November 25, 2014
Toronto Quantum Information Seminars
Fields Institute,
222 College St.


The Toronto Quantum Information Seminar 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.

Upcoming Seminars

Dec 5, 2014

RM 210

Hamed Majedi (University of Waterloo)

Jan 30, 2015

RM 210

Edward Taylor (University of Toronto)

March 20, 2015

RM 210

Joel Yuen (Massachusetts Institute of Technology)

Spring 2015

Demetrios Christodoulides (University of Central Florida)

Analogies between different disciplines provide a powerful tool in understanding nature. As such, quantum-classical optical similarities offer new opportunities in manipulating classical optical fields or quantum states. In recent years, many of the ramifications of these concepts have come to fruition on several fronts in the area of optics. What made this possible are new advances in structure fabrication and beam synthesis techniques. In this talk, we will provide an overview of our activities in this field. As an example, we will consider accelerating optical wavepackets in the form Airy beams as a means to bend light for applications in plasmonics, extreme nonlinear optics, and biology. Studying quantum inspired phenomena in artificial optical structures that would have been otherwise impossible to directly observe in their own habitat, like Anderson localization and parity-time (PT) symmetry. Finally, the possibility of quantum state engineering in periodic and random optical lattices will be reviewed in this talk.


Past Seminars

Nov 21, 2014

RM 210

Yaoyun Shi (University of Michigan)
How to generate the first secret, then as many as you like

Secrecy is randomness. A perfect secret is one for which all the alternatives are equally likely to the adversary. For secrecy to be possible, we have to assume that the world is not deterministic. Here we show how this necessary assumption, together with the validity of quantum mechanics and relativity, will allow us to generate the first and almost perfect secret, and then to expand it to be arbitrarily long. Unlike all existing solutions, the security of our construction is provable, unconditional (as opposed to computational), and verifiable. Our method can also be used for the important task of distributing cryptographic keys.

Technically speaking, we formulate a precise model of extracting randomness from quantum devices whose inner-workings may be imperfect or even malicious. We then construct such a "physical extractor" that needs only a single and arbitrarily weak classical source, and the output randomness can be arbitrarily long and almost optimally close to uniform. This is impossible to achieve for classical randomness extractors, which cannot increase entropy and requires two or more *independent* sources.

Our construction also differs from quantum-based random number generators in the market, as they all require that the users trust their quantum inner-workings. Such a trust threatens security when the devices are defective or were procured from an untrusted vendor. Several features of our construction, such as maximum noise tolerance and unit quantum memory requirement, have fundamentally lowered the implementation requirements.

Nov 14, 2014

RM 210

Nancy Makri (University of Illinois)
Quantum-Classical Path Integral

The path integral formulation of time-dependent quantum mechanics provides the ideal framework for rigorous quantum-classical or quantum-semiclassical treatments, as the spatially localized, trajectory-like nature of the quantum paths circumvents the need for mean-field-type assumptions. However, the number of system paths grows exponentially with the number of propagation steps. In addition, each path of the quantum system generally gives rise to a distinct classical solvent trajectory. This exponential proliferation of trajectories with propagation time is the quantum-classical manifestation of time nonlocality, familiar from influence functional approaches.

A quantum-classical path integral (QCPI) methodology has been developed. The starting point is the identification of two components in the effects induced on a quantum system by a polyatomic environment. The first, “classical decoherence mechanism” dominates completely at high temperature/low-frequency solvents and/or when the system-environment interaction is weak. Within the QCPI framework, the memory associated with classical decoherence is removable. A second, nonlocal in time, “quantum decoherence process” is also operative at low temperatures, although the contribution of the classical decoherence mechanism continues to play the most prominent role. The classical decoherence is analogous to the treatment of light absorption via an oscillating dipole, while quantum decoherence is primarily associated with spontaneous emission, whose description requires quantization of the radiation field. The QCPI methodology takes advantage of the memory-free nature of system-independent solvent trajectories to account for all classical decoherence effects on the dynamics of the quantum system in an inexpensive fashion. Inclusion of the residual quantum decoherence is accomplished via phase factors in the path integral expression, which is amenable to large time steps and iterative decompositions. Preliminary tests on dissipative two-level systems and fully atomistic simulations of charge transfer in solution suggest that the QCPI methodology is realistically applicable to many processes of chemical and biological interest.

