  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.
