Josephson parametric phase-locked oscillator: application to dispersive readout of superconducting qubits*

We present a new qubit readout scheme using a Josephson parametric phase-locked oscillator. The parametric oscillator is the same circuit as the flux-driven parametric amplifier used in Refs. 1 and 2, but is operated at the pump power above the oscillation threshold. The oscillator works as a sensitive binary phase detector and discriminates the dispersive phase shifts in the probe microwave field reflected from a resonator coupled to a qubit. The scheme offers fast and latching-type readout, but requires only a small number of photons in the resonator. Using this scheme, we achieved high-fidelity single-shot readout of a flux qubit with more than 90$\%$ contrast of Rabi oscillations. [1] T. Yamamoto {\it et al.}, APL {\bf 93}, 042510 (2008). [2] Z. R. Lin {\it et al.}, APL {\bf 103}, 132602 (2013).