SCIENTIFIC PROGRAMS AND ACTIVITIES

CQIQC/Toronto Quantum Information Seminars QUINF 2009-10 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 11 am unless otherwise indicated

PAST TALKS
Friday, June 18 Stewart Library, Fields Institute 11am	Hong Guo (Peking University, Beijing, China) Two Key Techniques for the Security of Practical Quantum Key Distribution System: Truly Random Number Generator and Passive Scheme of Source Monitoring Two key issues for the security of practical quantum key distribution (QKD) system, i.e., truly random number generator (TRNG) and monitoring of the QKD source, are addressed. For TRNG, two schemes based, respectively, on the detection of photon number statistics of diode laser, and on the continuous beat signal detection of a VCSEL, is reported, which can produce truly random numbers at 20 Mbit/s rate for any long time and the true randomness of which is primarily confirmed by 3 sigma-criteria up to 14 Gbit. In the security analysis of some QKD protocols, the photon-number distribution (PND) of QKD source is assumed to be fixed and known to Alice and Bob, while Eve cannot control or change it. In real-life experiment, the PND may deviate from this assumption (the source is untrusted) and so a monitoring for the source is needed. Previously, the active scheme for the monitoring is proposed but did not work well in the experiment. For monitoring the photon statistics of QKD source, we propose a passive scheme with a beam splitter and a PD detector and the experiment is realized in a real-life QKD system.
Thursday, June 3 Stewart Library, Fields Institute 2pm	Jiangbin Gong (National University of Singapore) Preserving Known or Unknown Entangled States by Uhrig's Dynamical Decoupling Sequence Decoherence effects can completely kill quantum entanglement within a very short time. Hence it will be of vast importance if we can actively preserve quantum entanglement with a high efficiency and with a universal scenario that does not require our knowledge of the bath or of the system-bath coupling. We show that this is indeed possible, by successfully extending Uhrig's dynamical decoupling sequence from one-qubit cases to general two-qubit systems. In particular, we explicitly construct control sequences to lock a known but arbitrary two-qubit state to the Nth order of time, using only N control pulses. We then show that three layers of Uhrig's dynamical decoupling sequence in a special order will be able to preserve an unknown two-qubit entangled state to the Nth order of time, using N^3 control pulses. The results may be also extended to general multi-level quantum systems.
**PLEASE NOTE DATE AND TIME** Wednesday, May 26 Location Room MP606	Alex Hayat (Department of Electrical Engineering, Technion, Haifa, Israel) Semiconductor Quantum Photonics Miniaturizing quantum photonics is a rapidly growing field. We introduced a concept of microcavity standing-wave nonlinear optics theoretically and experimentally, where the phasematching requirement is translated into a nonlinear mode overlap. We demonstrated experimentally the first observation of two-photon emission in semiconductors - a process, in which electron transition between energy levels occurs by the emission of a photon pair. We proposed this phenomenon as an electrically-driven room-temperature source of energy-entangled photons, much more efficient than the down-conversion schemes. We also proposed two-photon absorption interferometry for characterization of energy qubits. First observations of electrically-induced two-photon transparency and two-photon gain in semiconductors are demonstrated experimentally, and a scheme for a femtosecond-scale g(4) measurement is implemented.
Mon., April 26 11:10 am Room MP 606, 60 St. George Street	Prof. Tilman Pfau (Universität Stuttgart, Germany) Lecture 3: Ultracold Rydberg chemistry and how to excite Rydberg atoms in a microscopic glass box I will discuss how quantum chemistry in the ultracold world allows for novel binding mechanisms. The recently observed of ultralong-range Rydberg molecules (dimers and trimers) are based on quantum scattering of Rydberg electrons from polarizable ground state atoms. Furthermore, we show calculations that reproduce the observed binding energies remarkably well and reveal that some of the excited states are purely bound by quantum reflection at a shape resonance for p-wave scattering. Finally as an outlook on how long range interactions between neutral atoms could actually lead to practical quantum devices like single photon sources I report on our effort to observe the Rydberg blockade in micron sized thermal vapor cells.
