From Niels Bohr Institute DK: “Photon turnstile brings order to light”

University of Copenhagen DK

Niels Bohr Institute bloc

From Niels Bohr Institute DK

1 October 2020
Anders Søndberg Sørensen, Professor

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.

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.

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.”

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Niels Bohr Institute Campus

Niels Bohr Institute DK (Danish: Niels Bohr Institutet) is a research institute of the University of Copenhagen DK. The research of the institute spans astronomy, geophysics, nanotechnology, particle physics, quantum mechanics and biophysics.

The Institute was founded in 1921, as the Institute for Theoretical Physics of the University of Copenhagen DK, by the Danish theoretical physicist Niels Bohr, who had been on the staff of the University of Copenhagen since 1914, and who had been lobbying for its creation since his appointment as professor in 1916. On the 80th anniversary of Niels Bohr’s birth – October 7, 1965 – the Institute officially became The Niels Bohr Institute. Much of its original funding came from the charitable foundation of the Carlsberg brewery, and later from the Rockefeller Foundation.

During the 1920s, and 1930s, the Institute was the center of the developing disciplines of atomic physics and quantum physics. Physicists from across Europe (and sometimes further abroad) often visited the Institute to confer with Bohr on new theories and discoveries. The Copenhagen interpretation of quantum mechanics is named after work done at the Institute during this time.

On January 1, 1993 the institute was fused with the Astronomic Observatory, the Ørsted Laboratory and the Geophysical Institute. The new resulting institute retained the name Niels Bohr Institute DK.

The University of Copenhagen (UCPH) DK (Danish: Københavns Universitet) is the oldest university and research institution in Denmark. Founded in 1479 as a studium generale, it is the second oldest institution for higher education in Scandinavia after Uppsala University (1477). The university has 23,473 undergraduate students, 17,398 postgraduate students, 2,968 doctoral students and over 9,000 employees. The university has four campuses located in and around Copenhagen, with the headquarters located in central Copenhagen. Most courses are taught in Danish; however, many courses are also offered in English and a few in German. The university has several thousands of foreign students, about half of whom come from Nordic countries.

The university is a member of the International Alliance of Research Universities (IARU), along with University of Cambridge, Yale University, The Australian National University, and UC Berkeley, amongst others. The 2016 Academic Ranking of World Universities ranks the University of Copenhagen as the best university in Scandinavia and 30th in the world, the 2016-2017 Times Higher Education World University Rankings as 120th in the world, and the 2016-2017 QS World University Rankings as 68th in the world. The university has had 9 alumni become Nobel laureates and has produced one Turing Award recipient.