A coupled two-mode system with balanced gain and loss is a paradigmatic
example of an open quantum system that can exhibit real spectra despite being
described by a non-Hermitian Hamiltonian. We utilize a degenerate parametric
amplifier operating in three-wave mixing mode to realize such a system of
balanced gain and loss between the two quadrature modes of the amplifier. By
examining the time-domain response of the amplifier, we observe a
characteristic transition from real-to-imaginary energy eigenvalues associated
with the Parity-Time-symmetry-breaking transition.Comment: 6 pages, 4 figure