Hamburg 2009 – scientific programme

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Q: Fachverband Quantenoptik und Photonik

Q 30: Poster II

Q 30.29: Poster

Tuesday, March 3, 2009, 16:30–19:00, VMP 9 Poster

True Quantum Dephasing — •Julius Helm and Walter T. Strunz — Institut für Theoretische Physik, TU Dresden, 01062 Dresden

Doubly Stochastic channels (channels that are trace preserving and unital -- i.e., mapping the identity onto itself) can be distinct into two different classes. On the one hand there are the random unitary processes, which may always be written as a convex sum of unitary transformations, also known as random external fields [1]. On the other hand, and in contrast to the classical analogue, there exist doubly stochastic channels that do not allow for this representation [2]. In the context of quantum error correction this distinction obtains some practical significance, for it was shown that errors can be fully corrected if and only if they belong to the class of random unitary channels [3].

A simple yet crucial example of doubly stochastic channels are dephasing channels, characterized by their diagonal Kraus representation (according to a fixed basis). We study the notion of random unitarity in this context. Here we are able to give physical examples of "true quantum" dephasing channels, that is, channels that do not belong to the class of random unitary channels.

[1] R. Alicki and K. Lendi, "Quantum Dynamical Semigroups and Applications", Springer (1987)

[2] L. Landau and R.F. Streater, Lin. Alg. Appl. 193, 107 (1993)

[3] M. Gregoratti and R.F. Werner, J. Mod. Opt. 50, 915 (2002)