For the first time, scientists from Germany, Denmark, and Austria have managed to directly convert laser light in optical fibers into a stream of isolated photons using a new effect. The results of the experiment are reported by the journal Nature Photonics.
If you imagine laser light as a stream of light particles, the so-called photons, then they are completely independent of each other, and their exact arrival time depends on the case. In particular, two photons can simultaneously hit the receiver. However, in many situations, it is desirable that one photon is registered after another, that is, that the light particles line up like a string of pearls.
Such isolated photons, for example, are a basic requirement for a quantum network. Until now, single quantum emitters, such as a single atom or a single molecule, generally acted as sources of such streams of individual photons. If a quantum emitter is excited by laser light and fluorescence, it will always emit exactly one photon for each quantum jump. For this type of source, it is still a challenge to efficiently “feed” the emitted photons into the fiberglass to send as many of them as possible to the receiver.
Physicists from Germany, Denmark and Austria managed to create a kind of “turnstile” for light in glass fibers, which allows light particles to pass only one at a time. The proposal for the experiment came from theoretical physicists Dr. Sahand Mahmoudian and Professor Clemens Hammerer of the Leibniz University of Hanover and colleagues at the University of Copenhagen. The experiment was carried out with the help of the research group of Prof. Dr. Arno Rauschenbeutel at the Humboldt University of Berlin. For this purpose, the scientists used a powerful atom-light interface in which atoms are trapped next to an optical nanofiber and coupled with light guided in the nanofiber.
These special glass fibers are a hundred times thinner than a human hair, and the atoms are held 0.2 micrometers from the surface of the fiberglass. At the same time, they are cooled by laser light to temperatures of a few millionths of a degree above absolute zero. This system allowed researchers to precisely control the number of atoms along the laser beam. Then, during the experiment, the researchers analyzed how often the photons exited the fiber individually or in pairs.
When about 150 atoms were captured near the nanofiber, it turned out that the transmitted light consisted almost only of isolated photons. Thus, all together the atoms acted on the photons like a turnstile regulating the flow of people. Surprisingly, the effect was the opposite when the number of atoms was increased: then the atoms passed photons, preferably in pairs.
This discovery opens up a completely new way of creating bright single-photon sources with embedded fiber. At the same time, the principle of operation demonstrated by the researchers can be applied to a wide range of the electromagnetic spectrum (from microwaves to X-rays). This opens up the possibility of generating single photons in spectral ranges for which there are no sources yet. Researchers have already applied for a patent for this technology.