling of that electronic excitation energy within and between proteins to a reaction center. Isolated antenna proteins have proven to be important model systems enabling researchers to learn how excitation energy is transmitted by resonance energy transfer, and recent work has discovered a role of quantum-coherence in energy funneling for some antenna proteins at temperatures as high as 180K. Quantum-coherence means that light-absorbing molecules in the protein capture and funnel energy according to quantum-mechanical probability laws instead of classical laws. The subject has stimulated cross-disciplinary interest because it was previously thought that long-range quantum-coherence between molecules could not be sustained in complex biological systems, even at low temperature. Here we report observations of quantum coherence at ambient temperature in the energy funnel of the phycocyanin 645 (PC645) antenna protein isolated from the marine cryprophyte alga Chroomonas CCMP270. Electronic excitations interfere in a way to 'wire' distant molecules together in the PC645 photosynthetic antenna protein. We find that quantum-coherence is maintained for long enough that it may feasibly be employed within live cryptophyte marine algae to increase the spatial cross-section for light-harvesting. This work leads to new, more demanding, questions such as This opens up questions such as 'do quantum effects offer an evolutionary advantage in biology, and if so, how'?


Photon-Echo Quantum Memory and Controlled State Manipulation
Wolfgang Tittel
Institute for Quantum Information Science, University of Calgary
Coauthors: A. Delfan, E. Saglamyurek, and C. La Mela

Quantum memories, as a part of a quantum repeater, are key elements to extend quantum communication beyond its current distance limit of around 100 km. In addition to memories, quantum repeaters also require the distribution of entangled photons as well as state manipulation, which is generally accomplished by means of interferometric optical setups. We experimentally investigate a novel approach based on photon-echo type atom light-interaction that allows combining storage with controlled transformation of quantum states [1,2]. As an example, we perform a proof-of-principle demonstration of unambiguous state discrimination in an Er:LiNbO3 waveguides cooled to 3K using states encoded into pulses of light in superposition of different temporal modes. Our approach can easily be extended to any unitary transformation. The high robustness and flexibility compared to current optical setups for state manipulation makes it promising for quantum communication and computation protocols that require storage and manipulation of photons, in particular quantum repeaters.

[1] W. Tittel, M. Afzelius, T. Chanelière, R. Cone, S. Kröll, S.A. Moiseev, and M. Sellars, Laser & Photonics Reviews, DOI 10.1002/lpor.200810056 (2009).

[2] A. Delfan, C. La Mela and W. Tittel, in Proceedings of SPIE 6903: 690308 (6 pp.), San Jose, United States of America (SPIE , Bellingham, United States of America, 2008).


Quantum dots in photonic crystals: from quantum information processing to optical switching at a single photon level
Jelena Vuckovic, Andrei Faraon, Dirk Englund, Arka Majumdar, Pierre Petroff
Ginzton Laboratory, Stanford University

Quantum dots in photonic crystals are interesting both as a testbed for fundamental cavity quantum electrodynamics (QED) experiments, as well as a platform for quantum and classical information processing.
Quantum dot-photonic crystal cavity QED has been probed both in photoluminescence and coherently, by resonant light scattering from such a system [1]. In the latter case, both intensity and photon statistics of the reflected beam have been analyzed as a function of wavelength, leading to observation of effects such as photon blockade and photon induced tunneling - for the first time in solid state [2]. The system has also been employed to achieve a controlled phase and amplitude modulation between two modes of light at the single photon level [3] - nonlinearity observed so far only in atomic physics systems.

These demonstrations lie at the core of a number of proposals for quantum information processing, and could also be employed to build novel devices, such as optical switches controlled at a single photon level.

[1]. Dirk Englund, Andrei Faraon, Ilya Fushman, Nick Stoltz, Pierre Petroff, and Jelena Vuckovic, "Controlling cavity reflectivity with a single quantum dot," Nature, vol. 450, No. 7171, pp. 857-861, December 2007
[2]. Andrei Faraon, Ilya Fushman, Dirk Englund, Nick Stoltz, Pierre Petroff, and Jelena Vuckovic, "Coherent generation of nonclassical light on a chip via photon-induced tunneling and blockade," Nature Physics, Vol 4, pp. 859 - 863 (2008)

[3]. Ilya Fushman, Dirk Englund, Andrei Faraon, Nick Stoltz, Pierre Petroff, and Jelena Vuckovic, "Controlled phase shift with a single quantum dot," Science, vol. 320, number 5877, pp. 769-772 ( 2008)

Quantum Chemistry on a Quantum Computer: First Steps and Prospects
Andrew White
University of Queensland

We use a photonic quantum computer to simulate the hydrogen molecule. This is the first experimental demonstration of efficient quantum chemistry, which promises to be a powerful new tool in biology, chemistry, and materials science.

