  When an atom strongly couples to a cavity, it can undergo coherent vacuum
Rabi oscillations. Controlling these oscillatory dynamics quickly relative to
the vacuum Rabi frequency enables remarkable capabilities such as Fock state
generation and deterministic synthesis of quantum states of light, as
demonstrated using microwave frequency devices. At optical frequencies,
however, dynamical control of single-atom vacuum Rabi oscillations remains
challenging. Here, we demonstrate coherent transfer of optical frequency
excitation between a single quantum dot and a cavity by controlling vacuum Rabi
oscillations. We utilize a photonic molecule to simultaneously attain strong
coupling and a cavity-enhanced AC Stark shift. The Stark shift modulates the
detuning between the two systems on picosecond timescales, faster than the
vacuum Rabi frequency. We demonstrate the ability to add and remove excitation
from the cavity, and perform coherent control of light-matter states. These
results enable ultra-fast control of atom-cavity interactions in a nanophotonic
device platform.
