Coherence of a Josephson phase qubit under partial-collapse measurement

We discuss quantum evolution of a decaying state in relation to a recent experiment of Katz Based on exact analytical and numerical solutions of a simple model, we identify a regime where the qubit retains coherence over a finite time interval independently of the rates of the three competing decoherence processes. In this regime, the quantum decay process can be continuously monitored via a weak measurement without affecting the qubit coherence.