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.

Charged quantum dots containing an electron or hole spin are bright solid-state qubits suitable for quantum networks and distributed quantum computing. Incorporating such quantum dot spin into a photonic crystal cavity creates a strong spin-photon interface, in which the spin can control a photon by modulating the cavity reflection coefficient. However, previous demonstrations of such spin-photon interfaces have relied on quantum dots that are charged randomly by nearby impurities, leading to instability in the charge state, which causes poor contrast in the cavity reflectivity. Here we demonstrate a strong spin-photon interface using a quantum dot that is charged deterministically with a diode structure. By incorporating this actively charged quantum dot in a photonic crystal cavity, we achieve strong coupling between the cavity mode and the negatively charged state of the dot. Furthermore, by initializing the spin through optical pumping, we show strong spin-dependent modulation of the cavity reflectivity, corresponding to a cooperativity of 12. This spin-dependent reflectivity is important for mediating entanglement between spins using photons, as well as generating strong photon-photon interactions for applications in quantum networking and distributed quantum computing.Keyword: quantum dots, single electron spin, strong light-matter interaction, cavity quantum electrodynamics MainCharged epitaxially-grown Ⅲ-Ⅳ quantum dots that contain an electron or hole spin 1,2 have emerged as a promising solid-state qubit system. They exhibit a high radiative efficiency, 3 which is important for generating single or entangled photons of high brightness. In addition, they support fast all-optical coherent spin rotations on picosecond timescales, 4-6 necessary for high-speed quantum network and quantum information processing. Coupling such charged quantum dots to photonic crystal cavities enables strong spin-photon interfaces, 7 which are essential for solid-state quantum networks 8-10 and the generation of strong photon-photon interactions. [11][12][13] These applications require the spin-state of the quantum dot to modulate the cavity reflectivity, allowing the spin to control the state of the reflected photons (e.g., polarization and frequency).Several studies have demonstrated such strong spin-photon interfaces between the electron spin of a charged quantum dot and cavity, 14-17 enabling optical nonlinearities such as Kerr rotations 16,17 and single-photon transistors. 15 These studies have relied on probabilistically charged quantum AUTHOR INFORMATIONCorresponding Author

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