  We report on optical detection of a single photostable Ce3+ ion in an yttrium
aluminium garnet (YAG) crystal and on its magneto-optical properties at room
temperature. The quantum state of an electron spin of the emitting level of
cerium ion in YAG can be initialized by circularly polarized laser pulse.
Furthermore, its quantum state can be read out by observing temporal behaviour
of circularly polarized fluorescence of the ion. This implies direct mapping of
the spin quantum state of Ce3+ ion onto the polarization state of the emitted
photon and represents one-way quantum interface between a single spin and a
single photon.