In principle, it is possible to model any physical system exactly using quantum mechanics; in practice, it quickly becomes infeasible. Recognising this, Richard Feynman suggested that quantum systems be used to model quantum problems. For example, the fundamental problem faced in quantum chemistry is the calculation of molecular properties, which are of practical importance in fields ranging from materials science to biochemistry. Within chemical precision, the total energy of a molecule as well as most other properties, can be calculated by solving the Schrodinger equation. However, the computational resources required to obtain exact solutions on a conventional computer generally increase exponentially with the number of atoms involved. In the late 1990's an efficient algorithm was proposed to enable a quantum processor to calculate molecular energies using resources that increase only polynomially in the molecular size. Despite the many different physical architectures that have been explored experimentally since that time---including ions, atoms, superconducting circuits, and photons---this appealing algorithm has not been demonstrated to date.

Here we take advantage of recent advances in photonic quantum computing to present an optical implementation of the smallest quantum chemistry problem: obtaining the energies of H_2, the hydrogen molecule, in a minimal basis. We perform a key algorithmic step---the iterative phase estimation algorithm---in full, achieving a high level of precision and robustness to error. We implement other algorithmic steps with assistance from a classical computer, and explain how this non-scalable approach could be avoided. Finally we provide new theoretical results which lay the foundations for the next generation of simulation experiments using quantum computers. We have made early experimental progress towards the long term goal of exploiting quantum information to speed up quantum chemistry calculations.

Contributed Talks

Quantum computers: A new state of matter?
Stephen Bartlett
School of Physics, The University of Sydney
Coauthors: Andrew Doherty, Sean Barrett, Terry Rudolph, David Jennings

A recent breakthrough in quantum computing has been the realization that quantum computation can proceed solely through single-qubit measurements on an appropriate quantum state. One exciting prospect is that the ground or low-temperature thermal state of an interacting quantum many-body system can serve as such a resource state for quantum computation. The system would simply need to be cooled sufficiently and then subjected to local measurements.

It would be unfortunate, however, if the usefulness of a ground or low-temperature thermal state for quantum computation was critically dependent on the details of the system's Hamiltonian; if so, engineering such systems would be difficult or even impossible. A much more powerful result would be the existence of a robust ordered phase which is characterized by the ability to perform measurement-based quantum computation.

Using some simple toy models, we investigate the existence of such a computational phase of matter. In one model, we identify such a phase by using the fidelity of quantum gates as order parameters. In another, we are able to show the existence of a transition in quantum computational power – from a region in parameter space where every state is useful for measurement-based quantum computation, to a region where all local measurements can be efficiently simulated on a classical computer. Remarkably, this transition occurs despite there being no phase transitions in the model at all. Together, these results reveal that the characterization of computational phases of matter has a rich, complex structure – one which is still poorly understood.

Paper reference: arXiv:0802.4314 (accepted to PRL); arXiv:0807.4797

Scalable quantum computation via local control of only two qubits
Daniel Burgarth
Imperial College London
Coauthors: Koji Maruyama, Michael Murphy, Simone Montangero, Tommaso Calarco, Franco Nori, Martin B. Plenio

We apply quantum control techniques to control a large spin chain by only acting on two qubits at one of its ends, thereby implementing universal quantum computation by a combination of quantum gates on the latter and swap operations across the chain. It is shown that the control sequences can be computed and implemented efficiently. We discuss the application of these ideas to physical systems such as superconducting qubits in which full control of long chains is challenging.

Paper reference: arXiv:0905.3373

Flipping quantum coins
Félix Bussières
École Polytechnique de Montréal and University of Calgary
Coauthors: Guido Berlín, Gilles Brassard, Nicolas Godbout, Joshua A. Slater, Wolfgang Tittel

Coin flipping is a cryptographic primitive in which two distrustful parties wish to generate a random bit in order to choose between two alternatives. This task is impossible to realize when it relies solely on the asynchronous exchange of classical bits: one dishonest player has complete control over the final outcome. It is only when coin flipping is supplemented with quantum communication that this problem can be alleviated although partial bias remains. Unfortunately, practical systems are subject to loss of quantum data, which allows a cheater to force a bias that is complete or arbitrarily close to complete in all previous protocols. We report herein on the first implementation of a quantum coin-flipping protocol that is impervious to loss. Moreover, in the presence of unavoidable experimental noise, we propose to use this protocol sequentially to implement many coin flips, which guarantees that a cheater unwillingly reveals asymptotically, through an increased error rate, how many outcomes have been fixed. Hence, we demonstrate for the first time the possibility of flipping coins in a realistic setting.

