[論文レビュー] Enhancing qubit readout fidelity with two-mode squeezing of the coherent measurement signal

Baleegh Abdo, William Shanks+2 more|arXiv (Cornell University)|Mar 16, 2026

Quantum Information and Cryptography被引用数 0

ひとこと要約

The paper demonstrates a readout scheme that boosts qubit readout fidelity by simultaneously measuring two-mode squeezed outputs from a nondegenerate amplifier and coherently combining them to enhance SNR, compatible with frequency-multiplexed setups.

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.

研究の動機と目的

• Motivation to achieve high-fidelity quantum nondemolition (QND) readout for scalable superconducting quantum computers.
• Leverage two-mode squeezing of amplified readout signals to boost readout fidelity without major architectural changes.
• Demonstrate a processing protocol that coherently combines signal and idler outputs to enhance the qubit-encoded quadrature.
• Show compatibility with frequency-m multiplexed readout chains and realistic amplifier noise.
• Provide theoretical and experimental validation across practical amplifier gains and output-chain noises.

提案手法

• Use a nondegenerate Josephson mixer (JM) as the quantum-limited amplifier that outputs two entangled modes at signal and idler frequencies.
• Measure both outputs and rotate/scale the data to refer back to the JM output ports.
• Apply a relative rotation to the idler-channel data and coherently combine the two data sets to form an optimized combined quadrature.
• Quantify readout performance via power SNR R and assignment fidelity F, with analytical expressions for R and F under two-mode squeezing (Eqs. 9–13 in the text).
• Compare two-mode (ab) readout against standard phase-preserving amplification (mode a) across varying JM gains and output noise (N_sys).
• Demonstrate that R_ab,max exceeds R_a in practical regimes (nonzero N_sys and r>~0.35) and that F_ab > F_a under typical conditions.

Figure 1: Harnessing two-mode squeezing of amplified qubit readout signals. a Illustration of a dispersive qubit readout signal amplified by a nondegenerate quantum-limited amplifier (e.g., JM) connected to two output lines. Antisqueeezing in the $I$ ( $Q$ ) quadrature carrying (lacking) the qubit s

実験結果

リサーチクエスチョン

• RQ1 Can two-mode squeezing of the amplified readout signals at the JM signal and idler frequencies improve qubit readout fidelity beyond standard phase-preserving amplification?
• RQ2 How should the two outputs be processed (phase rotation and coherent combination) to maximize SNR and fidelity?
• RQ3 What is the dependence of SNR and fidelity on JM gain and added noise in the output chain, and is the scheme compatible with frequency-multiplexed readout?
• RQ4 Is the improvement robust across practical hardware constraints (e.g., insertion losses, backaction, and nondegenerate signal/idler frequencies)?

主な発見

• Two-mode squeezing readout yields higher SNR and assignment fidelity than standard phase-preserving readout for practical JM gains and output-noise levels.
• The combined mode ab with appropriate phase rotation achieves R_ab,max that scales favorably with squeezing parameter r and outperforms R_a under typical N_sys conditions.
• Assignment fidelity F_ab is generally higher than F_a, with a broad maximum at φ≈180 degrees for antisqueezing of I and φ≈0 for antisqueezing of Q.
• Theoretical expressions (Eqs. 9–13) agree with experimental data on R and F across gains G=1.3–4 dB and input powers, and show potential for >99.5% fidelity at higher JM gains.
• Enhancements are achieved without modifying the quantum chip and are compatible with frequency-multiplexed readout schemes.
• The improvements persist across a wide range of amplifier gains and output-noise values, with potential large gains in fidelity under practical conditions.

Figure 2: Qubit readout fidelity measured using mode a , b and the combined mode ab of the JM for different gains. The following applies to the various gains listed on the left side of the figure. a ( b ) Readout measurement shots in the $IQ$ plane taken with mode a ( b ) corresponding to initializa