Feedback-stabilization of an arbitrary pure state of a two-level atom - art. no. 003810

Unit-efficiency homodyne detection of the resonance fluorescence of a two-l evel atom collapses the quantum state of the atom to a stochastically movin g point on the Bloch sphere. Recently, Hofmann, Mahler, and Hess [Phys. Rev . A 57, 4877 (1998)] showed that by making part of the coherent driving pro portional to the homodyne photocurrent one can stabilize the state to any p oint on the bottom-half of the sphere. Here we reanalyze their proposal usi ng the technique of stochastic master equations, allowing their results to be generalized in two ways. First, we show that any point on the upper- or lower-half, but not the equator, of the sphere may be stabilized. Second, w e consider nonunit-efficiency detection, and quantify the effectiveness of the feedback by calculating the maximal purity obtainable in any particular direction in Bloch space.