Paper reference:

Multipartite entanglement for one photon shared among four optical modes
Kyung Soo Choi
Quantum Optics Group, Norman Bridge Laboratory of Physics 12-33, California Institute of Technology, Pasadena, CA 91125, USA
Coauthors: S. B. Papp, H. Deng, P. Lougovski, S. J. van Enk, H. J. Kimble

Access to genuine multipartite entanglement of quantum states enables advances in quantum information science and also contributes to the understanding of strongly correlated quantum systems. A critical requirement for realizing these extraordinary promises, however, is an efficient and unambiguous method to detect and characterize the purported entanglement. We report the detection and characterization of heralded entanglement in a multipartite quantum state composed of four optical modes that share one photon, a so-called W state [1]. By reducing the relative phase coherence between bipartite components of the W state, we observe the transitions from four- to three- to two-mode entanglement. These observations are possible for our system because our entanglement verification protocol makes use of quantum uncertainty relations to simultaneously detect the entangled states that span the Hilbert space of interest [2].

Paper reference: [1] Science 324, 764 (2009); [2] New J. Phys. 11, 063029 (2009).

Ultrafast optical spin echo for electron spins in semiconductors
Susan Clark
Stanford University
Coauthors: Kai-Mei C. Fu, Qiang Zhang, Thaddeus D. Ladd, Colin Stanley, Yoshihisa Yamamoto

Spin-based quantum computing and magnetic resonance techniques rely on the ability to measure the coherence time, T2, of a spin system. We report on the experimental implementation of all-optical spin echo to determine the T2 time of a semiconductor electron-spin system. We use three ultrafast optical pulses to rotate spins an arbitrary angle and measure an echo signal as the time between pulses is lengthened. Unlike previous spin-echo techniques using microwaves, ultrafast optical pulses allow clean T2 measurements of systems with dephasing times (T2*) fast in comparison to the timescale for microwave control. This demonstration provides a step toward ultrafast optical dynamic decoupling of spin-based qubits. Such a scheme could be used to extend the spin-memory time of a spin-based quantum computer and can be integrated into quantum bus schemes for quantum computing.

Ultrafast quantum state tomography
Steve Flammia
Perimeter Institute

Everybody hates tomography. And with good reason! Experimentalists hate it because it is inefficient and difficult. Theorists hate it because it isn't very "quantum." But because of our current lack of meso-scale quantum computers capable of convincingly performing non-classical calculations, tomography seems like a necessary evil. In this talk, I will attempt to banish quantum state tomography to the Hell of Lost Paradigms where it belongs. I hope to achieve this by introducing several heuristics for learning quantum states more efficiently, in some cases exponentially so. One such heuristic runs in polynomial time and outputs a polynomial-sized classical approximation of the state (in matrix product state form.) Another takes advantage of the fact that most interesting states are close to pure states to get a quadratic speedup using ideas from compressed sensing. Both algorithms come with rigorous error bounds.

This is joint work with S. Bartlett, D. Gross, R. Somma (first result), and S. Becker, J. Eisert, D. Gross, and Y.-K. Liu (second result).

Quantum-Bayesian Coherence
Christopher A. Fuchs
Perimeter Institute for Theoretical Physics
Coauthors: Christopher A. Fuchs and Ruediger Schack

In the quantum-Bayesian development of quantum mechanics, the Born Rule cannot be interpreted as a rule for setting measurement-outcome probabilities from an objective quantum state. But if not, what is the role of the rule? In this paper, we argue that it should be seen as an empirical addition to Bayesian reasoning itself. Particularly, we show how to view the Born Rule as a normative rule in addition to usual Dutch-book coherence. It is a rule that takes into account how one should assign probabilities to the consequences of various intended measurements on a physical system, but explicitly in terms of prior probabilities for and conditional probabilities consequent upon the imagined outcomes of a special counterfactual reference measurement. This interpretation is seen particularly clearly by representing quantum states in terms of probabilities for the outcomes of a fixed, fiducial symmetric informationally complete (SIC) measurement. We further explore the extent to which the general form of the new normative rule implies the full state-space structure of quantum mechanics. It seems to get quite far

Paper reference: arXiv:0906.2187v1 [quant-ph]

