  Mediated by the interaction with quantum vacuum fields, a laser field
propagating in a nonlinear optical medium can generate new light fields via
spontaneous parametric process. Such process is inherent independent of the
propagation direction of light and reciprocal thus far, due to the
direction-independent field-vacuum interaction. In this work, we experimentally
demonstrate a nonreciprocal spontaneous parametric four-wave mixing process in
sodium atomic vapors with dispersive nonlinearity and further broadband optical
isolation by unidirectionally coupling the probe field to an auxiliary quantum
vacuum field in another four-wave mixing process. Thanks to the broad bandwidth
of the spontaneous parametric process, in combination with the Doppler and
power-induced broadening of atomic energy levels, we achieve optical isolation
with a bandwidth larger than 100 GHz for isolation ratio >25 dB. Considering
that both spontaneous parametric processes and wave mixing in nonlinear medium
have been realized in diverse on-chip photonic platforms, our work paves the
way for integrated broadband optical isolations and thus can boost scalability
and function of photonic chips.
