  While gain-loss-coupled photonic platforms have achieved significant success
in studying classical parity-time (PT) symmetry, they encounter challenges in
demonstrating pure quantum effects due to incompatible operator transformations
and Langevin noise. Here, we present compelling evidence that a non-Hermitian
(NH) twin-beam system, undergoing phase-sensitive amplification (PSA) and
balanced loss, not only enables observing the usual eigenvalue-associated PT
phase transition but also exhibits distinctive features absent in classical NH
or Hermitian quantum scenarios, encompassing quadrature PT symmetry, anomalous
loss-induced quadrature squeezing, and dynamical and stationary
classical-to-quantum transitions in noise fluctuations. Furthermore, our
proposed bipartite open system promises optimal sensing, showcasing an improved
signal-to-noise ratio and sensitivity, constrained by quantum Cram\'{e}r-Rao
bound or Fisher information. These findings deepen the comprehension of
authentic quantum optical PT symmetry involving both gain and loss, addressing
contentious issues and illuminating new facets of the subject.
