Transmission of Entangled Photons
over a High-Loss Free-Space Channel
[This is an invited article based on a recently published work of the authors and their collaborators -- 2Physics.com]
Authors: Alessandro Fedrizzi1, Rupert Ursin1 and Anton Zeilinger1,2,
1 Institute for Quantum Optics and Quantum Information (IQOQI), Austria,
2 Faculty of Physics, University of Vienna, Austria
Entanglement is an essential phenomenon of quantum mechanics. Two entangled particles, photons for example, will individually yield random results upon being measured, but these results will always be perfectly correlated, no matter how far the two particles are separated from each other. Entanglement has been proven to be at the heart of a wide range of fundamental quantum effects and it drives exciting practical applications, such as quantum cryptography, quantum teleportation and quantum computing , which would be impossible in a world limited to classical physics. A team of researchers from the Institute for Quantum Optics and Quantum Information (IQOQI) in Vienna and the University of Vienna, led by Anton Zeilinger, has now reported the successful transmission of entangled photon pairs between two Canary Islands, bridging a distance of 144 km and a two-photon attenuation of almost ten million to one. The result, published in Nature Physics , is so far the most convincing demonstration to perform experiments with entangled photons in space.
More information on this endeavor can be found at http://www.quantum.at .
Past 2Physics articles by members of this collaboration:
"The Frontier of Quantum Communication is the Space" -- Paolo Villoresi,
"Entanglement and One-Way Quantum Computing" -- Robert Prevedel and Anton Zeilinger
Image 2: Canary islands (Google Map). Entangled photons were sent from La Palma to Tenerife (distance 144 km)
In their experiment, the researchers exploited a new design of a high-intensity source of entangled photons, which allowed photon-pair production rates of about one million per second . This photon source was located at the Island of La Palma, one of the Canary Islands (see image 2). From there, both photons of entangled pairs were transmitted through a home-built twin telescope (image 3) and sent to the 144 km distant island of Tenerife, where they were collected by the Optical Ground Station (OGS, image 4), a research observatory operated by the European Space Agency. This unique location was chosen because of the excellent experimental conditions for free-space experiments; clean air, an unobstructed view over a very long distance and the availability of a high-tech, large aperture telescope, which could be used as a receiver.
Once the primary mirror of the OGS collected the photon pairs, the photons were guided to an experimental chamber by a series of mirrors then split and individually analyzed and detected. The trickiest part in free-space experiments with single photons is certainly to find the individual photons in the background light. Fortunately, entangled photons are produced at exactly the same time. If therefore two detectors click at exactly the same time (in practice within a time window of 1 nanosecond, one billionth of a second), one can be quite sure that the clicks were really produced by two photons of an entangled pair. A series of polarization correlation measurements made it possible to show that the received photon pairs were still just as highly entangled as when they were produced by the source, with the entanglement quality limited only by background noise. This is astounding as the photons were experiencing a rough ride during their flight time of ½ of a millisecond through the turbulent atmosphere, the longest recorded lifetime for entangled photons so far.
Image 3: Thomas Herbst, aligning the homebuilt transmitter telescope at La Palma.
Previous proof-of-principle experiments for quantum information in space by the same group and their international collaborators* include the transmission of only one photon of an entangled pair over the same free-space link , quantum key distribution with weak coherent pulses  and an experiment in which single photons were bounced off a retro-reflecting mirror mounted on a satellite orbiting at an altitude of about 6000 km .
The conditions in the new experiment were very close to those expected for a downlink from a satellite to two separate receiver stations on the ground. In particular, the high attenuation (107:1 for photon pairs) the photons were exposed to was similar to that expected in a space scenario. The atmospheric turbulence along the 144 km flight path was in fact much larger, because the atmosphere thins out rapidly at higher altitude and the optical density of the atmosphere along a vertical trajectory into space is equivalent to a merely 7 km long horizontal path through the atmosphere. Moreover, the researchers have shown that observatories like the OGS, which was originally built for classical laser communication and is perfectly suited to track a fast-moving object in orbit, can be adapted for quantum optics experiments.
Image 4: Receiver telescope in the optical ground station, Tenerife. Incoming photons are collected by this 1-meter mirror telescope and then guided to the analysis and detection apparatus.
Moving entanglement from ground-based laboratories into space will eventually enable experiments on a much larger distance scale than currently possible on ground. A low flying space vessel such as the International Space Station ISS, would be able to transmit photons to ground observers separated by more than 1000 km. This would benefit tests of the foundations of quantum mechanics as well as practical applications of entangled photons in space, such as long distance quantum cryptography, the secure distribution of keys, which can be used to encrypt messages . Once this initial step into space has been mastered, experiments between two or more moving satellites will allow relativistic tests of quantum mechanics as well as experimental tests on entanglement in gravitational fields .
To eventually make this vision a reality, the group in Vienna and their international partners, which include universities, space industry and the European Space Agency, has already started working on a first demonstration prototype of an entangled-photon source that could be integrated into a space-borne terminal. The schedule is compatible with a launch into space within the next decade.
This work was supported by the Austrian Research Promotion Agency (FFG) and the European Space Agency (ESA).
* In collaboration with LMU Munich and MPQ Garching, Germany, the University of Bristol, UK, the University of Padova, Italy and the European Space Agency.
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