April 25, 2014

CQIQC/Toronto Quantum Information Seminars
QUINF 2010-11
held at the Fields Institute

The CQIQC/Toronto Quantum Information Seminar - QUINF - is held roughly every two weeks to discuss ongoing work and ideas about quantum computation, cryptography, teleportation, et cetera. We hope to bring together interested parties from a variety of different backgrounds, including math, computer science, physics, chemistry, and engineering, to share ideas as well as open questions.

Talks are held Fridays at 10 am unless otherwise indicated (Starting in September start time is 11 am)

Friday 10-June-2011
10:10 a.m.
Stewart Library
Thomas Jennewein, University of Waterloo
High transmission-loss and classical-quantum multiplexing QKD enabled with short wavelength photons

We will present recent demonstrations of QKD at short optical wavelengths (532nm, 810 nm), and how it offers unique applications such as operation with ultra-high channel losses of 60dB, or entanglement based QKD within existing - and fully active - IT networks.

Short-wavelength QKD has been around ever since experiments have been performed, and is therefore widely studied. One important benefit for systems operating at that wavelength is the use Si-avalanche-photo-diodes for the photon detection. These offer high detection efficiency, very low dark counts and free-running operation.

The visible wavelength range is ideally suited for free-space applications, because the transmission through atmosphere is very good and the beam diffraction is still small. The use of such systems is to exchange quantum keys or entangled photons between mobile sites, and in the future even satellites. We implemented a system using the best available Si-detectors, an advanced and high-precision timing analysis system, and an ultrafast and modulated faint-laser source to achieve operation at ultra-high losses of 60dB. Our system shows a viable approach for operating under difficult situations such as the uplink of photons to a satellite, as well high-background, and these possible applications will be briefly reviewed.

In terms of fiber optic based transmissions, the current systems usually favor telecom wavelength over the short wavelengths, because existing IT infrastructure is single mode for telecom signals. This comes at the cost of needing sophisticated, and noisier photon detectors based on InGaAs. However, it has been established that also short wavelengths can be utilized over telecom optical fibers. The use of spatial and temporal filters must be implemented to suppress higher order modes in the optical fibers. We will show our recent experimental demonstration of entanglement based QKD, where the photon pairs created from a simple continuous wave operated entangled photon source can be distributed symmetrically over telecom optical fiber, and generated high quality secure keys. In a further experiment we show that this system directly allows the parallel transmission of QKD and classical telecom signals over the same optical fibers.

Friday 03-June-2011
10:10 a.m.
Stewart Library
Claude Fabre, University Pierre et Marie Curie-Paris 6 and Ecole Normale Superieure
Quantum frequency combs : generation and use

Frequency combs, made of the coherent superposition of many longitudinal modes, are good examples of highly multimode quantum objects, that can be of interest in massively parallel quantum information processing and quantum metrology. We have shown theoretically that frequency combs generated by Synchronously Pumped Parametric Oscillators (SPOPO) exhibit nonclassical features such as multimode squeezing and multipartite entanglement, that can be tailored at will by adjusting the shape of the pump pulses. I will report on the first achievements of our SPOPO experiment and show how these "quantum frequency combs" can be used in the future to improve time measurements beyond the shot noise limit.
Friday 13-May-2011
2:00 PM to 3:00 PM,
Stewart Library

Evgeny Shapiro, University of British Columbia
Coherent nonlinear spectroscopy with noisy broadband laser pulses
As a laser pulse is applied to an opaque scattering sample - such as biological tissue, paint, suspension, or plastic - its structure breaks down. In space, a coherent beam breaks into a multitude of speckles. In the spectral domain, the pulse is strongly modified due to the random transmission of the sample. Both effects are deleterious for one's ability to use coherent techniques for the spectral analysis of the sample.

I will review our ongoing work aimed at implementing nonlinear spectroscopy with coherent broadband laser pulses that have passed through opaque samples. Our goal is to use the quasi-random spectrum of light for extracting spectral information [1,2]. At the same time, we use two-dimensional spatial light modulators to correct for the spatial and temporal distortions due to the multiple scattering in opaque samples.

