Quantum Bayesian approach to circuit QED measurement
Alexander N. Korotkov · 2014
Abstract This chapter introduces a puzzling property of the monitoring of a qubit by a coherent signal traversing a cavity dispersively coupled to the qubit. If the measurement of the signal is done by a fully efficient phase-sensitive amplifier, no noise is added by the amplifier and the backaction of the amplifier on the qubit induces the Bloch vector of the latter to diffuse along a line of longitude. When the measurement of the signal is done by a fully efficient phase-preserving amplifier, extra noise of quantum origin is added by the amplifier owing to the presence of the idler port, but the state of the qubit remains pure, the backaction-induced diffusion of the Bloch vector now occurring on the entire Bloch sphere.