Researchers realise single atom lasers

The findings, by Rainer Blatt's and Piet Schmidt's research team from the University of Innsbruck have been published in the journal Nature Physics. The Innsbruck researchers have demonstrated that a laser threshold can be achieved at the smallest possible building block of a laser: a single atom, which interacts with a single mode in an optical cavity. A single calcium ion is confined in an ion trap and excited by external lasers. A high-finesse optical cavity consists of two mirrors, which traps and accumulates the photons emitted by the ion into a mode. The ion is excited cyclically by an external laser and at each cycle a photon is added to the cavity mode, which amplifies the light. François Dubin, a French postdoc and first author of the publication, said that for strong atom-cavity coupling, the regime of atom and cavity shows quantum mechanical behaviour - only single photons can be introduced into the cavity. Dubin explained: "As a consequence, stimulated emission and threshold are absent." According to Dubin, by choosing the right parameter of the drive laser, the physicists were able to achieve stronger excitation and, consequently, add more photons to the cavity. Although there was still less than one photon in the cavity, the researchers observed stimulated emission in the form of a threshold. The Innsbruck researchers plant to further investigate the transition between quantum and classical lasers through the controlled addition of more and more ions interacting with the light field.