We show that the presence of a single photon deterministically controls the transmission of another photon. Their strong interactions are mediated by a single quantum dot spin that is strongly coupled to a nanophotonic cavity.
The field of quantum nanophotonics has recently been attracting a great deal of interest. In quantum nanophotonics, fermionic degrees of freedom such as atoms or quantum dots are coupled to traditional nanophotonic systems. As the fermionic components can only absorb a finite number of photons at a time, the interplay between the fermionic and photonic degrees of freedom can fundamentally alter the transport properties of photons and their correlations. In many conventional photonic systems, however, atom-photon interactions are too weak to realize efficient devices at ultra-low power levels. We show that in properly designed quantum nanophotonic geometries, we can create strong photon-photon interaction and quantum interference to realize efficient devices operating down to the single-photon power level. In particular, quantum nanophotonic geometry allows us to establish quantum interference to provide efficient single-photon frequency conversion and long-term photonic storage. Moreover, we use strong photon-photon interactions in cavity-QED to produce strongly antibunched streams of photons with shot noise below the classical limit which can allow fundamentally secret communication over large distances.
Single-photon switches and transistors generate strong photon-photon interactions that are essential for quantum circuits and networks. However, the deterministic control of an optical signal with a single photon requires strong interactions with a quantum memory, which has been challenging to achieve in a solid-state platform. We demonstrate a single-photon switch and transistor enabled by a solid-state quantum memory. Our device consists of a semiconductor spin qubit strongly coupled to a nanophotonic cavity. The spin qubit enables a single 63-picosecond gate photon to switch a signal field containing up to an average of 27.7 photons before the internal state of the device resets. Our results show that semiconductor nanophotonic devices can produce strong and controlled photon-photon interactions that could enable high-bandwidth photonic quantum information processing.