Single-shot qubit readout in circuit quantum electrodynamics

Abstract

The ‘transmon’ design for superconducting qubits is particularly promising, owing to the long coherence times that it enables. Now, high-fidelity single-shot readout of such qubits — necessary for operating a quantum processor — has been demonstrated The future development of quantum information using superconducting circuits requires Josephson qubits1 with long coherence times combined with a high-fidelity readout. Significant progress in the control of coherence has recently been achieved using circuit quantum electrodynamics architectures2,3, where the qubit is embedded in a coplanar waveguide resonator, which both provides a well-controlled electromagnetic environment and serves as qubit readout. In particular, a new qubit design, the so-called transmon, yields reproducibly long coherence times4,5. However, a high-fidelity single-shot readout of the transmon, desirable for running simple quantum algorithms or measuring quantum correlations in multi-qubit experiments, is still lacking. Here, we demonstrate a new transmon circuit where the waveguide resonator is turned into a sample-and-hold detector—more specifically, a Josephson bifurcation amplifier6,7—which allows both fast measurement and single-shot discrimination of the qubit states. We report Rabi oscillations with a high visibility of 94%, together with dephasing and relaxation times longer than 0.5 μs. By carrying out two measurements in series, we also demonstrate that this new readout does not induce extra qubit relaxation.