3,242 research outputs found
Non-negative Wigner functions in prime dimensions
According to a classical result due to Hudson, the Wigner function of a pure,
continuous variable quantum state is non-negative if and only if the state is
Gaussian. We have proven an analogous statement for finite-dimensional quantum
systems. In this context, the role of Gaussian states is taken on by stabilizer
states. The general results have been published in [D. Gross, J. Math. Phys.
47, 122107 (2006)]. For the case of systems of odd prime dimension, a greatly
simplified proof can be employed which still exhibits the main ideas. The
present paper gives a self-contained account of these methods.Comment: 5 pages. Special case of a result proved in quant-ph/0602001. The
proof is greatly simplified, making the general case more accessible. To
appear in Appl. Phys. B as part of the proceedings of the 2006 DPG Spring
Meeting (Quantum Optics and Photonics section
Practical characterization of quantum devices without tomography
Quantum tomography is the main method used to assess the quality of quantum
information processing devices, but its complexity presents a major obstacle
for the characterization of even moderately large systems. The number of
experimental settings required to extract complete information about a device
grows exponentially with its size, and so does the running time for processing
the data generated by these experiments. Part of the problem is that tomography
generates much more information than is usually sought. Taking a more targeted
approach, we develop schemes that enable (i) estimating the fidelity of an
experiment to a theoretical ideal description, (ii) learning which description
within a reduced subset best matches the experimental data. Both these
approaches yield a significant reduction in resources compared to tomography.
In particular, we demonstrate that fidelity can be estimated from a number of
simple experimental settings that is independent of the system size, removing
an important roadblock for the experimental study of larger quantum information
processing units.Comment: (v1) 11 pages, 1 table, 4 figures. (v2) See also the closely related
work: arXiv:1104.4695 (v3) method extended to continuous variable systems
(v4) updated to published versio
From Quantum Optics to Quantum Technologies
Quantum optics is the study of the intrinsically quantum properties of light.
During the second part of the 20th century experimental and theoretical
progress developed together; nowadays quantum optics provides a testbed of many
fundamental aspects of quantum mechanics such as coherence and quantum
entanglement. Quantum optics helped trigger, both directly and indirectly, the
birth of quantum technologies, whose aim is to harness non-classical quantum
effects in applications from quantum key distribution to quantum computing.
Quantum light remains at the heart of many of the most promising and
potentially transformative quantum technologies. In this review, we celebrate
the work of Sir Peter Knight and present an overview of the development of
quantum optics and its impact on quantum technologies research. We describe the
core theoretical tools developed to express and study the quantum properties of
light, the key experimental approaches used to control, manipulate and measure
such properties and their application in quantum simulation, and quantum
computing.Comment: 20 pages, 3 figures, Accepted, Prog. Quant. Ele
Real-time Quantum evolution in the Classical approximation and beyond
With the goal in mind of deriving a method to compute quantum corrections for
the real-time evolution in quantum field theory, we analyze the problem from
the perspective of the Wigner function. We argue that this provides the most
natural way to justify and extend the classical approximation. A simple
proposal is presented that can allow to give systematic quantum corrections to
the evolution of expectation values and/or an estimate of the errors committed
when using the classical approximation. The method is applied to the case of a
few degrees of freedom and compared with other methods and with the exact
quantum results. An analysis of the dependence of the numerical effort involved
as a function of the number of variables is given, which allow us to be
optimistic about its applicability in a quantum field theoretical context.Comment: 32 pages, 6 figure
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