Reconsidering Rapid Qubit Purification by Feedback

Details Reconsidering Rapid Qubit Purification by Feedback Reconsidering Rapid Qubit Purification by Feedback

2006-03-08

arxiv.org/abs/quant-ph/0603062v2

New J. Phys. 8 (2006) 90

Physics

Quantum Physics

11 pages, 3 figures. Final version, accepted for publication in New J. Phys

Scientific paper

10.1088/1367-2630/8/6/090

This paper reconsiders the claimed rapidity of a scheme for the purification of the quantum state of a qubit, proposed recently in Jacobs 2003 Phys. Rev. A67 030301(R). The qubit starts in a completely mixed state, and information is obtained by a continuous measurement. Jacobs' rapid purification protocol uses Hamiltonian feedback control to maximise the average purity of the qubit for a given time, with a factor of two increase in the purification rate over the no-feedback protocol. However, by re-examining the latter approach, we show that it mininises the average time taken for a qubit to reach a given purity. In fact, the average time taken for the no-feedback protocol beats that for Jacobs' protocol by a factor of two. We discuss how this is compatible with Jacobs' result, and the usefulness of the different approaches.

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