Quantum nonlinear magnonics: Magnon squeezing in the quantum regime

Squeezed states – quantum states with reduced fluctuations in one quadrature and amplified noise in its conjugate counterpart – serve as foundational resources for quantum-enhanced metrology and nonclassical state engineering [1]. These states can offer enhanced measurement sensitivities beyond the standard quantum limit in, e.g., gravitational-wave detectors [2] and axion dark matter searches [3], while also enabling advances in continuous-variable quantum computing [4, 5] and quantum gravity tests [6]. Here we report the first experimental observation of quantum-level magnon squeezing in a millimeter-scale yttrium iron garnet (YIG) sphere. By engineering a strong dispersive magnon-superconducting qubit coupling via a microwave cavity, we implement a significant self-Kerr nonlinearity to generate squeezed magnon states with their mean magnon number less than one. Harnessing a magnon-assisted Raman process, we perform Wigner tomography, revealing quadrature variances of ∼ 0.8 (∼ 1.0 dB squeezing) relative to the vacuum. The squeezed state exhibits a 400 ns lifetime, surpassing the intrinsic 145 ns magnon coherence time. These novel results lay the groundwork for quantum nonlinear magnonics and promise potential applications in quantum metrology.