Nonclassical phonon pair

Quantum-correlated photon pairs are crucial resources for modern quantum information science. Similarly, the reliable generation of nonclassical phonon pairs is vital for advancing engineerable solid-state quantum devices and hybrid quantum networks based on phonons. Here, we present a novel approach to generate quantum-correlated phonon pairs in a suspended silicon microstructure initialized in its motional ground state. By simultaneously implementing red- and blue-detuned laser pulses, equivalent high-order optomechanical nonlinearity–specifically, an effective optomechanical four-wave mixing process–is achieved for generating a nonclassical phonon pair, which is then read out via a subsequent red-detuned pulse. We demonstrate the nonclassical nature of the generated phonon pair through the violation of the Cauchy–Schwarz inequality. Our experimentally observed phonon pair violates the classical bound by more than 5 standard deviations and maintains a decoherence time of 132ns. This work reveals novel quantum manipulation of phonon states enabled by equivalent high-order optomechanical nonlinearity within a pulse scheme and provides a valuable quantum resource for mechanical quantum computing.