Ultrafast all-optical switching by single photons

An as yet outstanding goal in quantum optics is the realization of fast optical nonlinearities at the single-photon level. This would allow for the implementation of optical devices with new functionalities such as single-photon switches/transistors 1 , 2 or controlled-phase gates 3 . Although nonlinear optics effects at the single-emitter level have been demonstrated in a number of systems 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , none of these experiments showed single-photon switching on ultrafast timescales. Here, we perform pulsed two-colour spectroscopy and demonstrate that, in a strongly coupled quantum dot–cavity system, the presence of a single photon on one of the fundamental polariton transitions can turn on light scattering on a transition from the first to the second Jaynes–Cummings manifold. The overall switching time of this single-photon all-optical switch 14 is ∼50 ps. In addition, we use the single-photon nonlinearity to implement a pulse correlator. Our quantum dot–cavity system could form the building block of future high-bandwidth photonic networks operating in the quantum regime 15 , 16 , 17 , 18 . Researchers report the first demonstration of an ultrafast all-optical switch in the single-photon regime. The device, which consists of an InAs/GaAs quantum dot in a photonic crystal defect cavity, exhibits a coherent coupling constant of 141 meV and a quality factor of 25,000. The overall switching time is around 50 ps.