Entanglement and nonlocality in multiqubit pure states
Shohini Ghose
Wilfrid Laurier University
Coauthors: N. Sinclair, S. Debnath, P. Rungta and R. Stock

Multiqubit entanglement is a crucial ingredient for large-scale quantum information processing and can also play a role in quantum criticality phenomena in condensed matter systems. Entanglement between qubits can lead to violations of Bell-type inequalities, indicating the nonlocal nature of the correlations between qubits. We have derived relationships between genuine multiqubit entanglement and nonlocality for families of 3-qubit pure states. Our results show that these relationships are counterintuitive and can be quite different from the well-known relationship between 2-qubit entanglement and violation of the Bell-CHSH inequality. We identify tripartite entangled states that do not violate the Svetlichny inequality, which tests for genuine tripartite nonlocal correlations. On the other hand, we show that all members of a set of states called the maximal slice states violate the Svetlichny inequality and analogous to the 2-qubit case, the amount of violation increases with the amount of entanglement. The generalized GHZ states and the maximal slice states have unique tripartite entanglement and nonlocality properties in the set of all pure states. Our results can be simply generalized to analyze multiqubit entanglement and nonlocality in systems of 4 or more qubits.

Paper reference: arXiv:0812.3695

Non-adiabatic quantum control of multiple quantum dots embedded in cavities with global femtosecond optical pulses
Jordan Kyriakidis
Physics Department, Dalhousie University
Coauthors: Wayan Sudiarta

There are several proposals utilising quantum dots embedded in optical cavities as physical or logical qubits. The advantage of these systems is that distant qubits can be controllably coupled through virtual cavity modes. While there has been recent experimental progress in coherent manipulation in such systems, further progress is hampered by at least two limitations. One, is that transitions are accomplished adiabatically; since the light-qubit coupling is typically rather weak, switching times are not appreciably shorter than the relaxation time of these systems. Second, lasers typically need to address individual dots in the cavity, which is exceedingly difficult. We present results of our work showing how both these limitations can be simultaneously overcome. In the experimentally relevant case of dots of varying size, nonadiabatic transitions can be achieved using chirped pulses applied globally to the cavity. The nonadiabaticity enables switching times far quicker than either the relaxation time or (one over) the Rabi frequency. These global pulses further eliminate the need to address a single dot with a single pulse. We show results showing fast entangling operations on distant qubits even for arbitrarily closely-spaced energy levels. This level of quantum control has not yet been been demonstrated for multiple quantum dots embedded in cavities. Our scheme can be implemented with present-day experimental capabilities.

Designed photons from birefrigent waveguides
Jeff Lundeen
National Research Council, Institute for National Measurement Standards, 1200 Montreal Road, Ottawa, K1A 0R6, Canada
Coauthors: Offir Cohen, Pierre Mahou, Brian J. Smith, and Ian A. Walmsley

The capability to produce photon pairs and non-classical states of light, such as squeezed states, with controlled spatial and temporal mode structure is a crucial requirement for optical quantum technologies such as photonic quantum information processing, quantum cryptography, and quantum metrology. We develop a theoretical model of photon pair generation by spontaneous four-wave mixing in birefringent waveguides, such as optical fibers. The model demonstrates that a wide variety of spectral correlations can be designed into the photon pairs. We present experimental results in photonic crystal fiber and birefringent standard fiber where we eliminate all correlations, enabling the heralding of single photons in pure quantum state, which is a requirement for high-fidelity operation of photonic quantum logic gates.

Paper reference:

Applications of Four-Wave Mixing in Quantum Information
Alberto M. Marino
Coauthors: Raphael C. Pooser Vincent Boyer Paul D. Lett

One of the most important resources in quantum mechanics is entanglement, which is at the basis of applications such as quantum cryptography, quantum imaging, teleportation, etc. In order to fully take advantage of this resource, it is necessary to develop a number of tools to manipulate and control it. We show that non-degenerate four-wave mixing in rubidium vapor is a good candidate for the implementation of some of those tools. Its dispersive properties make it possible to control the propagation velocity of light. We have used this property to delay entanglement without a significant degradation, effectively implementing a short term quantum memory. In addition, its operation as an almost ideal amplifier has allowed us to clone one of the beams of an entangled state of light.

