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SOCS CQIL - Cryptography and Quantum Information Seminar Schedule

Date Category Seminar Info
2012/12/06 CQIL - Cryptography and Quantum Information Place: McConnell 320
Time: 10:00 - 11:00
Speaker: Winton Brown
Affiliation: University of Sherbrooke
Title: Scrambling speed of random quantum circuits

Random transformations are typically good at "scrambling" information. Specifically, in the quantum setting, scrambling refers to the process of mapping most initial pure product states under a unitary transformation to states which are macroscopically entangled, in the sense of being close to completely mixed on all or most subsystems containing a fraction fn of all n particles for some constant f. While the term scrambling is used in the context of the black hole information paradox, scrambling is related to problems involving decoupling in general, and to the question of how large isolated many-body systems reach local thermal equilibrium under their own unitary dynamics. I will discuss the speed at which various notions of scrambling/decoupling occur in a simplified but natural model of random two-particle interactions: random quantum circuits. For a circuit representing the dynamics generated by a local Hamiltonian, the depth of the circuit corresponds to time. We resolve a conjecture raised in the context of the black hole information paradox with respect to the depth at which a typical quantum circuit generates an entanglement assisted encoding against the erasure channel. In addition, we prove that typical quantum circuits of poly(log n) depth satisfy a stronger notion of scrambling and can be used to encode r1 n qubits into n qubits so that up to r2 n errors can be corrected, for finite rates r1 and r2 > 0.

2012/11/13 CQIL - Cryptography and Quantum Information Place: McConnell 320
Time: 14:30 - 15:30
Speaker: Nicolas Menicucci
Affiliation: University of Sydney
Area: Quantum information
Title: Optical continuous-variable cluster states

I will describe the theoretical underpinnings of one-way quantum computation using continuous-variable systems, as well as the pros and cons of several different methods of experimental implementation using lasers. Issues related to error correction and fault tolerance -- many of which remain open problems -- will also be discussed.

2012/10/18 CQIL - Cryptography and Quantum Information Place: McConnell 320
Time: 16:00 - 17:00
Speaker: Grant Salton
Affiliation: McGill University
Title: Measuring distance by harvesting entanglement

I will briefly introduce key topics in the field of relativistic quantum information, focussing mainly on the process by which entanglement can be 'harvested' from a quantum field. I will then show that entanglement harvested from a quantum field by interaction with local detectors undergoing anti-parallel acceleration can be used to measure the distance of closest approach between the two detectors. Information about the separation is stored nonlocally in the phase of the joint state of the detectors after the interaction; a single detector alone contains none. Although each detector alone sees the same thermal spectrum (due to Unruh radiation), the joint state between them may be entangled. In the vicinity of a critical distance of closest approach between the detectors, the phase of the entangled state depends sensitively on the distance. We will contrast this with the case of parallel acceleration, in which no such critical distance exists, and we will discuss the connection of this case with entanglement harvested from an expanding universe.

2012/10/04 CQIL - Cryptography and Quantum Information Place: McConnell 320
Time: 16:00 - 17:00
Speaker: Alex May
Affiliation: McGill University
Title: Summoning Information in Spacetime, or Where and When Can a Qubit Be?

If information is localized near a point, it should be possible in principle to quickly exhibit that information nearby. Building on this notion of localization, we characterize the propagation of quantum information through spacetime by giving a simple description of all the situations in which that information can be summoned to a set of spacetime points. The answer depends only the various causal relationships between the points at which the challenges occur and the points at which the information must be produced. In general, whenever summoning is possible, it can be achieved using a combination of quantum error correction and teleportation.

2012/09/20 CQIL - Cryptography and Quantum Information Place: McConnell 320
Time: 16:00 - 17:00
Speaker: Olivier Landon-Cardinal
Affiliation: University of Sherbrooke
Title: Local topological order inhibits thermal stability

We put severe constraints on the existence of a self-correcting quantum memory made of a two-dimensional (2D) array of particles. Such a memory would passively protect the encoded information thanks to its dynamics at low temperature. To be robust to perturbation, candidates for such devices encode information in topological degrees of freedom, which are impervious to local errors on a short timescale. However, we show that, for any topologically ordered 2D local commuting projector code, thermal excitations can accumulate and corrupt the encoded information. We thus prove a no-go theorem, extending the known results to non-stabilizer codes.

2012/09/12 CQIL - Cryptography and Quantum Information Place: MC103
Time: 13:30 - 14:30
Speaker: Tobias Fritz
Affiliation: Perimeter Institute
Area: information theory and mathematics
Title: Entropy as a functor

Shannon entropy is of one the most useful concepts in applied mathematics. I will explain how it can be regarded as a functor on the category of finite probability spaces, and how this can be turned into a characterization of Shannon entropy. This is joint work with John Baez and Tom Leinster.

Biography of Speaker:

Tobias Fritz received his doctorate from the Max Planck Institute of Mathematics, Bonn, Germany, has been a post-doctoral fellow at the Institute of Photonic Sciences, Barcelona, Spain, and has now joined the Perimeter Institute for Theoretical Physics in Waterloo, Canada.