A quantum optical valve in a nonlinear-linear resonator junction

Electronic diodes, which enable the rectification of an electrical energy flux, have played a crucial role in the development of current microelectronics after the invention of semiconductor p-n junctions. Analogously, signal rectification at specific target wavelengths has recently become a key goal in optical communication and signal processing. Here we propose a genuinely quantum device with the essential rectifying features being demonstrated in a general model of a nonlinear-linear junction of coupled resonators. It is shown that such a surprisingly simple structure is a versatile valve and may be alternatively tuned to behave as: a photonic diode, a single or two-photon rectified source turning a classical input into a quantum output depending on the input frequency, or a quantum photonic splitter. Given the relevance of non-reciprocal operations in integrated circuits, the nonlinear-linear junction realises a crucial building component in prospective quantum photonic applications.