Oct 17, 2014
11:00 a.m.

RM 210

Josh Combes (Perimeter Institute for Theoretical Physics)
Perimeter Institute and Institute for Quantum Computing

In 1988 Yakir Aharonov, David Albert, and Lev Vaidman wrote a paper provocatively titled "How the result of a measurement of a component of the spin of a spin-1/2 particle can turn out to be 100". In this paper they defined a quantity, similar to the expectation value of an operator, called the "weak value" of an operator. The weak value of an operator has many weird properties which has lead some researchers to: (1) think that quantum paradoxes are solved by this defined quantity, and (2) suggest that the weak value can be used to perform sensitive measurements. In this talk I will address both points. First, I argue that the phenomenon of anomalous weak values is not limited to quantum theory. In particular, I show that the same features occur in a simple model of a coin subject to a form of classical backaction with pre- and post-selection. Second, I will explain how rigorous estimation and detection theory imply that weak values do not aid quantum metrology. This is joint work with Chris Ferrie of the University of New Mexico.

Oct 10, 2014
11:00 a.m.

RM 210

Paola Cappellaro (Massachusetts Institute of Technology)
Quantum control strategies for imaging and spectroscopy

Quantum control techniques have proven effective to extend the coherence of qubit sensors, thus allowing quantum-enhanced sensitivity at the nano-scale. The key challenge is to decouple the qubit sensors from undesired sources of noise, while preserving the interaction with the system or field that one wishes to measure. In addition, tailoring the sensor dynamics can help reveal temporal and spatial information about the target.

In this talk I will show how we can use coherent control of quantum sensors to reconstruct the arbitrary profile of time-varying fields, while correcting the effects of unwanted noise sources. These control techniques can be further used to reveal information about classical and quantum noise sources. For example, they can achieve high frequency resolution, thus allowing precise spectroscopy and imaging of the spatial configuration of a spin bath.

I will illustrate applications of these strategies in experimental implementations based on the Nitrogen-Vacancy center in diamond.

Oct 3, 2014
11:00 a.m.

RM 210

Man-Duen Choi (University of Toronto)
The Principle of Locality made simple

In physics, the Principle of Locality states that an object is influenced directly only by its immediate surroundings. This could be transformed to a simple mathematical statement of NO wisdom at all. Nevertheless, with extravagent assumption (on the obvious truth) and fascinating explanation (of the ultimate nonsense), the Principle may become a big Law/Theory/Theorem or a tremendous Paradox to shake your heart/body.
This is an expository talk of my own adventure in the quantum wonderland (concerning the structure problems of direct sums and tensor products). No working knowledge of quantum information is required in this talk.

Sept 19, 2014
12:30 p.m
Fields RM 210

Boris Braverman (Massachusetts Institute of Technology)
Progress toward a spin squeezed optical atomic clock beyond the standard quantum limit

State of the art optical lattice atomic clocks have reached a relative inaccuracy level of $10^{-18}$, making them the most stable time references in existence. One limitation to the precision of these clocks is the projection noise caused by the measurement of the atomic state. This limit, known as the standard quantum limit (SQL), can be overcome by entangling the atoms. By performing spin squeezing, it is possible to robustly generate such entanglement and therefore surpass the SQL of precision in optical atomic clocks. I will report on recent experimental progress toward realizing spin squeezing in an ${}^{171}$Yb optical lattice clock. A high-finesse micromirror-based optical cavity mediates the atom-atom interaction necessary for generating the entanglement. By exceeding the SQL in this state of the art system, we are aiming to advance precision time metrology and expand the boundaries of quantum control and measurement.
Sept 12, 2014
11:00 a.m.
RM 210

