  We propose that Rydberg dressing of a single qubit atom can be used to
control a surrounding ensemble of three-level atoms and hereby the phase of
light reflected by an optical cavity. Our scheme employs an ensemble dark
resonance that is perturbed by the qubit state of a single atom to yield a
single-atom single-photon gate. We show here that off-resonant Rydberg dressing
of the qubit offers experimentally-viable regimes of operation that drastically
reduce error compared to schemes using shelved Rydberg population. Such low
errors (in the $10^{-3}$ range) are a necessary condition for fault-tolerant
optical-photon, gate-based quantum computation. We also demonstrate the
technique for microwave circuit-QED, where a strongly-coupled ancilla
superconducting qubit can be used in the place of the atomic ensemble to
provide high-fidelity coupling to microwave photons.
