High-Fidelity Measurement of a Superconducting Qubit using an On-Chip Microwave Photon Counter

TL;DR

This paper presents a high-fidelity, on-chip microwave photon counting method for superconducting qubit measurement, achieving over 98% fidelity in under 500 ns without quantum-limited amplifiers.

Contribution

It introduces a novel measurement protocol using a Josephson Photomultiplier to directly detect cavity photons, enabling fast, high-fidelity qubit readout at millikelvin temperatures.

Findings

Achieved >98% single-shot fidelity

Measurement times under 500 ns

Mitigated backaction and crosstalk

Abstract

We describe an approach to the high-fidelity measurement of a superconducting qubit using an on-chip microwave photon counter. The protocol relies on the transient response of a dispersively coupled measurement resonator to map the state of the qubit to "bright" and "dark" cavity pointer states that are characterized by a large differential photon occupation. Following this mapping, we photodetect the resonator using the Josephson Photomultipler (JPM), which transitions between classically distinguishable flux states when cavity photon occupation exceeds a certain threshold. Our technique provides access to the binary outcome of projective quantum measurement at the millikelvin stage without the need for quantum-limited preamplification and thresholding at room temperature. We achieve raw single-shot measurement fidelity in excess of 98% across multiple samples using this approach in total measurement times under 500 ns. In addition, we show that the backaction and crosstalk associated with our measurement protocol can be mitigated by exploiting the intrinsic damping of the JPM itself.