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 132 ns. 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.