Paper reference: Nature 457, 859 (2009) and PRL 103, 010501 (2009)


Quantum matchgate computation is as powerful as space-bounded quantum computation
Akimasa Miyake
Perimeter Institute
Coauthors: Richard Jozsa (University of Bristol) Barbara Kraus (University of Innsbruck) John Watrous (Institute for Quantum Computing and University of Waterloo)

Quantum matchgate computation is a family of uniform quantum circuits comprising only certain two-qubit unitary gates acting arbitrarily in each even and odd parity subspaces on any nearest neighboring pair of two qubits. This model has a tight connection to the time evolution of the free fermionic system ubiquitous in physics, for example, the dynamics of one-dimenisional spin chains via the so-called Jordan-Wigner transformation. Indeed, the matchgates constitute a strict subset of a universal set of elementary gates as a quantum circuit model, and the matchgate computation has been known to be efficiently simulatable by the classical computer, i.e., it is in P.

Here we prove that the matchgate computation is equally powerful as space-bounded quantum computation where, compared to the size n of the input, the size of the working register is exponentially small, i.e., logn, and thus that may be called quantum log-space computation, QL shortly. Since it can be said that QL captures computational capability of small-sized quantum computers which would be only available in practice in the next decade by our current technology, our result not only shows that the matchgate is in QL, but also provides explicitly how any computation in QL can be simulated by the matchgates conversely and characterizes a gap to the full-scaled quantum computers. Some implications to quantum simulation with a small quantum computer, such as that of one-dimensional quantum systems, are discussed.

Merging photonic crystal cavities and single quantum dots: a practical source of entangled photon pairs
P. K. Pathak
Department of Physics, Queen's University, Kingston, Ontario, Canada K7L 3N6
Coauthors: Stephen Hughes

There has been considerable progress for developing chip-based, and scalable, sources of entangled photons using single quantum dots (QDs)[1-3]. In semiconductor QDs, entangled photons are typically generated in a biexciton-exciton cascade decay. However, the entanglement between the generated photons is limited by inherent cylindrical asymmetries and various dephasing processes. The cylindrical asymmetries produce fine structure splitting (FSS) in the exciton states; as a result, the emitted x-polarized and y-polarized photon pairs become distinguishable in frequencies, and the entanglement between the photons is largely destroyed. Several methods have been employed to minimize the detrimental effects of FSS on the generated photons, for example, by spectrally filtering the indistinguishable photon pairs [1] and by suppressing the FSS using external fields [2] or thermal annealing [3]. Thus, the photons of different polarizations, generated within the same generations, are forced to match in their frequencies. An interesting alternate approach, insensitive to FSS, has been proposed recently [4], which suppressing the binding energy of the biexciton [5]. For a zero binding energy of the biexciton, photons of different polarizations match in frequencies in "across generations". Because of the different ordering in the emission for x-polarized and y-polarized photon pairs, the photons are distinguishable temporarily and remain unentangled, but the entanglement can be restored using a time delay between photons of different generations.
The effects of dephasing in the generated entangled state of photons can be minimized significantly by enhancing the emission rates of photons through the Purcell effect in a system comprised of a QD coupled with a microcavity; and various experiments have now demonstrated single QD strong coupling to miniaturized semiconductor cavities [6]. Recently, several cavity-QED schemes for generating entangled photons have also been proposed whereby the excitons are strongly coupled with the cavity modes and form degenerate polariton states [7, 8]. However, one major drawback of these proposed schemes is that because of the large biexciton binding energy, the biexciton remains uncoupled with cavity modes and thus the first generation of photons has a long life time, relative to the life time of the exciton-emitted photons. In this work, we will introduce a new scheme for the fast generation of entangled photons from a single QD coupled to a planar photonic crystal that supports two orthogonally polarized cavity modes [9]. We discuss, and develop a rigorous theory for, "within generation" and "across generation" of entangled photons when both biexciton to exciton, and exciton to ground state transitions, are coupled through cavity modes by manipulating the binding energy of the biexciton such that both biexciton to excitons and excitons to ground state are coupled with two cavity modes of orthogonal polarization; experimentally, manipulation of the binding energies of the biexciton can be realized by applying lateral electric field and by thermal annealing. The two photon concurrence is calculated to be greater than 0.7 and 0.8 using experimentally achievable parameters in across generation and within generation, respectively. We also show that the entanglement can be distilled in both cases using a simple spectral filter.

This work was supported by the National Sciences and Engineering Research Council of Canada and the Canadian Foundation for Innovation.

[1] N. Akopian et. al., Phys. Rev. Lett. 96, 130501 (2006).

[2] R. M. Stevenson et. al., Nature (London) 439, 179 (2006).

[3] R. Seguin et. al., Appl. Phys. Lett. 89, 263109 (2006); D. J. P. Ellis et. al., ibid. 90, 011907 (2007).