[1] E.A. Shapiro, S.O. Konorov, V. Milner, "Interference spectroscopy with coherent anti-Stokes Raman scattering of noisy broadband pulses", arXiv: 1104.1164.

[2] X.G. Xu, S.O. Konorov, J.W. Hepburn, V. Milner, "Noise autocorrelation spectroscopy with coherent Raman scattering", Nature Physics 4, 125 (2008).

Wednesday, Apr. 27
Stewart Library
Fields Institute
Gregor Weihs (University of Innsbruck)
Quantum Optics With Exciton-Polaritons in Semiconductor Microcavities

Since their discovery in 1992 by Weisbuch and others, exciton-polaritons have opened the world of semiconductor quantum optics. An exciton-polariton is a quasiparticle formed as a superposition of a bound electron-hole state (exciton) and a photon. Owing to the light effective mass of a microcavity photon, exciton-polaritons exhibit peculiar dispersion characteristics that have enabled a variety of applications. Exciton-polaritons can interact via their exciton component and they have shown parametric scattering, polariton lasing, Bose condensation at temperatures of a few Kelvin and many of the effect associated with superfluid behavior.

In our group we study the use of polariton parametric scattering for the generation of entangled photon pairs. To this end we have constructed a versatile experiment based on spatial light modulators, so that we can explore various momentum-conservation, i.e. phase-matching schemes. I will present results on the parametric scattering of polaritons and ideas on how to suppress bacground scattering mechanisms for generating clean entanglement.

Friday, Apr. 15
Stewart Library
Fields Institute
Robert Hadfield, Heriot-Watt University, Edinburgh, UK
Quantum photonics with superconducting single-photon detectors

Advanced optical quantum information processing applications place stringent demands on the performance of key components such as single-photon detectors [1]. A new class of single-photon detectors based on superconducting nanowires has recently emerged, offering telecom-wavelength sensitivity, combined with low dark counts, short recovery times and low timing jitter. I will describe the basic operating principle of this type of device, the current state-of-the-art and prospects for improvements. I will also discuss implementations of these devices in quantum information processing applications such as quantum key distribution [2], characterization of quantum emitters and operation of quantum waveguide circuits [3].

[1] Hadfield R. H. Nature Photonics 3 696 (2009)
[2] Takesue H. et al Nature Photonics 1 343 (2007)
[3] Natarajan C. M. et al Applied Physics Letters 96 211101 (2010)

Friday, Apr. 8
Stewart Library
Fields Institute
Moshe Shapiro (University of British Columbia and Weizmann Institute of Science)
Coherent Control of Entanglement and Spin Alignment

Coherent Control using two (H and V) perpendicular polarizations is shown to give rise to a direct way of writing arbitrary "cluster states" of entangled atoms or atom-ion pairs. The method relies on our proven ability to control the directionality of motion of (valence) electrons in dissociation processes, giving rise to either the "forward" |f> or "backward" |b> products. By irradiating a one dimensional array of molecules (Na2) or molecular-ions (Ca2+) in an optical lattice with a combination of two perpendicular (H and V) polarizations and another source of light at half their frequency, one can entangle light with matter AND craft clusters of entangled states composed of arbitrary superpositions of sequences of atom (or atom-ion) pairs, such as |H>|f>|b>...+|V>|b>|f>...

We then present a similar method to create a beam of spin aligned atoms (Iodine or Na) or molecules.

Friday, Apr. 1
Stewart Library
Fields Institute

David Tannor (Weizmann Institute of Science)
Designing Laser Pulses to Control Photochemical Reactions: Two New Pieces of the Puzzle

Since the invention of the laser, chemists have dreamed of designing specially tailored laser pulses to control the outcome of chemical reactions. This talk will start with a brief overview of the field, introducing the theoretical concepts and the current experimental status. Then two recent developments in our group will be described. The first is a method based for complete reconstruction of the excited state wave packet. Significantly, the method applies to polyatomics as well as diatomics, and has the potential to provide systematic mapping of excited state potentials. This information is critical to the ab initio design of pulses to control chemical reactions. The second development is the use of the von Neumann time-frequency representation to represent phase and amplitude shaped, ultrashort laser pulses. The von Neumann basis provides an efficient and intuitive representation that we show has significant practical advantages for distilling pulse mechanisms, mapping quantum control landscapes and for designing efficient closed-loop laser control based on mechanistic building blocks. The talk will conclude with some speculation about the prognosis for laser control of chemical bond-breaking in the next 5-10 years.


