Enhancing qubit readout fidelity with two-mode squeezing of the coherent measurement signal

TL;DR

This paper introduces a novel qubit readout method using two-mode squeezing and coherent signal combination to significantly improve fidelity in superconducting qubit measurements, compatible with large-scale quantum processors.

Contribution

It presents a new readout scheme that enhances fidelity by measuring and combining two-mode squeezed states, surpassing traditional amplification methods.

Findings

Enhanced readout fidelity across various amplifier gains.

Compatibility with frequency multiplexed quantum processors.

Effective noise reduction through two-mode squeezing and coherent processing.

Abstract

The ability to perform high-fidelity quantum nondemolition qubit readout is pivotal for the realization of large and powerful quantum computers. Such readout of superconducting qubits is generally enabled by amplifying the weak dispersive measurement signals using phase-preserving quantum-limited Josephson amplifiers with sufficient gain to dilute the contribution of the added noise by the output chain. Here, we further enhance the qubit readout fidelity by (1) simultaneously measuring the two-mode squeezed states of the amplified readout signals at the signal and idler frequencies of the nondegenerate amplifier and (2) coherently combining them at the classical processing stage following a relative rotation that maximizes the signal to noise ratio of the qubit-encoded readout quadrature. Such readout scheme exhibits enhancement in the readout fidelity for all practical values of amplifier gain and noise added by the output chain and is fully compatible with frequency multiplexed setups used in large quantum processors.