  Single excitons in semiconductor microcavities represent a solid-state and
scalable platform for cavity quantum electrodynamics (c-QED), potentially
enabling an interface between flying (photon) and static (exciton) quantum bits
in future quantum networks. While both single-photon emission and the strong
coupling regime have been demonstrated, further progress has been hampered by
the inability to control the coherent evolution of the c-QED system in real
time, as needed to produce and harness charge-photon entanglement. Here, using
the ultrafast electrical tuning of the exciton energy in a photonic crystal
(PhC) diode, we demonstrate the dynamic control of the coupling of a single
exciton to a PhC cavity mode on a sub-ns timescale, faster than the natural
lifetime of the exciton, for the first time. This opens the way to the control
of single-photon waveforms, as needed for quantum interfaces, and to the
real-time control of solid-state c-QED systems.
