  Recent advances in nanostructure fabrication and optical control, suggest
that it will soon be possible to prepare collections of interacting two-level
systems (i.e. qubits) within an optical cavity. Here we show theoretically that
such systems could exhibit novel phase transition phenomena involving
spin-glass phases. By contrast with traditional realizations using magnetic
solids, these phase transition phenomena are associated with both matter and
radiation subsystems. Moreover the various phase transitions should be tunable
simply by varying the matter-radiation coupling strength.