[4] J. E. Avron et. al., Phys. Rev. Lett. 100, 120501 (2008); see also P. K. Pathak and S. Hughes, Phys. Rev. Lett., In Press (2009) [arXiv:0905.4420v1].

[5] M. E. Reimer et. al., Phys. Rev. B 78, 195301 (2008).

[6] See, e.g., J. P. Reithmaier et. al., Nature (London) 432, 197 (2004); T. Yoshie et. al., ibid. 432, 200 (2004).

[7] R. Johne et. al., Phys. Rev. Lett. 100, 240404 (2008).

[8] P. K. Pathak and S. Hughes, Phys. Rev. B 79, 205416 (2009).

[9] P. K. Pathak and S. Hughes, arXiv:0906.3035 (1999).

Paper reference: P. K. Pathak and S. Hughes, arXiv:0906.3035

Quantum Process Discrimination, Waveguides, and Fault Tolerant Quantum Processes
Anthony Laing
University of Bristol

Given the emerging potential of quantum information science where devices including photonic quantum circuits are being miniaturized, making their identification challenging, quantum process discrimination (QPD) has pragmatic, as well as foundational, considerations. Discrimination between unknown quantum processes chosen from a finite set is experimentally shown to be possible even in the case of nonorthogonal processes. We demonstrate unambiguous deterministic QPD of nonorthogonal processes using properties of entanglement, additional known unitaries, or classical communication.
Single qubit measurement and unitary processes, and multipartite unitaries (where the unitary acts nonseparably across two distant locations) acting on photons are discriminated with a confidence of at least 97% in all cases.

In principle these discrimination protocols can be realised with 100% confidence, however the usual imperfect input states and imperfect processes contribute to experimental errors and our discrimination confidence of 97% is not perfect. In this spirit, we go on to discuss recent improved tests of quantum photonic devices (waveguides) and report unprecedented fidelities, which demonstrate that these devices can operate within the fault tolerant regime, by some accepted measures.


The structure of spin
Bryan Sanctuary
McGill University

There are a number of experiments on particles with spin that quantum mechanics cannot explain. These include the double slit experiment and coincidence experiments using photons. It is proposed that intrinsic spin angular momentum has a two dimension structure and this leads to a new angular momentum state of magnitude 1/sqrt(2).

In order to form this new spin state it is necessary to relax the hermitian postulate of quantum mechanics and admit non-hermitian states, which is manifest as a quantum phase. However for both isolated and entangled spins, hermitian states naturally result.


"Piecewise" vs. "Coherently controlled" adiabatic passage.
Evgeny Shapiro
The University of British Columbia
Coauthors: Moshe Shapiro, Valery Milner

We develop a technique for executing robust and selective transfer of populations between pre-selected superpositions of energy eigenstates. Viewed in the frequency domain, our methods stem from the idea of Coherently Controlled Adiabatic Passage [1], in which several adiabatic passage pathways coherently add up to provide the desired population transfer. Viewed in the time domain, the methods work by piecewise accumulation of the wavefunction in the target wave packet, applying the Piecewise Adiabatic Passage technique [2] in the multi-state regime. The presentation will introduce the basic concepts behind the technique and will discuss its recent theoretical and experimental developments.

[1] P. Kral, I. Thanopulos, M. Shapiro, Rev. Mod. Phys. 79, 53 (2007). [2] E.A. Shapiro, Phys. Rev. Lett. 99, 033002 (2007).

Quantum Repeaters
Christoph Simon
University of Calgary
Coauthors: N. Sangouard, H. de Riedmatten, M. Afzelius, N. Gisin, M. Staudt, J. Minar, H. Zbinden, B. Zhao, Y.-A. Chen, J.-W. Pan, R. Dubessy

I will briefly describe recent progress on the development of practical quantum repeater architectures, where the most immediate goal is to outperform the direct transmission of quantum states. I will focus on architectures using solid-state atomic ensembles as quantum memories, which may allow very efficient temporal multiplexing through the implementation of multimode memories. I will also discuss a promising approach based on trapped ions, which builds on the impressive experimental progress achieved with the goal of quantum computation in mind.