Fri., April 23 11:10 am Stewart Library	Prof. Tilman Pfau (Universität Stuttgart, Germany) Lecture 2: Strongly interacting Rydberg atoms I will introduce Rydberg atoms and their mutual interaction which can be of van der Waals or dipolar character. This interaction leads a blockade mechanism which is observed in a BEC. We will see that this strong interaction will allow us to emulate the ground state properties of spin Hamitonians as they are discussed in condensed matter physics. Universal scaling behavior in the quantum critical region of the underlying phase diagram is observed. The laser excitation to the Rydberg state can be coherent despite strong interactions. To prove this experimentally rotary echo sequences are applied.
Wed., April 21 11:10 am Room MP 606, 60 St. George Street	Prof. Tilman Pfau (Universität Stuttgart, Germany) Lecture 1: A purely dipolar quantum gas I will report on the realization of a purely dipolar quantum gas, where the long-range and anisotropic interaction between magnetic chromium atoms is determining the physical properties. We will discuss the stability diagram of a dipolar gas and the dipolar collapse dynamics. In the outlook I will show how spin orbit coupling in dipolar gases can give rise to a quantum version of the Einstein de Haas effect. The same coupling can also be used for demagnetization cooling, an idea that dates back to the first laser cooling proposal by Alfred Kastler in 1950.
**PLEASE NOTE DATE AND TIME** Monday, April 19 11:10 am Stewart Library	**CANCELED** Nicolas Brunner (University of Bristol) Why is quantum non-locality limited? Quantum mechanics is a non-local theory, however not a maximally non-local one according to relativity. More precisely, there exist alternative theories containing more non-locality than quantum mechanics that still respect the no-signaling principle. Why these theories are unlikely to exist in nature, and what physical principle limits quantum non-locality is still not known today, despite an intensive research effort. After briefly reviewing general non-signaling theories, in particular focusing on non-local boxes, I will present recent work which aims at recovering quantum correlations from information-theoretic principles, such as communication complexity and information causality.
Friday, Apr.16 11:10 am FIELDS ROOM 210	Barry Sanders (University of Calgary) Machine Learning for Precise Quantum Measurement Quantum measurement schemes aim to surpass the standard quantum limit (essentially partition noise) and strive to reach the quantum limit (precision inversely proportional to number of injected particles). One particularly promising category of quantum measurement schemes employs a feedback mechanism: leading particles are detected with the resultant information used to control the instrument in order to extract progressively more information during passage of the pulse. Clever quantum feedback schemes have been devised but are restricted to ideal conditions. In general quantum feedback schemes are challenging to design so we decided to adapt machine learning theory to quantum information inputs and employ our theory to devise adaptive-feedback quantum measurement schemes. In particular our approach replaces guesswork in quantum measurement by a logical, fully-automatic, programmable routine. We show that our method yields schemes that outperform the best known adaptive scheme for interferometric phase estimation. Furthermore our approach can be adapted to the real-world case where the instrument would learn through trial and error an effective quantum feedback routine.
Friday, April 9 11:10 am Stewart Library	Steve Flammia (Perimeter Institute for Theoretical Physics) Ultra Fast Quantum State Tomography 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 methods for learning quantum states more efficiently, in some cases exponentially so. The first method runs in polynomial time and outputs a polynomial-sized classical approximation of the state (in matrix product state form), together with a rigorous bound on the fidelity. The second result takes advantage of the fact that most interesting states are close to pure states to get a quadratic speedup using ideas from compressed sensing. I'll also show simulations of this second method that demonstrate how well it works in practical situations. Both of these results are heralded, and require no a priori assumptions about the state. This is joint work with S. Bartlett, D. Gross, R. Somma (first result), and D. Gross, Y.-K. Liu, S. Becker, J. Eisert, (second result; arXiv:0909:3304).
