Bistable squeezing in a multimode circuit-quantum-acoustodynamics system
Phys. Rev. A 114, 043720 – Published 15 October, 2026
DOI: https://doi.org/10.1103/1cbf-c3z1
Abstract
Recent experiments in circuit quantum acoustodynamics (cQAD) have demonstrated on-chip multimode phononic interactions and single-mode phonon squeezing [von Lüpke et al., Nat. Phys. 20, 564 (2024)], yet the generation of bistable squeezing and the redistribution of squeezing across multiple mechanical modes remain largely unexplored. Here we propose a scheme to generate and controllably redistribute bistable phonon squeezing in a cQAD system consisting of a superconducting transmon qubit coupled to two high-overtone bulk acoustic resonator modes. By exploiting the intrinsic Kerr nonlinearity of the qubit under a single-tone microwave drive, we uncover a robust bistability in the steady states of both the qubit and the two phonon modes. We systematically map out distinct stability regimes and show that, within the bistable region, the degree and orientation of squeezing become branch-dependent—a direct manifestation of bistable quantum fluctuations. Remarkably, both the qubit and the second phonon mode can surpass the standard quantum limit, achieving up to 3 dB of steady-state squeezing with experimentally accessible parameters. Moreover, by tuning the mode detunings, the squeezing resources of the qubit can be efficiently redistributed to the two phonon modes, enabling targeted control over phononic squeezing. Our proposal is fully compatible with current cQAD platforms and provides a practical route toward engineering nonclassical states of motion for quantum information processing and sensing, where bistable squeezing offers an additional degree of freedom for noise suppression and quantum-state switching.
