Abstract

High radiation rates are usually associated with macroscopic lasers. Laser radiation is “coherent”—its amplitude and phase are well-defined—but its generation requires energy inputs to overcome loss. Excited atoms spontaneously emit in a random and incoherent fashion, and for <em font-size:="font-size:" style="box-sizing: inherit; color: rgb(51, 51, 51); font-family: Roboto, ">N</em><span font-size:="font-size:" style="color: rgb(51, 51, 51); font-family: Roboto, "> such atoms, the emission rate simply increases as </span><em font-size:="font-size:" style="box-sizing: inherit; color: rgb(51, 51, 51); font-family: Roboto, ">N</em><span font-size:="font-size:" style="color: rgb(51, 51, 51); font-family: Roboto, ">. However, if these atoms are in close proximity and coherently coupled by a radiation field, this microscopic ensemble acts as a single emitter whose emission rate increases as </span><em font-size:="font-size:" style="box-sizing: inherit; color: rgb(51, 51, 51); font-family: Roboto, ">N</em>2<span font-size:="font-size:" style="color: rgb(51, 51, 51); font-family: Roboto, "> and becomes “superradiant,” to use Dicke's terminology (</span><em font-size:="font-size:" style="box-sizing: inherit; color: rgb(51, 51, 51); font-family: Roboto, ">1</em><span font-size:="font-size:" style="color: rgb(51, 51, 51); font-family: Roboto, ">). On page 662 of this issue, Kim </span><em font-size:="font-size:" style="box-sizing: inherit; color: rgb(51, 51, 51); font-family: Roboto, ">et al.</em><span font-size:="font-size:" style="color: rgb(51, 51, 51); font-family: Roboto, "> (</span><em font-size:="font-size:" style="box-sizing: inherit; color: rgb(51, 51, 51); font-family: Roboto, ">2</em><span font-size:="font-size:" style="color: rgb(51, 51, 51); font-family: Roboto, ">) show the buildup of coherent light fields through collective emission from atomic radiators injected one by one into a resonator field. There is only one atom ever in the cavity, but the emission is still collective and superradiant. These results suggest another route toward thresholdless lasing.</span>