Optical Kerr effect with less than one photon

We emulated the self-phase modulation typical of strong optical Kerr nonlinearity on coherent light pulses containing less than a single photon (on average). These results just appeared in PRL.

Strong nonlinearity at the single-photon level represents a crucial enabling tool for quantum technologies because, by allowing single photons to interact with each other, it could be the basis of all-optical quantum logical gates for future processors.

Unfortunately, most materials present nonlinearities that are orders of magnitude smaller than those required for operating at the level of single photons.

Now, in a collaboration with our theoretical colleague Jaromír Fiurášek from Olomouc (CK), we learned how to use our photon addition and subtraction tools to emulate the effect of a strong optical Kerr nonlinearity on weak quantum states of light. The observed change of sign of specific Fock components was the signature of this strong effect on input coherent light pulses emitted from a laser and containing on average less than a single photon.