Patent ID: 8217724

Claim:
An apparatus comprising: a condensed matter medium comprising at least one system that has at least a pair of states, said states comprising a first state and a second state with respective energy levels, said energy levels having an energy difference therebetween, wherein the energy difference varies as a function of applied magnetic field; a magnet device arranged to apply an adjustable magnetic field to the medium; an excitation device arranged to cause the at least one system to undergo transitions between said pair of states; and a detection device arranged to detect the response of the at least one system induced by the excitation device and to produce an output; and a controller for receiving the output of the detection device and arranged to control the magnet device such that the magnetic field applied to the medium has a value at which the rate of change of said energy difference with change in magnetic field is substantially zero, and to derive oscillations at a frequency determined by the energy difference between said pair of states between which the at least one system is caused to undergo transitions.