Paper reference: PRL 98, 190503; PRA 76, 050301; PRA 77, 062301; Nature 456, 773; PRA 79, 042340; PRA 79, 052329

Decoherence and the quantum-to-classical transition of a symmetry breaking coherent control scenario in an optical lattice
Michael Spanner
Coauthors: Ignacio Franco

An experimentally accessible way to study the quantum-to-classical (hbar ? 0) transition of a symmetry breaking coherent control scenario, in both isolated systems and in the presence of tunable amounts of decoherence, is proposed. The setup exploits the experimental control over the depth of the potential wells in optical lattices to define an effective hbar that, in principle, can be experimentally manipulated. Simulations of the transition show that the symmetry breaking effect survives in the classical limit and hence that matter interference effects are not required for the emergence of coherent laser control. Even when the average photoinduced momentum (i.e. the net degree of symmetry breaking) approaches smoothly its classical limit, the probability distribution of the observable does not, having an extremely fine oscillatory structure superimposed on the classical background that has little effect on the average. This fine structure due to quantum coherences is extremely fragile to environmental decoherence in the small hbar limit and a very small amount of decoherence is required to ensure the classical limit. We conclude that the detrimental effects induced by interaction with environmental degrees of freedom in this coherent laser control scheme are due to a decay of the temporal correlations in the system's dynamics, and not due to a decay of matter interference effects due to decoherence.

Measuring High-Order Coherences of Chaotic and Coherent Optical States
Martin J. Stevens
National Institute of Standards and Technology (NIST)
Coauthors: Burm Baek, Eric A. Dauler, Andrew J. Kerman, Richard J. Molnar, Scott A. Hamilton, Karl K. Berggren, Richard P. Mirin and Sae Woo Nam

We demonstrate a new approach to measuring high-order temporal coherences that uses a four-element superconducting nanowire single-photon detector (SNSPD) in which four independent, single-photon-sensitive elements are interleaved over a single spatial mode of the optical beam. We show the power of this technique by measuring nth-order coherences (n = 2,3,4) both of a chaotic, pseudo-thermal source that exhibits high-order photon bunching (up to n!), and of a coherent state source for which all coherences are ~1. Our results demonstrate that using multiple detector elements to parse an optical beam over dimensions smaller than the minimum diffraction-limited spot size can be equivalent-and in some cases superior-to using multiple beamsplitters and discrete detectors that each sample a replica of the entire mode.


Unconditionally secure entanglement-based quantum key distribution experiment
Hiroki Takesue
NTT Basic Research Laboratories, NTT Corporation
Coauthors: Ken-ichi Harada, Kiyoshi Tamaki, Hiroshi Fukuda, Tai Tsuchizawa, Toshifumi Watanabe, Koji Yamada, Sei-ichi Itabashi

Entanglement-based quantum key distribution (QKD) [1,2] is expected to achieve long distance secure communication with a relatively simple setup. One promising entanglement-based QKD technique is the Bennett-Brassard-Mermin 1992 (BBM92) protocol [2] where Alice and Bob use threshold detectors with linear optics. Recently, Koashi et al. provided an unconditional security proof that can be directly applied to a practical implementation of BBM92 [3]. In this presentation, we report the first unconditionally secure BBM92 QKD experiment based on this theory.

The experimental setup is explained below. We used an entangled photon-pair source based on spontaneous four-wave mixing in a silicon photonic wire waveguide [4] with which we generated high-purity 1.5-µm band time-bin entangled photon pairs. The clock frequency and the pulse interval were 100 MHz and 2 ns, respectively. The generated signal (idler) photon was sent to Alice (Bob) via an optical fiber. Alice and Bob input the received photons into 1-bit delayed interferometers. The two output ports of each interferometer were connected to 500-MHz gated-mode single photon detectors based on InGaAs/InP avalanche photodiodes operated with the sinusoidal gating technique [5]. In addition to the sifted key generation based on coincidence measurements between Alice’s and Bob’s detectors, we also counted the “double clicks” either on Alice’s or Bob’s side. From the error rate of the sifted key and the fraction of double click events, we can calculate the upper bound of the information that is possibly leaked to an eavesdropper. As a result, we can distill an unconditionally secure key by applying an appropriate amount of privacy amplification, provided that the observed bit error rate and the double click fraction are not too high [3].

At a fiber length of zero, we observed a sifted key rate of 510 bit/s, an error rate of 4.7%, and a double click fraction of 2.9 x 10^-4, which implies that we can distill unconditionally secure keys with a rate of 207 bit/s. When we transmitted the photons over 100 km (50 km x 2) of fiber, the experimental result showed that we were able to generate unconditionally secure keys with a rate of 0.16 bit/s. Thus, we successfully demonstrated unconditionally secure key distribution over 100 km of fiber. Also note that the secure key rate at 0 km was 10 times the previous record rate for 1.5-µm band entanglement-based QKD (with a security model based on general individual attacks) [6], which was realized thanks to the high photon collection efficiency of the silicon-based entanglement source and the use of fast single photon detectors with relatively large detection efficiencies.

