April 18, 2014
Toronto Quantum Information Seminars
at the Fields Institute,
Fridays, 11:10 a.m.


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

April 14
Stewart Library


Dr. Ben Fortescue, (Southern Illinois University)
Tolerating Qubit Loss in Quantum Error Correction

Standard fault-tolerant quantum error correction (QEC) schemes can protect quantum information against arbitrary errors on individual qubits, but with the implicit assumption that the underlying physical systems remain within the qubit subspace. Fault-tolerance may therefore be lost under "leakage" errors (such as a photonic qubit being absorbed) which violate this assumption, and techniques for modifying QEC to cope with generic leakage can be very costly in terms of the extra operations required. I will discuss a more specific "loss" model of leakage, applicable to many implementations (photonic ones especially), and show that it allows for fault-tolerance to be analysed and achieved at much less cost in common Calderbank-Shor-Steane (CSS) codes.

April 25
Room 210
Prof. Marko Loncar, Harvard University
May 2,2014
11:00 a.m.
Dr. Raul Garcia (Max Plank Institute)
May 7, 2014
Room 210
Prof. Sven Höfling (University of St Andrews)

May 9, 2014
Room 210

Prof. Norbert Lütkenhaus (University of Waterloo)
May 16, 2014
Room 210

Prof. Mile Gu (Tsinghua University)
Discord as a Consumable Resource

Correlations lie at the heart of our capacity to manipulate information. The fewer the constraints on the correlations we can exploit, the greater our capacity to manipulate information in ways we desire. The rapid development of quantum information science is a testament to this observation. Quantum systems may be so correlated that they are `entangled', such that each of its subsystems possesses no local reality. Exploitation of such uniquely quantum correlations has led to many remarkable protocols that would otherwise be either impossible or in feasible. However, the absence of entanglement does not eliminate all signatures of quantum behaviour. Coherent quantum interactions between separable systems that result in negligible entanglement could still lead to exponential speed-ups in computation. The potential presence of discord within such protocols motivated speculation that discord might prove a better quantifier of the `quantum resource' that coherent interactions exploit to deliver a `quantum advantage'.

In this presentation, I will give a brief tutorial of quantum discord. I then introduce and demonstrate an operational method to use discord as a physical resource. I show that under certain measurement constraints, discord between bipartite systems can be consumed to encode information that can only be accessed by coherent quantum interactions. The inability to access this information by any other means allows us to use discord to directly quantify this `quantum advantage'. I will outline recent experiments done at the Australian National University and the University of Queensland, where we experimentally encoded information within the discordant correlations separable states. The amount of extra information recovered by coherent interaction is quantified and directly linked with the discord consumed during encoding. I survey the potential applications of this phenomena, in both certification of entangling operations, and protecting the benefits of entanglement in entanglement breaking noise.

July 25
Room 210
Prof. Gershon Kurizki, Weizmann Institute of Science
Past Seminars

April 4
Room 210


Angela Sestito, Università della Calabria, Italy
Understanding Quantum Mechanics: a formal analysis of non-locality theorems

Per se, quantum theory entails no violation of the locality principle; conflicts between them arise if further conditions, which do not
belong to the genuine set of quantum postulates, are required to hold. In this talk we explore consequences of adopting the criterion of reality (R) stated by Einstein, Podolsky and Rosen in 1935. In
particular, we will analyze two possible interpretations of (R) and implications that follow if we adopt them in connection with the nonlocality theorem of Hardy: the contradiction - arising if a "wide" interpretation is adopted- does not arise in connection with a "strict" interpretation of (R). We conclude that if the strict interpretation of (R) is adopted the theorem fails in proving the inconsistency between quantum mechanics and locality. Finally, we propose an ideal experiment enabling the simultaneous assignment of the objective values of two incompatible properties of a system made up of two separated non-interacting spin particles when the strict interpretation of (R) is adopted.

11:10 a.m.
Room 210
Jianming Cai, Ulm University
Nano-scale quantum sensing with color centers in diamond

Color centers are atomic defects in diamond that possess electronic and nuclear spins. The rapid progress of experiments with color centers in diamond indicates that they are promising systems for quantum information processing, and more important for quantum sensing (imaging) under ambient conditions.
We have devised strategies to achieve highly sensitive measurement of weak signals, such as magnetic field, electric field, pressure, and temperature in the presence of ambient noise, while achieving nanometer spatial resolution. We are exploring various applications of these strategies ranging from quantum information processing, fundamental physics, to material science as well as biology including the structure of proteins and the possible role of quantum effects in biological functions.

Mar. 7
Stewart Library

Prof. Gerd Leuchs, Max Planck Institute for the Science of Light, Erlangen, Germany
Distributing entanglement via separable states

Whether or not entanglement is necessary pre-­requisite for quantum information protocols had been debated ever since the first experiments on NMR quantum computing, which were performed successfully with separable systems. There were several hints that entanglement is sufficient in such applications but that it is not absolutely necessary. The first was that the entanglement in a Werner state [1] vanishes discontinuously as the state is gradually tuned towards more mixedness. Yu and Eberly [2] found another example of a discontinuous disappearance of entanglement. In a seminal paper Ollivier and Zurek [3] introduced quantum discord as a measure for quantum correlations, which does not show such discontinuities. Quantum discord can be seen as a measure for the entanglement, which can be extracted from a separable system in a mixed state. Along this line, three experiments recently demonstrated distributing entanglement with separable states [4,5,6].

