Stephen Barnett has made significant contributions to theoretical quantum optics and electrodynamics. He is well known for his solution, with David Pegg, of the longstanding problem of identification of the Hermitian operator that represents the phase of the electromagnetic field, now known as the Pegg–Barnett phase operator.
He also developed the first formalism for the field quantisation in dispersive and lossy dielectrics, with subsequent applications to the spontaneous emission by an embedded atom and the propagation of light through attenuating and amplifying media. This led, in particular, to his resolution of the century-old Abraham–Minkowski dilemma of the nature of optical momentum in a medium.
Stephen’s wide-ranging interests include the foundations of quantum theory and of quantum information, most especially the nature and application of generalised measurements. He was a pioneer of the study of optical angular momentum and is currently working on applying these ideas to chiral chemistry. When not working in the quantum domain, he is a keen student of general relativity and cosmology.