Quantum-to-classical transition enabled by quadrature-PT symmetry

Quantum Langevin noise makes experimental realization of genuine quantum-optical parity-time (PT) symmetry in a gain-loss-coupled open system elusive. Here, we challenge this puzzle by exploiting twin beams produced from a nonlinear parametric process, one undergoing phase-sensitive linear quantum amplification (PSA) and the other engaging balanced loss merely. Unlike all previous studies involving phase-insensitive amplification (PIA), our PSA-loss scheme allows one quadrature pair to experience PT symmetry, a unique quantum effect without any classical counterpart. Such symmetry showcases many radical noise behaviors beyond conventional quantum squeezing and inaccessible to any PIA-based platform. Importantly, it is the only non-Hermitian system hitherto that enables the emergence of non-Hermiticity-induced quantum-to-classical transition for the same quantum observable when crossing exceptional point. Utilizing this quadrature-PT structure, we have further studied its potential in quantum sensing by exploring the quantum Cram\'er-Rao bound or Fisher information. Besides, the proposed quadrature PT symmetry also sheds new light on protecting continuous-variable (CV) qubits from decoherence in lossy transmission, a long-standing conundrum for various CV-based quantum technologies.