Nonreciprocal Frequency Domain Beam Splitter

The canonical beam splitter-a fundamental building block of quantum optical systems-is a reciprocal element. It operates on forward- and backward-propagating modes in the same way, regardless of direction. The concept of nonreciprocal quantum photonic operations, by contrast, could be used to transform quantum states in a momentum- and direction-selective fashion. Here we demonstrate the basis for such a nonreciprocal transformation in the frequency domain through intermodal Bragg scattering four-wave mixing (BSFWM). Since the total number of idler and signal photons is conserved, the process can preserve coherence of quantum optical states, functioning as a nonreciprocal frequency beam splitter. We explore the origin of this nonreciprocity and find that the phase-matching requirements of intermodal BSFWM produce an enormous asymmetry (76x) in the conversion bandwidths for forward and backward configurations, yielding similar to 25 dB of nonreciprocal contrast over several hundred GHz. We also outline how the demonstrated efficiencies (similar to 10(-4)) may be scaled to near-unity values with readily accessible powers and pumping configurations for applications in integrated quantum photonics.