Monday, Mar.14
BA6183, 40 St. George Street

Man-Duen Choi (Mathematics Department, University of Toronto)
Special Seminar in Quantum Information - What are quantum channels through my old dream?

In 1970, I started off my adventure in the mathematical wonderland of completely positive linear maps (alias, quantum channels). Now, I am awaken to the new era of quantum computers, with all sorts of information processes in the setting of non-commutative analysis. As the time runs backwards in an alternating world through the looking glass, I have to come back to the same old scene to release myself from quantum entanglements. In particular, I shall give a MODERN report on my 1975 paper (Completely positive linear maps on complex matrices, Linear Algebra Appl. 10 (1975), pp. 285-290) which has been cited in more than 500 recent research articles (as shown in Google Scholars of March 2011).

This is an expository talk in the simple language of mathematics; no background knowledge of information theory will be assumed for this talk.

Friday, Feb. 18
Stewart Library
Fields Institute
Darin Ulness (Concordia)
Noisy Light Spectroscopy: Putting Noise to Good Use

Noisy light spectroscopy is an alternative approach, distinct from short-pulse and continuous wave methodologies, for probing ultra-fast dynamics via nonlinear optical processes. Although not a mainstream technique, noisy light does offer a new perspective on interesting physical phenomena. This talk will provide a overview of both the theoretical and experimental aspects of noisy light spectroscopy. Applications of noisy light to hydrogen and halogen bonding will be presented as well.

Tuesday, Feb. 1
Stewart Library
Fields Institute
Vadim Makarov (Norwegian University of Science and Technology)
Cracking commercial quantum cryptography

Quantum cryptography, unbreakable in principle, can currently be hacked through implementation loopholes. I present a loophole recently explored by researchers from the Norwegian University of Science and Technology (Trondheim, Norway) and Max Planck Institute for the Science of Light (Erlangen, Germany).

Most of today's quantum cryptography systems use single-photon detectors based on avalanche photodiodes. These detectors operate part-time in the linear regime, in which they respond deterministically to a short bright-light pulse producing a click when the pulse peak power exceeds a certain threshold. Furthermore, these detectors can be blinded to single photons by bright-light illumination, through several different mechanisms connected to detector electronic and thermal properties. We show how this killer superposition of loopholes can be used to launch a perfect attack against a quantum key distribution system, eavesdropping the complete secret key without alerting the legitimate users. We have shown experimentally that this vulnerability is fully present in commercial quantum cryptosystems, Clavis2 from ID Quantique and QPN 5505 from MagiQ Technologies. We propose how to build a plug-and-play eavesdropper for both cryptosystems, using off-the-shelf components. In a separate experiment on an entanglement-based research cryptosystem, we have built a full eavesdropper and actually demonstrated 100% eavesdropping of the 'secret' key. This class of loopholes should be patchable, but how to do it in practice remains an
open question.

The talk will include an equipment demonstration of full detector control by an eavesdropper.

Friday, Jan. 21
Stewart Library
Fields Institute
Guillaume Gervais (McGill)
Non-Abelian quantum statistics and the Moore-Read Pfaffian state
In 1937, italian physicist Ettore Marojana modified the Dirac equation so that it would admit only real solutions (as opposed to complex-valued solutions). Such solutions are known as Majorana fermions, a class of particles that are interestingly their own anti-particles. Recently, Majorana fermions have migrated into condensed matter physics, as they have been predicted to occur as elementary excitations of systems containing many interacting
fermions. In particular, they are predicted to exist in chiral p-wave superconductors, in superfluid 3He, and for the so-called Moore-Read Pfaffian state thought to be realized for some fractional quantum Hall states. The interest for the Majoranafermions is largely driven by that they obey to non-abelian braiding quantum statistics, a necessary property for the construction of a topological quantum computer that would, in principle, be immune to local perturbations.