Raphael Pooser (Oak Ridge National Labs)
Quantum Sensors: Data at the information frontier of physics

Quantum information processing has a host of applications, including quantum key distribution and quantum computing as some of the most prominent. In all of these applications, sensing and control are needed in order to maintain the fidelity of quantum information. In quantum sensors, information stored in quantum mechanical systems is extracted and put to use, either in subsequent control signals, or in general information processing applications. Some famous examples of quantum sensors include atomic clocks, cold atom interferometers, or Bose-Einstein condensates used in gravitometers, accelerometers, etc. Some of the original proposals for quantum sensors involved optical fields. In particular, sensors that exploit continuously variable degrees of freedom have been of interest since the discovery of quantum noise reduction. One of the first examples proposed by Caves is the use quantum noise reduction to achieve interferometric sensitivity in the quantum regime. Advanced LIGO is an example of an upcoming application. In addition to LIGO, in recent years continuous variables have seen renewed interest. In this talk we will discuss quantum sensors and their applications with a focus on the sensors developed at ORNL. We use quantum noise reduction to produce sub-shot-noise limited sensing devices, particularly in quantum plasmonic sensors and displacement sensors using MEMS cantilevers. Some applications for these devices include trace detection or quantum information applications, such as removing bias from QRNGs through adaptive control. We will also discuss other sensing types that use both discrete and continuous variables, such as quantum compressive imaging, and single photon detection applications.

Aug 29, 2014
11:00 a.m. RM 210

Robert Boyd (University of Ottawa)
Menzel's Experiment: Violation of Complementarity?

In 2012, the group of Ralf Menzel in Potsdam, Germany published an article in PNAS that appeared to violate the accepted quantum mechanical notion of complementarity. Specifically, they observed interference with good fringe visibility in a Young's two-slit experiment, even though, through use of a quantum protocol, they were able to deduce through which slit each photon had passed. Our group has recently articulated an explanation for these unexpected results (Bolduc et al., PNAS 2014). Our explanation is that the Potsdam group had inadvertently violated a fair-sampling assumption by means of the manner in which they collected and analyzed their data.

Aug 8, 2014
11:00 a.m.

Ioannis Thanapoulos (National Hellenic Research Foundation)
Quantum dynamics by the Effective Modes Differential Equations method

We show that the non-Markovian quantum dynamics of a system comprised of a subspace Q coupled to a much larger subspace P can be described by a set of Effective Modes Differential Equations (EMDE). The computational efficiency of the method is demonstrated by investigating the 24-mode decay dynamics and laser control of the radiationless transitions from the second to the first singlet electronic excited state of the pyrazine molecule.

Aug 7, 2014
11:00 a.m.

Thomas Monz (University of Innsbruck)
Topological qubits

Arbitrarily long quantum computation requires techniques to overcome errors accumulated during the operation. Here, different approaches have been proposed, with topological quantum computation yielding one of the highest thresholds against errors. In this talk I will first provide a brief introduction into topological quantum computation, in particular the color code. Subsequently I will show how, for the first time, a qubit has been topologically encoded using an ion-trap based quantum computer. The presented experimental data illustrates how we can detect all physical single-qubit errors, perform the entire set of Clifford operations on this logical qubit and investigate its coherence properties. The presentation is concluded by an outline on upcoming milestones and their experimental as well as theoretical challenges.

Aug 7, 2014
2:00 p.m.

