Realizing a Catch-Disperse-Release read-out of a qubit

Fast read-out is an essential piece of measurement based error correction codes. The usual technique of driving a dispersively coupled resonator presents some limitations such as finite reset time. To overcome these limits Sete and al. [1] proposed a catch, disperse and release scheme that we recently realized.

It uses a resonator with a tunable coupling to the transmission line. That resonator is coupled to the qubit in the dispersive coupling limit. First, we do brief unconditional coherent displacement of the resonator. Then the phase of the stored coherent state grows linearly in time at a rate depending on the state of the qubit. Finally, we release the resonator's state into the transmission line and measure the phase of the outgoing signal.
Our experiment implements that scheme by using a Josephson Parametric Converter as a tunable coupler between a low and a high Q factor resonator [2] to measure a transmon qubit in CPW geometry.

We demonstrate a state-of-the-art read-out with a fidelity of 97.5% in a total of 240 ns. The fidelity is limited by the qubit lifetime. We demonstrate the quantum non-demolition, reset, and Purcell protection granted by this scheme.

[1] : Sete and al., PRL 2013
[2] : Flurin and al., PRL 2015