**PLEASE NOTE DATE AND TIME** Monday, March 15 11:10 am Room MP 606, 60 St. George Street	Lev Vaidman, Tel Aviv University Where is the Quantum Particle between two Measurements? Wheeler Delayed Choice experiment, Elitzur-Vaidman Interaction-free Measurement, and Hosten-Kwiat Counterfactual Computation will be discussed to answer Bohr's forbidden question: "Where is a quantum particle while it is inside a Mach-Zehnder Interferometer?" I will argue that the naive Wheeler's approach fails to explain a weak trace left by the particle and that the two-state vector description is required.
Friday, 26 Feb 11:10am Stewart Library	Arjendu Pattanayak, Carleton College Non-monotonicity in the quantum-classical transition The transition between a system behaving completely classically and behaving completely quantum-mechanically is complicated. The two kinds of behaviors can be very different particularly when the system is nonlinear. We know the transition depends on the size of the system, the temperature and environmental effects, and on the nonlinear dynamics, so it is a multi-parameter landscape. But what is the shape of this landscape? In this talk I present evidence for non-monotonicity in the quantum-classical transition in two different systems: A damped driven double-well oscillator, as well as the simple harmonic oscillator. I will also discuss prospects for experimental verification of these predictions.
Tuesday, 23 Feb 2pm Room MP 713, 60 St. George Street	Morgan Mitchell, ICFO, Barcelona Quantum metrology with cold atoms: quantum non-demolition measurements, spin squeezing, and non-linear metrology Quantum metrology studies the use of quantum states, interference, and entanglement in precision measurement. Originally developed for interferometric measurement of gravitational waves, in recent years quantum metrology has expanded both theoretically and experimentally to become a general technique with application in several areas. I will describe experimental work using ensembles of cold rubidium, an interesting quantum system for measurement of magnetic fields. Using optical probes and paramagnetic Faraday rotation, we demonstrate quantum non-demolition measurement of spins at the projection-noise limit. If time permits, I will discuss the use of nonlinear Faraday rotation to make measurements with scaling better than the "Heisenberg limit" of linear measurements.
Tuesday, 23 Feb 11am Davenport East Seminar Room, 80 St. George Street, Toronto	Moshe Shapiro (Weizmann Institute of Science and University of British Columbia) Non destructive state reconstruction and the resolution of the spectroscopic phase problem We discuss the problem of non destructively reconstructing unknown time evolving vibrational wave packets and show how one can solve this problem and also reconstruct in a "point-by-point" manner the potential that governs the motion of such wave packets, using as input only the power spectrum of the light emitted from a small minority of replicas of the unknown states and the potential to which the light emission occurs.
Friday, Feb. 12, 2010 11:10 am Stewart Library	Ioannis Thanopoulos Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation Quantum dynamics of large molecules and control of multi-channel processes We show that the quantum dynamics of a system comprised of a subspace Q coupled to a much larger subspace P can be recast as a reduced set of ordinary differential equations with constant coefficients. These equations can be solved by a single diagonalization of a general complex matrix. The efficiency of the method is demonstrated via computations on large molecular systems, as the radiationless transitions in pyrazine. We also present a solution to the "Multi-Channel Quantum Control" problem, where selective and complete population transfer from an initial bound state to M energetically degenerate continuum channels is achieved. The control is affected by coherently controlled Adiabatic Passage proceeding via N bound intermediate states. We illustrate the viability of the method by computationally controlling the multi-channel photodissociation of methyl iodide.
Friday, Jan. 8, 2010 11:10 am Stewart Library	*** CANCELLED *** Vladimir Korepin Physics Department, State University of New York at Stony Brook Entanglement and Correlation Functions: Bethe Ansatz vs Valence Bond Solid States I will consider entanglement entropy and correlation functions in two different set states [wave functions]: Bethe Ansatz and Valence Bond Solid states [discovered by Affleck, Kennedy, Lieb and Tasaki]. I will argue that these states are essentially different. As measure of entanglement we will use von Neumann entropy or Renyi entropy. Main results obtained analytically by means of FisherHarwig formula, desribing Toeplitz determinants. Information about the speaker: http://en.wikipedia.org/wiki/Vladimir_Korepin http://insti.physics.sunysb.edu/~korepin
Friday, Dec. 11, 2009 11:10 am Stewart Library	Joe Altepeter, Northwestern University Entangled Photon Polarimetry The Polarimeter is a useful tool in nearly every branch of modern optics, providing a complete, real-time, graphical measurement of the polarization of an optical field. From the perspective of quantum information processing, a polarimeter actively monitors the complete quantum state of a single qubit system. Here we present advances in two-qubit state visualization and quantum state tomography which have made it possible to realize a complete, real-time, graphical measurement of a two-qubit system. In addition, we present work on the fiber-based, telecom-band entanglement sources for which this type of tool will be immediately useful.