Prof. Charlie Ironside (Curtin University)
A surface-patterned chip as a strong source of ultra-cold atoms for quantum technologies

Aug 1, 2014
11:00 am

Prof. Lianao Wu (University of Basque Country)
One Component Dynamical Equation and a Universal Control Theory

We use a Feshbach P-Q partitioning technique to derive a closed one- component integro-differential equation. The resultant equation properly traces the footprint of the target state in quantum control theory. The physical significance of the derived dynamical equation is illustrated by both general analysis and concrete examples. We show that control can be realized by fast-changing external fields, even fast noises. We illustrate the results by quantum memory and controlled adiabatic paths.

July 25, 2014
Room 210

Prof. Gershon Kurizki, Weizmann Institute of Science
A thermal bath: more friend than foe?

Traditionally, the interaction of quantum systems with a thermal bath is viewed as detrimental to their quantumness. Yet this is not always the case, as the bath may actually promote quantumness, particularly when system-bath interactions are subject to control. I will review our recent results concerning different types of control capable of generating or enhancing quantum processes via the bath:
1. Control by modulation: By periodically modulating the energy of two-level or multilevel systems we may purify the state of the systems or the bath they couple to, upon tailoring the modulation to the bath spectrum. An intriguing consequence of such purification is the possibility to cool a bath consisting of coupled spins down to absolute zero, in apparent violation of Nernst's third law of thermodynamics. The thermal bath may also mediate the transfer of quantum information between distant systems, at a rate and fidelity controllable by the modulation.
2. Control by state preparation: The quantum state of an oscillator coupled to a thermalized qubit determines the amount and efficiency of work extractable from the thermal bath, thereby retaining its quantum features over surprisingly long time scales. Remarkably, certain quantum states yield higher efficiency than allowed by the Carnot bound, yet in full compliance with the second law of thermodynamics. In N-level systems, appropriate state preparation allows for N-fold enhancement of work extractable from the bath at steady state.
3. Control by bath engineering: The ability to control the coupling of quantum systems to appropriately designed, axially-guided modes of the bath, may drastically enhance the range of entanglement mediated by the bath, or lead to giant enhancement of bath-induced dispersion forces, colloquially known as van der Waals and Casimir forces.

July 11, 2014
Stewart Library

Matthew Broome, University of New South Wales
My Quantum Optics Show and Tell: Topology, complexity and biology

Progress in optical quantum computation has started to slow in recent times due to the problems associated with probabilistic quantum gates, lack of good single photon sources and poor non-linear optical materials. However, by looking at other applications besides a fully scalable quantum computer, we see that linear optics alone (beam splitters and phase shifters) is a powerful tool for simulation or emulation of interesting physical systems. In this talk I will discuss some recent results from the University of Queensland's Quantum Technology Lab that employ purely linear optical schemes for this purpose. In particular, I will focus the talk around single- and multi-particle quantum walks for investigating areas from condensed matter science to complexity theory.

July 4, 2014
Room 210

Katja Ried, Perimeter Institute, Waterloo
How drug trials are simpler if your subjects are quantum (and other applications of quantum causal models)

A fundamental question in trying to understand the world -- be it classical or quantum -- is why things happen. We seek a causal account of events, and merely noting correlations between them does not provide a satisfactory answer. In classical statistics, a better alternative exists: the framework of causal models has proven useful for studying causal relations in a range of disciplines. We try to adapt this formalism to allow for quantum variables, and in the process discover a new perspective on how causality is different in the quantum world. One of the peculiarities that arise in this context can be harnessed to solve a task of causal inference -- inferring the causal relation between variables based on observed statistics -- that is impossible for classical variables. I will report on a recent experimental realization of this scheme.

Time permitting, I will also discuss a more realistic approach to the problem of characterizing quantum processes in the presence of initial
correlations with an environment, viz non-Markovian dynamics. Another application of quantum causal inference arises in the context of quantum field theory: if one couples two detectors to a quantum field at different points throughout space-time, this may allow one of them to causally influence the other, via the field. We explore how different variables of the model, such as the acceleration of the detectors and the ultraviolet cutoff of the field theory, are reflected in the strength and quality of the causal influence.

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