99.9%-fidelity in measuring a superconducting qubit

TL;DR

This paper presents a novel superconducting qubit measurement architecture achieving over 99.9% fidelity in 202 ns, significantly improving measurement accuracy without amplification, crucial for quantum error correction.

Contribution

The authors introduce a new architecture with genuine longitudinal interaction and tailored nonlinearity, reducing errors and enabling high-fidelity, amplification-free quantum state measurement.

Findings

Achieved 99.8% measurement fidelity in 202 ns

Pure measurement fidelity exceeds 99.9% after error correction

Compatible with multiplexing readout and quantum error correction

Abstract

Despite the significant progress in superconducting quantum computation over the past years, quantum state measurement still lags nearly an order of magnitude behind quantum gate operations in speed and fidelity. The main challenge is that the strong coupling and readout signal used to probe the quantum state may also introduce additional channels which may cause qubit state transitions. Here, we design a novel architecture to implement the long-sought longitudinal interaction scheme between qubits and resonators. This architecture not only provides genuine longitudinal interaction by eliminating residual transversal couplings, but also introduces proper nonlinearity to the resonator that can further minimize decay error and measurement-induced excitation error. Our experimental results demonstrate a measurement fidelity of 99.8% in 202 ns without the need for any first-stage amplification. After subtracting the residual preparation errors, the pure measurement fidelity is above 99.9%. Our scheme is compatible with the multiplexing readout scheme and can be used for quantum error correction.