In this talk, I will briefly review the recent progress in the field of non-abelian quantum statistics with a strong emphasis on the physics of the "5/2" fractional quantum Hall state where such statistics are thought to occur. In particular, I will discuss the conundrum of the spin polarization for that state, as well as discussing the hypothetical adiabatic cooling of non-abelian states which could be used as an experimental proof of "non-abelianness".

Friday, Dec. 17
Stewart Library
Fields Institute

Christoph Simon (University of Calgary)
Towards efficient photon-photon gates
The implementation of efficient quantum gates between individual photons is an important goal in quantum information processing. Achieving this goal poses two complementary challenges. On the one hand, it requires strong interactions between individual photons. On the other hand, the gate operations have to be realized in such a way that they preserve the single-mode character of the photons despite those strong interactions. I will give a brief overview over different approaches that have been proposed, and discuss where they stand with respect to these two challenges. In particular I will explain the limitations due to transverse multi-mode effects, and show how they could be overcome in a scheme based on dipole-dipole interactions between Rydberg state polaritons [1]. Finally I will present a recent proposal for photon-photon gates in Bose-Einstein condensates [2], which exploits the long storage times and collisional interactions available in this system, where the interactions are enhanced by the adiabatic compression of the condensate and the use of a Feshbach resonance. I will conclude by arguing that we may have reached a critical mass of technology and theoretical understanding to allow successful experiments on efficient photon-photon gates in the foreseeable future.

[1] B. He, A. MacRae, Y. Han, A.I. Lvovsky, and C. Simon, Transverse
multi-mode effects on the performance of photon-photon gates,
[2] A. Rispe, B. He, and C. Simon, Photon-photon gates in
Bose-Einstein condensates, arXiv:1010.0037s.


Friday, Dec. 10
Stewart Library
Fields Institute
Joseph Emerson (Department of Applied Math and Institute for Quantum Computing, University of Waterloo)
Robust benchmarking of quantum processes via randomization

I will describe an experimental method for efficiently benchmarking quantum information processes. The method is based on characterizing the survival probability under sequences of random quantum processes drawn from a unitary 2-design. I will show how the protocol provides a reliable estimate of the average error-rate for a set of target operations (eg quantum gates) under a very general noise model that allows for both time and gate-dependent errors. I will discuss the conditions under which this estimate remains valid and illustrate features of the protocol through numerical examples.
Friday, Nov. 5
Stewart Library
Fields Institute

Mark M. Wilde (McGill University)
Entanglement boosts quantum turbo codes

One of the unexpected breakdowns in the existing theory of quantum serial turbo coding is that a quantum convolutional encoder cannot simultaneously be recursive and non-catastrophic. These properties are essential for a quantum turbo code to have an unbounded minimum distance and for its iterative decoding algorithm to converge, respectively. Here, we show that the entanglement-assisted paradigm gives a theoretical and practical "turbo boost" to these codes, in the sense that an entanglement-assisted quantum (EAQ) convolutional encoder can possess both of the aforementioned desirable properties, and simulation results indicate that entanglement-assisted turbo codes can operate reliably in a noise regime 5.5 dB beyond that of standard quantum turbo codes. Entanglement is *the* resource that enables a convolutional encoder to satisfy both properties because an encoder acting on only information qubits, classical bits, gauge qubits, and ancilla qubits cannot simultaneously satisfy them. We give several examples of EAQ convolutional encoders that are both recursive and non-catastrophic and detail their relevant parameters. Finally, simulation results demonstrate that interleaved serial concatenation of EAQ convolutional encoders leads to a powerful code construction with excellent performance on a memoryless depolarizing channel.

Friday, Oct. 22
Stewart Library
Fields Institute

Alireza Shabani (Princeton University)
Compressed Tomography of Quantum Dynamical Systems

A fundamental problem in characterization of complex quantum systems is the exponential growth in the required physical resources with the size of system. We develop an efficient method for complete estimation of an unknown quantum process/Hamiltonian with a polynomial number of experimental configurations via employing techniques known as compressed sensing. We demonstrate that by O(s \log d) random local preparations and measurements settings one can fully identify a quantum process/Hamiltonian for a d-dimensional system, if it is known to be nearly s-sparse in a basis. We present the first experimental implementation of this method for two- and four-photon quantum optical systems with a significant reduction in physical resources compare to known tomography techniques. Moreover, we demonstrate robustness of this technique by performing efficient high-fidelity estimation of two-qubit photonic phase gates under various decoherence strengths.

