1 October 2020

Photon turnstile brings order to light

QUANTUM OPTICS:

With the creation of a turnstile for light in glass fibers, quantum optics researchers from Germany, Denmark and Austria have succeeded in directly converting laser light in optical fibers into a single file of isolated photons. According to Anders Søndberg Sørensen from the Niels Bohr Institute at the University of Copenhagen, who was involved in the theoretical phase of the experiment, this creation of isolated photons can prove essential in the exploration of quantum communication. The results of the experiment are published in Nature Photonics this week.

Light particles pass through a glass fiber and meet a cloud of atoms. Like a turnstile, the atoms ensure that light particles only pass through one by one. Photo: Humboldt University
Light particles pass through a glass fiber and meet a cloud of atoms. Like a turnstile, the atoms ensure that light particles only pass through one by one. Photo: Humboldt University

Physicists have long studied the interaction of light and matter and the way in which light particles, so called photons, are affected when they meet clouds of atoms. Quantum optics researchers are particularly interested in this because it can help them find more secure ways to process information, for example by sending information in the form of single photons.

Until now, the challenge has been how to ‘feed’ emitted photons in a glass fiber in such a way that they come out in an sorted manner, one after the other. "This is crucial in making quantum technologies where we encode information in individual photons and atoms", explains Professor Anders Søndberg Sørensen, leader of the Theoretical Quantum Optics groups at the Niels Bohr Institute at Copenhagen University. To send encoded information in an undistorted way, you need to be able to send the photons in an isolated way. "If you can do that, you work towards dramatic new ways of processing information. Single photons can be for instance be used to send encrypted messages which cannot be eavesdropped" Søndberg Sørensen adds.

150 is the magic number

In their experiment, the researchers explored how many atoms a photon should meet for it to come out isolated at the other end by precisely controlling the number of atoms along the laser beam in the glass fiber. The proposal for the experiment came from Søndberg Sørensen and theoretical physicists at the Leibniz University Hannover. The research group of Prof. Dr. Arno Rauschenbeutel at Humboldt University of Berlin then carried out the experiment using a powerful atom-light interface in which atoms are trapped near a so-called optical nanofiber, which is one hundred times thinner than a human hair.

With the use of tweezers of laser lights, the atoms were held in place at precisely 0.2 micrometers from the glass fiber surface, while laser lights were cooling the atoms down to a temperature of a few millionths of a degree above absolute zero. The researchers found that when there were about 150 atoms trapped near the nanofiber, the photons would come out one by one. If they would use less atoms, the photons would be unaffected by the atoms; if they would use more, the photons would come out in pairs.

An unexpected result

Søndberg Sørensen is excited about the results of the experiment. Not only was it unexpected that the researchers found the exact interval that leads to the transmission of single photons, but also that it was possible to make it work on weakly coupled atoms. "The beauty of this interface is that it’s fairly simple and that it works with weakly coupled atoms, which means it could also be applied to for example x-rays in the future", Søndberg Sørensen explains.

What this could mean for the future is an open question. "Such sources have never been available before, so we do not yet understand the full range of applications for them," says Søndberg Sørensen. "But potentially, they could be used for ultra-precise sensing and allow for much broader exploration of quantum technologies."

Link to the scientific publication
“Correlating photons using the collective nonlinear response of atoms weakly coupled to an optical mode”, Adarsh S. Prasad, Jakob Hinney, Sahand Mahmoodian, Klemens Hammerer, Samuel Rind, Philipp Schneeweiss, Anders S. Sørensen, Jürgen Volz, and Arno Rauschenbeutel, Nature Photonics, 21/09/2020,

DOI 10.1038/s41566-020-0692-z: https://www.nature.com/articles/s41566-020-0692-z