  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.
