High-Fidelity Qubit Readout Using Interferometric Directional Josephson Devices

Abstract

Nonreciprocal microwave devices, such as circulators and isolators, are needed in high-fidelity qubit readout schemes to unidirectionally route the readout signals and protect the qubits against noise coming from the output chain. However, cryogenic circulators and isolators are prohibitive in scalable superconducting architectures because they rely on magnetic materials. Here we report a fast (750ns) high-fidelity (95%) quantum nondemolition readout of a coherent superconducting qubit (T1=52μs, T2E=35μs) without any nonreciprocal magnetic devices. We use in our readout chain a microwave-controlled qubit readout multichip module (QRMCM) that integrates interferometric directional Josephson devices consisting of an isolator and a reconfigurable isolator or amplifier device, and an off-chip low-pass filter. Using the QRMCM, we demonstrate isolation up to 45dB within 13MHz, when both directional devices are operated as isolators, and low-noise amplification in excess of 10dB within a dynamical bandwidth of 10MHz, when the reconfigurable device is operated as an amplifier. We also investigate the dependence of the qubit coherence times Tφ and T2E on the isolation response of the QRMCM, which we control in situ using the microwave drives feeding the isolators. Furthermore, by directly comparing the QRMCM performance with that of a state-of-art configuration (with T2E≈2T1) that uses a pair of wideband magnetic isolators, we find that the excess pure dephasing measured with the QRMCM (for which T2E≈T1) is likely limited by the residual thermal photon population in the readout resonator. Improved versions of the QRMCM could replace magnetic circulators and isolators in large superconducting quantum processors.

Similar papers