High-visibility single-shot readout for superconducting qubits in cavities
The future development of quantum information using superconducting circuits requires Josephson qubits with long coherence times and a high-fidelity quantum state readout. Major progress in the control of coherence has recently been achieved with a new circuit design where a transmon qubit is measured by a coplanar waveguide resonator (CPWR), yielding reproducibly long coherence times. Indeed, the transmon is insensitive to major decoherence sources in superconducting qubits, while the CPWR provides a well-controlled electromagnetic environment. However, a high-fidelity single-shot readout of the transmon quantum state, highly desirable for running simple quantum algorithms or measuring quantum correlations in experiments with a few qubits, is still lacking. Indeed, the readout method based on the qubit-state-dependent phase shift of a microwave pulse transmitted through (or reflected by) the resonator, lacks the required signal-to-noise to discriminate the two qubit states in a time shorter than the qubit relaxation time. To overcome this limitation we use instead a sample-and-hold detector that allows fast measurement and single-shot discrimination. This detector is a non-linear CPWR\ operated as a Josephson Bifurcation Amplifier (JBA), capacitively coupled to the qubit. We report Rabi oscillations with high visibility (94%) together with long coherence times, above 1s. By performing two successive measurements, we also demonstrate that the readout process does not induce extra qubit relaxation.