  We report an experimental demonstration of anti-parity-time (anti-PT)
symmetric optical four-wave mixing in thermal Rubidium vapor, where the
propagation of two conjugate optical fields in a double-$\Lambda$ scheme is
governed by a non-Hermitian Hamiltonian. We are particularly interested in
studying quantum intensity correlations between the two conjugate fields near
the exceptional point, taking into account loss and accompanied Langevin noise.
Our experimental measurements of classical four-wave mixing gain and the
associated two-mode relative-intensity squeezing are in reasonable agreement
with the theoretical predictions.
