High-fidelity readout of bosonic qubits via a dispersively coupled ancilla

High-fidelity qubit measurements play a crucial role in quantum computation, communication, and metrology. In recent experiments, readout fidelity has been improved by performing repeated quantum non-demolition (QND) readouts of a qubit’s state through an ancilla. For a qubit encoded in a two-level system, however, the fidelity of such schemes is limited by the fact that a single error can destroy the information in the qubit. In bosonic systems, this fundamental limit could be overcome by utilizing higher levels such that a single error still leaves states distinguishable. In this talk, we present a robust readout scheme, applicable to bosonic systems with dispersively coupled ancilla, that leverages both repeated QND readouts and higher-level encodings to asymptotically suppress the effects of qubit/cavity relaxation and individual measurement infidelity. We characterize the fidelity of the scheme and show how it may be calculated in terms of known experimental parameters.