Friday, 4-Dec.2009 11:10am Stewart Library	Dmitry Gavinsky NEC Labs, Princeton, NJ, U.S.A. Predictive Quantum Learning We give the first unconditional separation of quantum and classical learning. We demonstrate a relational concept class that is efficiently learnable in a quantum predictive analogue of PAC, while in any reasonable classical model exponential amount of training data would be required. We show that our separation is the best possible in several ways, in particular there is no analogous result for a functional class, as well as for several weaker versions of quantum learning.
Nov. 23, 2009 MP 606, 60 St. George St.	Joint Quantum Optics CQIQC Seminar Pablo Londero (Cornell University) Nonlinear Optics in Rubidium-Filled Hollow-Core Photonic Band-Gap Fibers Much of the success in optical quantum computing and quantum key distribution has come from linear techniques, in part due to the difficulty of developing systems which exhibit optical nonlinearities at low photon numbers. Hollow-core photonic band-gap fibers, when filled with Rb vapor, provide a system with strong coupling to resonant light. This opens the door to a variety of interesting quantum-optical experiments where low-photon number states can induce measurable nonlinearities, and where moderate amounts of light can produce unusually strong nonlinear effects. I will present recent experiments in our Rb-loaded fibers on electromagnetically-induced-transparency in the presence of buffer gas, four-wave mixing with gain >100 and bandwidths >300 MHz at microwatt power levels, and all-optical switching with a few thousand photons, as well as some thoughts on future directions.
Nov. 9, 2009 MP 606, 60 St. George St.	Joint Quantum Optics CQIQC Seminar Markus Buttiker (Universite de Geneve) Traversal time for tunneling The tunneling of particles through classically forbidden regions is a basic quantum phenomenon. We review discussions which investigate the dynamics of this process and in particular attempt to answer the question: “What is the time a particle needs to traverse a classically forbidden region”. Some approaches lead to superluminal velocities. Similar to Brillouin and Sommerfeld we are interested in approaches which yield causal subluminal velocities. In particular we discuss in some detail the propagation of monochromatic fronts of Stevens and Buttiker and Thomas [1]. The use of such fronts to find a sharp traversal time is not possible, however, using frequency band limited sources or short-time Fourier analysis allows to determine a traversal time with the accuracy of the time itself [2]. [1] Markus Buttiker and Harry Thomas, Ann. Phys. (Leipzig) 7, 602 (1988). [2] Gonzalo Muga and Markus Buttiker, Phys. Rev. A 62, 023808 (2000).
October 30, 2009 Stewart Library Fields Institute	TALK POSTPONED Steve Flammia, Perimeter Institute for Theoretical Physics Ultra Fast Quantum State Tomography 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 methods for learning quantum states more efficiently, in some cases exponentially so. The first method runs in polynomial time and outputs a polynomial-sized classical approximation of the state (in matrix product state form), together with a rigorous bound on the fidelity. The second result takes advantage of the fact that most interesting states are close to pure states to get a quadratic speedup using ideas from compressed sensing. I'll also show simulations of this second method that demonstrate how well it works in practical situations. Both of these results are heralded, and require no a priori assumptions about the state. This is joint work with S. Bartlett, D. Gross, R. Somma (first result), and D. Gross, Y.-K. Liu, S. Becker, J.Eisert, (second result; arXiv:0909:3304).

 

 

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