Science 361, 57–60 (2018)

Shuo Sun and co-workers from the USA have realized a single-photon switch and transistor using a solid-state quantum memory. The device consists of a semiconductor spin qubit strongly coupled to a nanophotonic cavity that is formed by a two-dimensional GaAs photonic crystal (PC), with a three-hole defect, containing a single layer of InAs quantum dots. A magnetic field of 5.5 T is applied to the PC so that one of the four optical transitions is resonant with the PC defect-cavity; a gate pulse and a signal optical pulse are resonant with the PC defect-cavity. The gate pulse first sets the spin state as a quantum memory of the switch, and the spin state is subsequently read by the transmittance contrast of the signal pulse. The spin qubit consists of a single electron in a charged InAs quantum dot. The team reports that the spin qubit enables a single 63-ps gate photon to switch a signal field containing up to an average of 27.7 photons. The maximum gain of 3.3 is obtained for a signal field containing an average of 29.2 photons.