Robust measurement of a qubit encoded in the bosonic mode of a superconducting cavity*

Reliable single-shot readout is a necessary DiVincenzo criterion for quantum computation. The readout fidelity of any two-level system is bounded by the rate of a single error. In contrast, if qubit states are encoded using higher occupation levels of a bosonic mode, a single error does not completely destroy the information: the basis states are still distinguishable. We present experimental results for a high-fidelity readout scheme in which information is encoded in higher photon numbers of a superconducting cavity. By performing repeated QND measurements utilizing an ancillary transmon, we can asymptotically reduce the effects of both photon loss and infidelity of individual measurements. We demonstrate the capability of high-fidelity state readout in our system and discuss limits on the achievable fidelity.