Researchers in Austria have successfully realized a single-atom laser, an important step toward a fundamental understanding of laser operation in the few-atom limit, including systems based on semiconductor quantum dots or molecules. A single calcium ion is confined in an ion trap and excited by external lasers. (Photo: C. Lackner) A laser normally consists of a gain medium, which is electrically or optically pumped, inside a highly reflective optical cavity (or resonator). The light in the cavity bounces back and forth in the form of modes whereby it is amplified repeatedly. One of the distinctive features of a classical laser is the steep increase of output power when a certain pumping threshold is reached. At this point the gain (amplification by the medium) equals the losses as the light circulates through the cavity. This is caused by the amplification of the interaction between light and atoms: The more photons are present in a mode the stronger the amplification of the light in the mode. This stimulated emission is usually observed in macroscopic lasers comprising of many atoms and photons. "Quantum" lasers have shown thresholdless lasing when there is strong coupling between an atom and a radiation field, but the existence of a threshold has been predicted. Rainer Blatt and Piet Schmidt and their team at the University of Innsbruck demonstrated and characterized a single-atom laser with and without threshold behavior by changing, or tuning, the strength of atom/light-field coupling. "We have demonstrated that a laser threshold can be achieved at the single-atom level, although much less pronounced compared with classical lasers as a consequence of the low photon number in the lasing mode," the researchers wrote in their paper on the work, which appears this week in the journal Nature Physics. In their research, 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. 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. (Illustration: Schmidt) 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 trapped-ion laser could be useful to study new types of spectroscopy based on narrow-band optical excitation with quantum/classical statistical distributions, the researchers wrote. It is also suited for further investigations of the transition between quantum and classical lasers through controlled addition of more and more ions interacting with the light field. This research work is supported by the Austrian Science Fund, the European Commission and the Federation of Austrian Industry Tirol. For more information, visit: www.nature.com/nphys/index.html