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Quantum Langevin noise makes experimental realization of genuinequantum-optical parity-time (PT) symmetry in a gain-loss-coupled open systemelusive. Here, we challenge this puzzle by exploiting twin beams produced froma nonlinear parametric process, one undergoing phase-sensitive linear quantumamplification (PSA) and the other engaging balanced loss merely. Unlike allprevious studies involving phase-insensitive amplification (PIA), our PSA-lossscheme allows one quadrature pair to experience PT symmetry, a unique quantumeffect without any classical counterpart. Such symmetry showcases many radicalnoise behaviors beyond conventional quantum squeezing and inaccessible to anyPIA-based platform. Importantly, it is the only non-Hermitian system hithertothat enables the emergence of non-Hermiticity-induced quantum-to-classicaltransition for the same quantum observable when crossing exceptional point.Utilizing this quadrature-PT structure, we have further studied its potentialin quantum sensing by exploring the quantum Cram\'er-Rao bound or Fisherinformation. Besides, the proposed quadrature PT symmetry also sheds new lighton protecting continuous-variable (CV) qubits from decoherence in lossytransmission, a long-standing conundrum for various CV-based quantumtechnologies.

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