We present a simple formalism describing evolution of a qubit in the process
of its measurement in a circuit QED setup. When a phase-sensitive amplifier is
used, the evolution depends on only one output quadrature, and the formalism is
the same as for a broadband setup. When a phase-preserving amplifier is used,
the qubit evolution depends on two output quadratures. In both cases a perfect
monitoring of the qubit state and therefore a perfect quantum feedback is
possible.Comment: 11 pages; Contribution to Proceedings of Les Houches summer school
"Quantum Machines" (2011
