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Single photons from weakly nonlinear photonic structures

It is commonly understood that, to cast light into a state with sub-Poissonian statistics, a strong optical nonlinearity is needed. The most common examples are resonance fluorescence of a two-level system, or a Kerr medium where the Kerr energy per photon U is much larger than the optical linewidth κ. In both cases, a single-photon nonlinearity is realized, whereby the presence of a single photon strongly modifies the optical response to a second incoming photon. Recently however it was shown that sub-Poissonian light can arise in presence of arbitrarily weak optical nonlinearity [1]. The general mechanism is radically different from the conventional photon blockade, in which the probability of having n>1 photons is uniformly suppressed. In the unconventional photon blockade (UPB), the interplay of optical phase and nonlinearity can selectively suppress the probability of having n=2 photons, so that sub-Poissonian statistics arises for a weakly displaced optical field (roughly corresponding to less than one photon on average) for which the probability of n>2 photons is originally very small. A very insightful alternative explanation of UPB [2] shows that sub-Poissonian statistics arises from optimizing the squeezing parameter of a squeezed-displaced field, for a given value of the displacement. UPB was theoretically studied for many configurations of optical resonators and mechanisms for optical nonlinearity. Recent experiments [3, 4], though not achieving a sub-Poissonian regime, suggest that the phase mechanism underlying UPB can determine to a large extent the photon statistics.