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Reconsidering Rapid Qubit Purification by Feedback

H. M. Wiseman·J. F. Ralph
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Abstract

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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H. M. Wiseman, J. F. Ralph. 2006-05-13. Reconsidering Rapid Qubit Purification by Feedback. https://doi.org/10.1088/1367-2630%2F8%2F6%2F090

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