This meeting will beheld at the University of St Andrews, Physics and Astronomy department, 1030 to 1700. Attendance is free. For catering purposes, please email Dr Natalia Korolkova nvk@st-andrews.ac.uk by the 16th May 2011.

Abstracts:

Quantum information processing on tensor-network states
Tomoyuki Morimae (Univ. Paris-Est)

Tensor-network representation is a way to efficiently describe exponentially large quantum many-body states in condensed matter physics. In this talk, I will first briefly review how to perform the measurement-based quantum computation on tensor-network states [1]. This is a generalization of the well-known one-way model on the cluster state to other many-body resource states.

Next I will explain the method of "uploading" a quantum state into the virtual space of tensors where quantum computation is performed [2]. I will finally show one recent application of the measurement-based quantum computation on tensor-network states to secure quantum computation, namely the blind quantum computation on the Affleck-Kennedy-Lieb-Tasaki state [3], which has long been studied in condensed matter physics.

[1] D. Gross and J. Eisert, PRL98, 220503 (2007).
[2] TM, PRA83, 042337 (2011).
[3] TM, arXiv: 1009.3486; TM, V. Dunjko, and E. Kashefi, in preparation.

Diagnosing Optical Implementations of Quantum Computing
Flaviu Cipcigan (Edinburgh)

In this talk I will present novel ways of visualising and analysing the raw data (represented by a set of measurements on a prepared quantum state) produced by optical implementations of quantum computing. The main aim is to present the data in an intuitive way, allowing the scientists running the experiment to quickly check the correctness of their implementation even using an incomplete (in the sense of quantum state tomography) set of measurements.

The project was done under the supervision of Dr. Elham Kashefi and Prof. Tony Kennedy (University of Edinburgh) and in collaboration with Ms. Stefanie Barz and Dr. Philip Walther (University of Vienna) and looked at an experimental implementation of Blind Quantum Computation.

Universal Quantum Computation with Continuous-Variable Abelian Anyons
Darran Milne (St Andrews)

We describe how continuous-variable abelian anyons, created on the surface of a continuous-variable analogue of Kitaev's lattice model can be utilized for quantum computation. In particular, we derive protocols for the implementation of quantum gates using topological operations. We find that the topological operations alone are insufficient for universal quantum computation which leads us to study additional non-topological operations such as offline squeezing and single mode measurements. It is shown that these in conjunction with a non-Gaussian element allow for universal quantum computation using continuous-variable abelian anyons.

An hetero-dimensional approach to nonlocality in a microscopic-macroscopic system
Mauro Paternostro (QUB)

Taking issue from a recent spur of work addressing the violation of non -locality and non-local realism under coarse-grained measurements, I will propose a hybrid approach to the assessment of the genuine quantum features of a general system consisting of a microscopic and a macroscopic part. As a benchmark, I will explicitly refer to a bipartite entangled state composed of a single-photon and a multi-photon field. My analysis shows that, under ideal conditions, maximal violation of a Bell-CHSH inequality is achievable regardless of the number of photons in the macroscopic part of the state. The difficulty in observing non-locality when losses and detection inefficiency are included can be overcome using a hybrid entanglement witness that allows to efficiently correct for losses in the few photon regime.

Non-classical Correlations Beyond Entanglement in Continuous Variable Systems
Richard Tatham (St Andrews)

I shall discuss methods for examining non-classical correlations in Gaussian states. I shall introduce a new measure of quantum correlations, the Gaussian ameliorated measurement-induced disturbance, defined by optimizing over all bi-local Gaussian positive operator valued measurements. By evaluating and comparing quantum discord, measurement-induced disturbance, and this new measure on Gaussian states, a better insight into quantum correlations beyond entanglement in continuous variable bipartite systems is provided.

The findings reaffirm the genuinely quantum nature of Gaussian states, and yet reveal that non-Gaussian measurements can play a crucial role for the optimized extraction and potential exploitation of non-classical correlations in Gaussian states.

All non-classical correlations can be activated into distillable entanglement
Gerardo Adesso (Nottingham)

We devise a protocol in which general non-classical multipartite correlations produce a physically relevant effect, leading to the creation of bipartite entanglement. In particular, we show that the relative entropy of quantumness, which measures all non- classical correlations among subsystems of a quantum system, is equivalent to and can be operationally interpreted as the minimum distillable entanglement generated between the system and local ancillae in our protocol. We emphasize the key role of state mixedness in maximizing non-classicality: Mixed entangled states can be arbitrarily more non-classical than separable and pure entangled states.

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