Monday, Oct. 18
Room MP606
(60 St. George St.)
Fields Institute
Alan Migdall (NIST)
Single-photon detector, source, and metrology efforts at NIST

NIST has ongoing efforts to advance single-photon and photon-number-resolving detectors, single-photon and entangled-photon sources, and the metrology of those devices for applications in quantum communications and quantum information processing. The detector efforts include improved circuitry for both Si and InGaAs single-photon avalanche photodiodes to improve efficiency and count rate, while reducing dark counts and afterpulsing, and to improve the compatibility of these devices with gigahertz-rate single and correlated-photon sources. We are also working on improved processing of the output signal from photon-number-resolving detectors to maximize the information extracted. Another scheme uses active optical switching and an array of detectors to increase allowable detection rates and reduce detection deadtime. Source efforts include bright and efficient sources based on parametric downconversion and fourwave mixing. Metrology efforts in the photon counting regime include both conventional metrology tied to detector-based standards and correlated-photon metrology using photon pair sources, with our ultimate goal to make higher accuracy metrology available to the photon-counting community at large.

Monday, Sept. 27
Stewart Library,
Fields Institute

Elanor Huntington (University of New South Wales)
Quantum Hindsight: Quantum Parameter Estimation Using Smoothing

Quantum parameter estimation has many applications, from gravitational wave detection to quantum key distribution. The most commonly used technique for this type of estimation is quantum filtering, using only past observations. We present the first experimental demonstration of quantum smoothing, a time symmetric technique that uses past and future observations, for quantum parameter estimation. We consider both adaptive and nonadaptive quantum smoothing, and show that both are better than their filtered counterparts. For the problem of estimating a stochastically varying phase shift on a coherent beam, our theory predicts that adaptive quantum smoothing (the best scheme) gives an estimate with a mean-square error up to 2-root-2 times smaller than nonadaptive filtering (the standard quantum limit). The experimentally measured improvement is 2.24.

Friday, Sept. 24
Stewart Library
Fields Institute
Ivan Deutsch (University of New Mexico)
Extreme Spin Squeezing in Cold Trapped Atomic Ensembles

Squeezing of the collective spin of an ensemble of atoms is intimately related to the creation of entanglement and nonlinear evolution of the many-body system. We describe a new approach to achieving a high degree of squeezing based on coherent quantum feedback in a double-pass Faraday interaction between an optical probe and an optically dense atomic sample. A quantum eraser is used to remove residual spin-probe entanglement, thereby realizing a single-axis twisting unitary map on the collective spin. This interaction can be phase-matched, resulting in *exponential* enhancement of squeezing as a function of optical density for times short compared to the decoherence time. In practice the scaling and peak squeezing depends on decoherence, technical loss, and noise. Including these imperfections, our model indicates that 10 dB of squeezing should be achievable with laboratory parameters. With such squeezing as a base, we can explore the atomic ensemble as a platform for continuous-variable quantum information processing.

Friday, Sept. 17
Stewart Library, Fields Institute

Alessandro Fedrizzi (University of Queensland)
Discrete- and continuous-time quantum walks with single photons

Quantum walks describe the evolution of quantum particles on a graph. Due to their rich dynamics, they can emulate a wide range of phenomena in real-world systems. In the first part of my talk, I will present a stable and reasonably scalable optical implementation of a discrete-time quantum walk on a line. Single photons, encoded in polarisation, walk through an interferometric network based on calcite beam displacers and half-wave plates. We demonstrate full control of the decoherence in the system and have access to all lattice sites at any given time step. This allows us to investigate a host of scenarios, such as the observation of signatures of topological phases in artificial 1D systems. We implemented phase transitions for topological phases of a certain symmetry class, distinguished by their winding number. For transitions in phase space with different topological invariants, we observe distinct bound states which are not present otherwise. In the second part of my talk, I present results on continuous-time quantum walks with periodic boundary conditions in direct-write waveguides with single photons and two-photon states.