  The much sought after optical transistor --the photonic counterpart of the
electronic transistor-- is poised to become a central ingredient in the
development of optical signal processing. The motivation for using photons
rather than electrons not only comes from their faster dynamics but also from
their lower crosstalk and minor environmental decoherence, which enable a high
degree of integration and the realization of quantum operations. A
single-molecule transistor has been recently demonstrated at cryogenic
temperatures. Here, we demonstrate that a single NV centre at room temperature
can operate as an optical switch under non-resonant CW illumination. We show
optical modulation of more than 80% and time response faster than 100 ns in the
green-laser-driven fluorescence signal, which we control through an independent
near-infrared (NIR) gating laser. Our study indicates that the NIR laser
triggers a fast-decay channel of the NV mediated by promotion of the excited
state to a dark band. This simple concept opens a new approach towards the
implementation of nanoscale optical switching devices.
