Programme

11.00 Robust Quantum Memory in the Solid State
Brendon Lovett, Heriot-Watt University

I will first discuss an experiment showing that is it possible to extend the coherence time of an electron spin qubit, in the solid state, to over three seconds [1]. This is done by transferring the quantum information from the electron to a nearby nuclear spin, before retrieving it for readout. I will also show that the coherence limiting process here is relaxation of the electron when the information is imprinted on the nucleus. In order to overcome this, I will go on to discuss how to remove the electron spin optically during the storage phase, and present experimental evidence that this is possible in a fullerene derivative system [2]. This could allow for even more robust quantum memories, and even optically generated nuclear spin entanglement.

[1] J. J. L. Morton, A. M. Tyryshkin, R. M. Brown, S. Shankar, B. W. Lovett, A. Ardavan, T. Schenkel, E. E. Haller, J. W. Ager and S. A. Lyon, Nature 455 1085 (2008)

[2] M. Schaffry, V. Filidou, S. D. Karlen, E. M. Gauger, S. C. Benjamin, H. L. Anderson, A. Ardavan, G. A. D. Briggs, K. Maeda, K. B. Henbest, F. Giustino, J. J. L. Morton, and B. W. Lovett, Phys. Rev. Lett. 104 200501 (2010)

11.45 Heisenberg groups in quantum information and communication
Manas Patra, University of York

Heisenberg groups, also known as Weyl-Heisenberg-Gabor groups, which play an important role in communication theory, phase space methods (in quantum mechanics) and harmonic analysis, among others, have received less-than-deserved attention. In this talk I will try to explain this underlying theme in these different areas. Our starting point will be "phase-space" representation of finite quantum systems and classical signals. The group algebra of Heisenberg groups play a crucial role in these representations. Some important operations on these systems like Fourier transform and Radon transform correspond to automorphisms on the group. This provides us with group theoretic tools for analyzing the former. I will then discuss some possible applications in quantum computing and information.

12.30 Lunch

13.30 Branching Bisimulation and Congruence for Quantum Processes
Tim Davidson, University of Warwick

The use of formal methods in classical computer science has been recognised as a very successful approach for discovering flaws and proving the correctness of systems. Process calculus is one area of formal methods, which focuses on the analysis of concurrent and communicating systems. An important concept in process calculus is the notion of bisimulation, which can be used to identify systems that produce the same behaviour. A standard approach to verification using process calculus is to show that a system model is bisimilar to a high-level specification process.

Communicating Quantum Processes (CQP) is a quantum process calculus that is based on the pi-calculus and includes primitives for quantum information. We describe and motivate the modelling of external interactions in CQP, and we present a notion of equivalence, namely probabilistic branching bisimilarity, for quantum processes. Congruence is an important and highly desirable property of an equivalence relation that provides the foundation for equational reasoning. Previous work on congruence relations for quantum processes excluded the classical information arising from measurements, and was therefore unable to analyse many interesting known quantum communication protocols. Developing a congruence relation for full quantum process calculus is difficult because of the interaction between quantum measurement, entanglement, and parallel composition. We show that, using our modelling approach, full probabilistic branching bisimilarity is a congruence. As an application, we show that quantum teleportation is equivalent, in all contexts, to a simple specification process.

14.15 Gaussian secrecy distribution by non-secret correlations and bound information
Ladislav Mista, Palacky University, Czech Republic

We show that Gaussian entanglement can be distributed between two systems with infinitely- dimensional Hilbert state space (CV systems) by sending a third separable CV system between them [1] in analogy with the case of two-level systems [2]. This phenomenon is possible owing to existence of tripartite states that are separable with respect to two bipartitions. These states are bound entangled as no entanglement can be distilled between any two parties by local operations and classical communication nevertheless the state cannot be prepared by these operations at the same time. Based on the known relationship between entanglement and secret correlations one can derive a classical analog of the quantum protocol, that is, a protocol for distribution of Gaussian classical secret correlations by non-secret correlations being a continuous variable analog of the protocol for discrete random variables [3]. This classical protocol relies on tripartite Gaussian distribution that can be prepared by local operations and public communication with respect to two bipartitions and therefore no secret key can be distilled between any two parties but contains secret correlations across the third bipartition and therefore cannot be prepared by these operations. The distribution is thus an example of what is known as multipartite bound information [4] in Gaussian scenario which is a classical analog of multipartite Gaussian bound entanglement.

[1] L. Mista, Jr. and N. Korolkova, Phys. Rev. A 80, 032310 (2009)

[2] T. S. Cubitt et al., Phys. Rev. Lett. 91, 037902 (2003)

[3] J. Bae et al., Phys. Rev. A 79, 032304 (2009)

[4] A. Acin et al., Phys. Rev. Lett. 92, 107903 (2004)

15.00 Break

15.30 Classical simulation of quantum computing
Flaviu Cipcigan, University of Edinburgh

The topic of my presentation will be the classical simulation of quantum computing, both in the circuit model and the measurement based model. I will provide a review of notions and algorithms for classical simulation in both models, and show that there is a need for both new notions and new algorithms in order to account for the classical simulation of matchgate circuits -- a result well known for the circuit model, which this summer we have extended for the measurement based model.

16.15 Automatic parallelisation of quantum circuits using the measurement based quantum computing model
Einar Pius, University of Edinburgh

In [1] a method for parallelising quantum circuits via applying optimising rewrite rules to the circuits representation in the Measurement Based Quantum Computing (MBQC) model was introduced. We wrote an application that uses this method to parallelise quantum circuits. A brief description of the parallelisation process and its implementation is given. We demonstrate what the implemented method does to some of the Quantum Circuits, known from the literature, and show some less known circuits that can be parallelised using this method.

[1] Anne Broadbent and Elham Kashefi. Parallelizing quantum circuits. Theoretical Computer Science, 410(26):2489-2510, 2009.

17.00 End

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