  Strong interactions between single spins and photons are essential for
quantum networks and distributed quantum computation. They provide the
necessary interface for entanglement distribution, non-destructive quantum
measurements, and strong photon-photon interactions. Achieving spin-photon
interactions in a solid-state device could enable compact chip-integrated
quantum circuits operating at gigahertz bandwidths. Many theoretical works have
suggested using spins embedded in nanophotonic structures to attain this
high-speed interface. These proposals exploit strong light-matter interactions
to implement a quantum switch, where the spin flips the state of the photon and
a photon flips the spin-state. However, such a switch has not yet been realized
using a solid-state spin system. Here, we report an experimental realization of
a spin-photon quantum switch using a single solid-state spin embedded in a
nanophotonic cavity. We show that the spin-state strongly modulates the cavity
reflection coefficient, which conditionally flips the polarization state of a
reflected photon on picosecond timescales. We also demonstrate the
complementary effect where a single photon reflected from the cavity coherently
rotates the spin. These strong spin-photon interactions open up a promising
direction for solid-state implementations of high-speed quantum networks and
on-chip quantum information processors using nanophotonic devices.
