  Nonreciprocal optical systems have found many applications altering the
linear transmission of light as a function of its propagation direction. Here
we consider a new class of nonreciprocity which appears in photon pair
correlations and not in linear transmission. We experimentally demonstrate and
theoretically verify this nonreciprocity in the second-order coherence
functions of photon pairs produced by spontaneous four-wave mixing in a silicon
microdisk. Reversal of the pump propagation direction can result in substantial
extinction of the coherence functions without altering pump transmission.