1. R.F. Werner, Phys. Rev. A 40, 4277 (1989)
2. H. Ollivier, W.H. Zurek, Phys. Rev. Lett. 88, 017901 (2002) 3. T. Yu and J.H. Eberly, Phys. Rev. Lett. 93, 140404 (2004) 4. A. Fedrizzi et al., Phys. Rev. Lett. 111, 230504 (2013) 5. C.E. Vollmer et al., Phys. Rev. Lett. 111, 230505 (2013) 6. Ch. Peuntinger et al., Phys. Rev. Lett. 111, 230506 (2013)


Feb. 28
Room 230

Prof. Guifre Vidal, Perimeter Institute
Tensor Networks for Quantum Many-body Systems

Quantum many-body systems are hard to study because the associated Hilbert space, containing all possible many-body states, is huge: its dimension grows exponentially in the system size. In recent years, however, progress in our understanding of quantum entanglement has revealed that a large class of many-body states of interest are highly atypical and such that we can actually efficiently represent them with a mathematical structure called tensor network. As a result, it is now possible to accurately simulate, say, a quantum spin chain made of thousands of interacting spins. In this Colloquium I will (i) review the computational challenge posed by quantum many-body systems; (ii) summarize our current understanding of many-body entanglement; and (iii) give a gentle introduction to tensor networks as an efficient description of many-body states.

Jan. 31
Room 210

Prof. Arjendu Pattanayak, Carleton College
Surprises in the quantum-classical transition for computed Lyapunov exponents

Completely classical behavior is very different from completely quantum-mechanical behavior, particularly for nonlinear or chaotic systems, even though the transition between the two happens as a function of controllable parameters, such as the size of the system or environmental effects. I report on recent work exploring this multi-parameter transition. The first set of results is on the behavior of calculated quantum Lyapunov exponents Lambda for a Duffing oscillator system as a function of effective action beta as well as the system damping parameter Gamma. In general Lambdas decrease as beta increases (chaos decreases as the system becomes more quantal, as expected). However, we identify anomalous regions where Lambdas
increase with beta, including going from negative to positive with increasing beta; and also regions where the quantum results do not tend smoothly to the classical results. All anomalous results correspond to windows of regularity embedded in a larger chaotic parameter regime, which inverts the usual paradigm: The classically regular behavior is the most challenging for quantum-classical correspondence. I also report on progress on various other projects including (a) studying how classical control algorithms work in controlling quantum systems across this parameter regime and (b) on the control of the quantum chaos.

Nov. 22
Room 210

Dr. Pascale Senellart, CNRS
Cavity quantum electrodynamics with semiconductor quantum dots

A semiconductor quantum dot is a promising system to develop a solid-state quantum network. Like real atoms, quantum dots can emit single photons, polarization entangled photon pairs, indistinguishable photons... Moreover, the spin of a carrier trapped in a quantum dot can present long coherence times and be used as a stationary quantum bit that one can optically manipulate and measure. However, the scalability of a quantum dot based quantum network requires implementing a highly efficient single photon-single quantum dot interface so as to collect every photon emitted by a quantum dot and symmetrically, to ensure that every photon sent onto the device interacts with the quantum dot. Controlling the spontaneous emission of a quantum dot in a cavity is an efficient way to build such an interface. In this talk, we will present our recent results along this research line.
We have developed an in-situ lithography technique to deterministically insert a single quantum dot into a pillar optical microcavity. In the light-matter weak coupling regime, we obtain ultrabright sources of quantum light. We demonstrate sources of indistinguishable single photons with brightness as large as 79 % collected photon per pulse. With coupled pillar cavities, we also fabricate bright sources of polarization entangled photon pairs. The potential of these sources for quantum information processing is demonstrated by implementing an entangling controlled-NOT gate. In the light matter strong coupling regime, we demonstrate optical non-linearities for only 8 incident photons per pulse. Finally, we present a novel photonic structure and a technology allowing the electrical control of the devices, a critical step for the scalability of a quantum network based on semiconductor quantum dots.

Nov 04, 12:00 PM
60 St. George Street, MP 606
Robin Côté, University of Connecticut
Quantum information with ultracold Rydberg atoms and molecules

Oct. 4
Room 210

Dr. Nicolas Menicucci (The University of Sydney)
A gigantic, 10,000-mode, continuous-variable cluster state

Cluster states are an entangled resource state that enable quantum computing using adaptive measurements alone. This is surprising when one considers what this means: one can quantum compute simply by *looking* at a quantum systems in a particular way! The continuous-variable incarnations of these states are simple to make using lasers and can be scaled up with ease. In this talk, I will describe the theoretical underpinnings of measurement-based quantum computation using continuous-variable systems, and I will report on their experimental realization, including the recent demonstration of a 10,000-mode (!) cluster state. This is the largest entangled state ever created to date in which every constituent quantum system is individually addressable. Issues related to error correction and fault tolerance -- many of which remain open problems -- will also be discussed.


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