Simple quantum feedback of a solid-state qubit

We propose an experiment on quantum feedback control of a solid-state qubit, which is within the reach of the present-day technology. Similar to an earlier proposal fR. Ruskov and A. N. Korotkov, Phys. Rev. B 66, 041401sRd s2002dg, the feedback loop is used to maintain coherent oscillations in a qubit for an arbitrarily long time; however, this is done in a significantly simpler way, which eases the bandwidth problem. The main idea is to use the quadrature components of the noisy detector current to monitor approximately the phase of qubit oscillations. The price for simplicity is a less-than-ideal operation: the fidelity is limited to about 95%. The feedback loop operation can be experimentally verified by the appearance of a positive in-phase component of the detector current relative to an external oscillating signal used for synchronization.