09 November, 2006

John Morton, Oxford University

Quantum computing with molecular electron spins

27 October, 2006

Leo Kouwenhoven, Delft University, Netherlands

Spin Qubits with Quantum Dots

26 October, 2006

Rob Spekkens, University of Cambridge

Quantum coherence: fact or fiction?

A controversy that has arisen many times over in disparate contexts is whether quantum coherences between eigenstates of additively conserved quantities are fact or fiction. I present a pedagogical introduction to the debate in the form of a hypothetical dialogue between proponents from each of the two camps: a factist and a fictionist. A resolution of the debate can be achieved, I argue, by recognizing that quantum states do not only contain information about the intrinsic properties of a system but about its extrinsic properties as well, that is, about its relation to other systems external to it. Specifically, the coherent quantum state of the factist is the appropriate description of the relation of the system to one reference frame, while the incoherent quantum state of the fictionist is the appropriate description of the relation of the system to another, uncorrelated, reference frame. The two views, I conclude, are alternative but equally valid paradigms of description. This conclusion has implications for a variety of conceptual puzzles including whether it is possible to lift superselection rules, whether synchronized clocks and aligned Cartesian frames are a source of entanglement, and whether a single particle can exhibit Bell correlations.

09 August, 2006

Jeremy Metz, Imperial College, London

Quantum Computing with Macroscopic Quantum Jumps

We propose a cluster state generation scheme that requires neither the coherent evolution of product states nor the detection of a single photon. Instead, the preparation of the desired entangled state is heralded by the observation of a classical fluorescence signal [1]. This is achieved by driving the system in such a way that it exhibits macroscopic light and dark periods with an entangled state as the dark state. A basic building block of our scheme is the maximally entangled state of the electronic ground states of two atoms trapped inside an optical cavity. Preparing this state with a fidelity above 0.9 is possible even when the single-atom cooperativity parameter C is as low as 10 and when using a photon detector with an efficiency as low as 0.2.
[1] J. Metz, M. Trupke, and A. Beige, Phys. Rev. Lett. 97, 040503 (2006).

07 August, 2006

Rob Adamson, Centre for Quantum Information & Quantum Control, University of Toronto

Characterization of Multiphoton States

I will discuss our work underway for creating and characterizing arbitrary three-photon polarization states in a single temporal and spatial fiber mode. When generating such states the main source of experimental error is unwanted distinguishability due to the three photons being in slightly different modes. We present a tomography scheme that addresses this issue by including measurements that can be used to determine the distinguishability of the photons, and generates the most complete description possible of the polarization state. For more than two photons a group-theoretic approach is required to determine what information can be extracted from the state. Some information about the state cannot be written as single density matrix element in any basis, but rather represents an average over possible permutations of the photons. I also examine other techniques for describing the state. Several different kinds of Wigner quasi-probability distributions exist for describing finite-dimensional spaces such as polarization. We will review these different distribution functions and look at how our experimentally generated states appear in these various descriptions.

15 June, 2006

Pieter Kok, QuNaT Group

Relativistic aspects of Quantum Information Theory

In this coffee talk I present a technique to describe relativistic effects in quantum information theory. Previous methods used the Wigner Little Group (WLG) formalism, which is instructive in its own right, but slightly restrictive and hard to use. I will show how the results from the WLG formalism can be derived in a straightforward way using ordinary creation and annihiilation operators. Time permitting, I will apply this technique to the problem of communicating continuous quantum variables between two inertial observers with unknown relative velocity.

12 June, 2006

Ed Hinds, Imperial College

Cold Atoms on Atom Chips

02 June, 2006

Peter Zoller, University of Innsbruck

Quantum Information and Condensed Matter Physics with Polar Molecules

01 June, 2006

Peter Zoller, University of Innsbruck

Engineering manybody potentials and many body quantum phases with cold molecules

This will be an informal / discussion-type seminar. As the speaker puts it: We are working on a problem right now involving dipole like 2D interactions. We have interesting ideas on the AMO side to engineer potential interactions. But on the cond-mat side I would be very interested in getting input.

01 June, 2006

Keith Hannabuss, Balliol college

Coffee talk: Models of dissipation and decoherence

The dissipation of energy from a classical system has links with decoherence in a quantum system. Theoretical tools, which were developed to study how dissipative behaviour at the macroscopic scale arises from the time-reversible equations of motion of the constituent molecules, have quantum versions in which decoherence appears naturally. This talk will try to give an introduction to some of the ideas and an outline of the applications.

29 May, 2006

Seth Lloyd, MIT

A theory of quantum gravity based on quantum computation

This talk proposes a theory of quantum gravity based on quantum information processing. In this theory, the geometry of spacetime is a construct, derived from an underlying quantum computation. The theory provides explicit predictions for the back-reaction of the metric to computational `matter,' black-hole evaporation, holography, and quantum cosmology.

23 May, 2006

Sougato Bose, Phillip Hyllus, Elham Kashefi, Barry Sanders and Marcus Silva,

QUOXIC meeting

18 May, 2006

Carlos A. Perez, University of Waterloo

Quantum Cellular Automata and Single Spin Measurement

We propose a method for single spin measurement. Our method uses techniques from the theory of quantum cellular automata to correlate a huge amount of ancillary spins to the one to be measured. It has the distinct advantage of being very efficient, and to a certain extent fault-tolerant. Under ideal conditions, it requires the application of only O(\sqrt[3]{N}) external radio frequency pulses to create a system of N correlated spins. It is also fairly robust against pulse errors, imperfect initial polarization of the ancilla spin system, and does not rely on entanglement. We study the scalability of our scheme through extensive numerical simulation.

12 May, 2006

Theodor W. Haensch, Max Planck Institute, Munich

A passion for precision

08 May, 2006

Vlatko Vedral, University of Leeds

Untangling Entanglement in Phase Transitions

20 February, 2006

Martin Plenio, Imperial College

Entanglement in Many Body Systems