[1] A. Ekert, Phys. Rev. Lett. 67, 661 (1991). [2] C. H. Bennett et al. Phys. Rev. Lett. 68, 557 (1992). [3] M. Koashi et al., arXiv:0804.0891. [4] H. Takesue et al., Appl. Phys. Lett. 91, 201108 (2007). [5] N. Namekata et al., Opt. Express 14, 10043 (2006). [6] T. Honjo et al., Opt. Express 16, 19118 (2008).

Complete process tomography of experimental one-way quantum computation
Yuuki Tokunaga
NTT, Osaka University
Coauthors: Satoru Okamoto, Rikizo Ikuta, Takashi Yamamoto, Masato Koashi, and Nobuyuki Imoto

We present full quantum process tomography of photonic cluster-state quantum computation. We demonstrated basic gates of one-way quantum computation and reconstructed these process matrices by using maximum likelihood estimation. From the completely reconstructed process matrices, we can evaluate several gate performances such as process fidelity, purity, and entanglement capability. We also discuss the relation between experimentally obtained process matrices and fault-tolerant threshold theory with several error models such as independent stochastic Pauli errors, independent stochastic completely-positive trace-preserving errors, and general local unitary errors (e.g. slight over-rotation).

Quantum Computer Simulations of Time Dependent Hamiltonians
Nathan Wiebe
University of Calgary
Coauthors: Dominic Berry, Peter Hoyer, Barry Sanders

Feynman's original motivation for the quantum computer resulted from a conjecture that quantum computers could efficiently simulate any quantum system, whereas classical computers cannot. Since then many quantum simulation schemes have verified his conjecture for sparse time independent Hamiltonians. However all proposals that have been put forward so far for simulating time dependent Hamiltonians only address sparse Hamiltonians and have complexity that scales as O(t^{3/2}). We address these issues by presenting a quantum computer simulation scheme that can simulate many non-sparse time-dependent Hamiltonians that uses arbitrary-order decomposition formulae to achieve complexity of O(t^{1+epsilon)) for arbitrary epsilon>0, provided the time dependence is suitably smooth. Since O(t) complexity has been proven to be optimal, our simulation scheme demonstrates near optimal performance for the broadest class of Hamiltonians considered so far.

Looking into the relation between quantum phase transition and entanglment via density functional theorey
Lian-Ao Wu
Theoretical Physics and History of Science, University of the Basque Country
Coauthors: M. S. Sarandy, D. A. Lidar, L. J. Sham

Density functional theory (DFT) is shown to provide a novel conceptual and computational framework for entanglement in interacting many-body quantum systems. DFT can, in particular, shed light on the intriguing relationship between quantum phase transitions and entanglement. We use DFT concepts to express entanglement measures in terms of the first or second derivative of the ground state energy. We illustrate the versatility of the DFT approach via a variety of analytically solvable models. As a further application we discuss entanglement and quantum phase transitions in the case of mean field approximations for realistic models of many-body systems.

Disentanglement and Control of Qubit Systems
Ting Yu
Department of Physics and Engineering Physics, Stevens Institute of Technology, New Jersey 07030, USA

Coherence dynamics of quantum systems is a generic paradigm that has been widely discussed in research fields ranging from atomic and optical physics to condensed matter physics and to quantum information science. In this talk I will present highlights of our recent work on several key issues in entanglement dynamics including evolution of spin entanglement under phonon noise, the sudden death of entanglement, two-body open system entanglement, probing many-body entanglement subject to thermal noise, and entanglement control.

Long-lived Quantum Memories
Ran Zhao
Georgia Institute of Technology
Coauthors: Y. O. Dudin, S. D. Jenkins, C. J. Campbell, D. N. Matsukevich, A. Kuzmich, and T. A. B. Kennedy

A memory based on hyperfine atomic coherences (spin waves) which can be read out optically at the single photon level, when classical noise sources have been eliminated, is a quantum memory. The spin waves are generally sensitive to ambient magnetic fields that limit their storage time to tens of microseconds. By optical pumping of the atoms and use of the clock coherence sensitivity to magnetic fields can be greatly reduced. Even in ultra-cold atomic samples motional dephasing becomes important on a scale of hundreds of microseconds. We present results of our work which circumvent both of these difficulties to achieve an atomic memory with a lifetime of several milliseconds. We will discuss various applications of the long-lived atomic memory, including deterministic single photon sources, matter qubit rotations, and matter-light entanglement.

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