220 research outputs found
Symmetrizing Evolutions
We introduce quantum procedures for making -invariant the dynamics of
an arbitrary quantum system S, where is a finite group acting on the
space state of S. Several applications of this idea are discussed. In
particular when S is a N-qubit quantum computer interacting with its
environment and the symmetric group of qubit permutations, the
resulting effective dynamics admits noiseless subspaces. Moreover it is shown
that the recently introduced iterated-pulses schemes for reducing decoherence
in quantum computers fit in this general framework. The noise-inducing
component of the Hamiltonian is filtered out by the symmetrization procedure
just due to its transformation properties.Comment: Presentation improved, to appear in Phys. Lett. A. 5 pages LaTeX, no
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Bures metric over thermal state manifolds and quantum criticality
We analyze the Bures metric over the manifold of thermal density matrices for
systems featuring a zero temperature quantum phase transition. We show that the
quantum critical region can be characterized in terms of the temperature
scaling behavior of the metric tensor itself. Furthermore, the analysis of the
metric tensor when both temperature and an external field are varied, allows to
complement the understanding of the phase diagram including cross-over regions
which are not characterized by any singular behavior. These results provide a
further extension of the scope of the metric approach to quantum criticality.Comment: 9 pages, 4 figures, LaTeX problems fixed, references adde
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