Nat. Nanotech. http://doi.org/bncj (2016)

A device that can sort photons in terms of photon number and polarization has been developed by scientists in the UK. Anthony Bennett and co-workers constructed their photon sorter from a pillar microcavity featuring a single InGaAs quantum dot emitting at 1.332 meV. The transition between the trion state and a single hole was activated by a weak non-resonant optical pump. This weak illumination yields excitons, allowing a hole to be captured into the ground state of the transition. The changes of single- and multi-photon components in the reflected beam were measured using autocorrelation spectroscopy. When the input was blue-detuned from the transition, a single photon was added in phase with the coherent light. On the other hand, when the input was red-detuned, resonantly Rayleigh scattered light was subtracted from the coherent light. Photon sorting by polarization was implemented by using the spin of the hole. Only when the spin of the hole was aligned along the cavity axis, was polarization correlation observed. The results pave the way for semiconductor optical switches operated by single quanta of light.