  The study of individual quantum systems in solids, for use as quantum bits
(qubits) and probes of decoherence, requires protocols for their
initialization, unitary manipulation, and readout. In many solid-state quantum
systems, these operations rely on disparate techniques that can vary widely
depending on the particular qubit structure. One such qubit, the
nitrogen-vacancy (NV) center spin in diamond, can be initialized and read out
through its special spin selective intersystem crossing, while microwave
electron spin resonance (ESR) techniques provide unitary spin rotations.
Instead, we demonstrate an alternative, fully optical approach to these control
protocols in an NV center that does not rely on its intersystem crossing. By
tuning an NV center to an excited-state spin anticrossing at cryogenic
temperatures, we use coherent population trapping and stimulated Raman
techniques to realize initialization, readout, and unitary manipulation of a
single spin. Each of these techniques can be directly performed along any
arbitrarily-chosen quantum basis, removing the need for extra control steps to
map the spin to and from a preferred basis. Combining these protocols, we
perform measurements of the NV center's spin coherence, a demonstration of this
full optical control. Consisting solely of optical pulses, these techniques
enable control within a smaller footprint and within photonic networks.
Likewise, this approach obviates the need for both ESR manipulation and spin
addressability through the intersystem crossing. This method could therefore be
applied to a wide range of potential solid-state qubits, including those which
currently lack a means to be addressed.
