A proper description of the quantum-mechanical interplay between an atom and the electromagnetic modes of a cavity is fundamental to lasers and other quantum systems. In Physical Review Letters, experimenters use optical waveguides to mimic this atom-field interaction, even in a regime where standard theoretical approximations are not valid.
The well-known quantum Rabi model describes the interaction between two energy levels of an atom and an energy ladder representing an increasingly excited electromagnetic-cavity mode (See 29 August 2011 Viewpoint ). Usually, the interaction is weak enough that the model can be simplified to include only energy transfer from the atom to the field or vice versa. But theory suggests new phenomena when this interaction is instead strong.
To create an experimental analog, Andrea Crespi at the Polytechnic Institute of Milan and colleagues used short, intense laser pulses to write parallel, gently curving waveguides of denser material within a silica substrate, where successive waveguides correspond mathematically to an increased excitation of the cavity mode. They made various sets of waveguides, adjusting their properties and spacings to match the energies and coupling appropriate to different model parameters. Looking down on the waveguides showed that light that was launched into the first waveguide (corresponding to an unexcited cavity) sloshed into the other waveguides and back again. With waveguides close enough together, simulating strong coupling, the team confirmed that the light spreads in a way analogous to simultaneous excitation of both atom and field, which doesn’t happen in the familiar “Rabi oscillations.” – Don Monroe