Miniaturized lasers can emit quantum light

In the sixties one of its pioneers, the Nobel Prize winnerRoy Glauber, suggested to characterize light sources according to the sequenceof their emitted photons. But the realization of this idea has been verylimited up to now. In modern lasers the light emission can take place within apicosecond, a 1000 times faster than has ever been detected before in photonsequence measurements. A team at the Faculty of Physics at TU Dortmund withProf. Manfred Bayer in charge made a major breakthrough in cooperation withtheir colleagues from Bremenand Würzburg: in several years of work the worldwide first detector has beendeveloped which has the necessary temporal resolution.

At first the detector was able to show that light has thestatistic characteristics which Roy Glauber theoretically predicted. For thispurpose a common laser structure was stimulated to emit light. When thestimulation is low the structure still emits classic light. But when thestimulation is stronger the laser crosses its threshold and starts to emitlaser light. The experimentally detected sequence of photons clearlysubstantiated that.

During their examinations of miniaturized lasers the Dortmund physicists MarcAssmann, Thorsten Berstermann, Franziska Veit and Manfred Bayer obtained surprisingresults.

One of the goals of the intensive worldwide effortsconcerning the miniaturization of lasers is the realization of a thresholdlesslaser which can convert power entirely into laser light. That would be a veryenergy-efficient method. For their examinations the Dortmundteam had miniaturized lasers from Würzburg and Bremen available. These lasers are result ofcutting-edge technology and come already close to thresholdless lasers.

They pushed such lasers across the threshold and thedetector showed that the sequence of two photons does not change from classiclight to laser light, as expected. At this crossover rather quantum lightemission occurred: the photons avoid each other, i.e. the probability to findtwo photons in direct succession is reduced compared to big displacement times.In the case of laser light, on the other hand, all random temporal delaysbetween two photons are equally probable, while in case of classic light thephotons preferentially arrive together.

When the Dortmund physicistspresented their results to their Bremenproject partners, who develop elaborate theoretical models for describing suchlasers, they could hardly believe it but immediately started modeling theexperimental data. And in fact, the model calculations confirmed the results.Moreover, oscillations in the probability to find two photons with a certaintemporal displacement were predicted which could experimentally be totallyconfirmed. Certain displacements, causing quantum light emissions, are more likelythan others.

This behavior can qualitatively be understood as follows:the laser can roughly be compared with the starship Enterprise which has two drive modes: thenormal rocket propulsion and the Warp-drive. The rocket propulsion isequivalent to the emission of classic light beneath the threshold. TheWarp-drive is equivalent to the laser operation. When one wants to switch fromclassic to Warp and does not produce enough propulsion, the power unit startsto splutter and the Enterprisehops through space – the final frontier. The oscillations which can be observedin the photon statistic of the miniaturized lasers are very similar: the laserjumps in its emission characteristics. Only when there is enough propulsion,there is regular laser operation. This work was enabled by the generous fundingof the Deutsche Forschungsgemeinschaft and was published in the latest editionof Nature.