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Scientistis from the Universität Stuttgart use hollow core fibre for future quantum technologies

07.07.2014 - (idw) Universität Stuttgart

Room-temperature single-photon turnstile Secure data communication, quantum computation, and sensing devices benefit from light sources, which emit only one photon at a time. These light sources operate in a mode that is similar to a turnstile, allowing only one person to pass through at a time. Such so-called single photon sources exist already, but with the technical drawback that they only work at ultracold temperatures close to absolute zero. Scientists of the center for Integrated Quantum Science and Technology (IQST) at the University of Stuttgart have now developed a microscopic platform, which could allow for such a turnstile operation, even at room temperature. They use atom-filled hollow-core optical fibres. Their results were presented in Nature Communications on June 19th 2014.

It is unimaginable to live without the benefits of light in our modern world. We take advantage of light to transmit information around the world through optical fibres, to read out BlueRay discs, and to perform surgeries. That light can be used for many diverse applications lies in its versatile nature. Different light sources produce different types of light. The subtle differences between light sources, relies on how the light particles, so called photons, leave the light source. Ordinary light bulbs produce quite irregular light. Here, the photons tend to bunch and to exit in groups. Laser light is more regular, but still groups of photons can appear. For some novel technologies researchers ask for even more regular light. The photons shall leave the light source one by one, like behind a turnstile. Only with these well-separated photons can one fully exploit the quantum mechanical properties of single photons, e.g. for secure data communication. Such sources do exist already, but they can only be operated at extremely low temperatures with lab-filling setups, that are not compatible with real life technologies.
Researchers from IQST at the University of Stuttgart and the Max Planck Institute for the Science of Llght in Erlangen have now made an important step towards a photon-turnstile that can operate at room temperature.

Robert Löw and his team want to exploit an already known scheme to realize their room-temperature photon turnstile. Scientists working with Sebastian Hofferberth, a researcher also from the University of Stuttgart, have done the following experiment: An ultracold gas of atoms is irradiated with ordinary laser light at a specific wavelength and is absorbed by the gas. A second laser beam, set to a different wavelength, is then turned on and sent through the gas, making the gas transparent for the first laser beam. The crucial step to create single photons is to excite the atoms in the gas to very high energy levels. These so-called Rydberg atoms are typically 1000s of times larger than ordinary atoms and are very sensitive. Due to their sensitivity they strongly influence each other, which reduces the transparency effect caused by the second laser beam. More precisely, quantum mechanics takes care that only one photon per time can pass.

This is the starting point for the experiments with hollow core photonic crystal fibres, which are now filled with a room-temperature atomic gas. The laser light is collimated inside the fibre providing the necessary high-intensity light for the optical non-linear response over longer distances than would be possible in free space. Therefore many more Rybergs atoms can be created along the length of the fibre. The scientists are convinced that the usage of the fibre will make the observation of the turnstile-effect possible even at room temperatures. The problem is the high velocity of the atoms whizzing back and forth in the fibre and eventually colliding with the fibre walls, which ruins the turnstile effect. The hope is that the many more Rydberg atoms in the fibre could compensate for this short lifetime and still sort the photons as desired.

One major concern towards this goal has now been ruled out by the Stuttgart team: The core of the fibre with 19 microns diameter is only marginally larger than the Rydberg atoms and is very likely to perturb the Rydberg atoms and by this the turnstile-effect. The question has been if caged Rydberg atoms behave differently than free range Rydberg atoms and the answer is no. The Rydbergs atoms do not feel the wall until they crash into them. Collisions happen very often, but the time in between wall collisions should be sufficient to realize a photon turnstile.


*Originalpublikation: G. Epple, K. S. Kleinbach, T. G. Euser, N. Y. Joly, T. Pfau, P. St.J. Russell, R. Löw: "Rydberg atoms in hollow-core photonic crystal fibres", Nature Communications 5 4132 (2014)
Weitere Informationen: Robert Löw, Universität Stuttgart, 5. Physikalisches Institut,
Tel. +49 711 685 64954, E-Mail: r.loew@physik.uni-stuttgart.de
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