A single-photon channel to space
According to the Institute of Physics (IOP), European researchers have successfully identified individual returning photons from space 'after firing and reflecting them off of a space satellite in orbit almost 1,500 kilometers above the earth.' This experiment could lead to a global quantum-encrypted communications network in the future. In other words, we could one day have access to completely secure channels for global communications via satellites in space. But read more...
You can see above a scheme of the satellite single-photon link. "A fraction of the beam in the uplink path irradiates the satellite (LAGEOS shown). The corner cubes on the satellite retro-reflect back to the Earth a small portion of the photons in the laser pulse (downlink) and the gathered portion, according to the receiver field of view (FOV) is indicated in green. This, according to the experimental parameters, is the single-photon channel." (Credit: 11 researchers from Austria and Italy) The researchers also used other satellites for their experiment were the most visible ones from MLRO: Ajisai (perigee height of 1485 km), Lageos II (5625 km), Topex-Poseidon (1350 km) and Beacon-C (927 km).
This research effort has been led by Paolo Villoresi, associate professor of Physics and Cesare Barbieri, professor of Astronomy, both from the University of Padova, Italy. Anton Zeilinger, professor of Experimental Physics at the University of Vienna, Austria, and the 2008 winner of the Newton Medal award given by the IOP was also involved in the research.
For their experiments, the researchers used the Matera Laser Ranging Observatory (MLRO), located at the Centre for Space Geodesy (CGS) in Matera, Italy, operated by the (ASI) Agenzia Spaziale Italiana -- or Italian Space Agency. The best results were obtained by communicating with the Experimental Geodetic Satellite Ajisai belonging to the Japan Aerospace Exploration Agency (JAXA).
This research work has been published in the New Journal of Physics, one of the journals of the Institute of Physics, under the title "Experimental verification of the feasibility of a quantum channel between Space and Earth" (Volume 10, Article 033038, March 2008).
Here is a link to the abstract. "Extending quantum communication to space environments would enable us to perform fundamental experiments on quantum physics as well as applications of quantum information at planetary and interplanetary scales. Here, we report on the first experimental study of the conditions for the implementation of the single-photon exchange between a satellite and an Earth-based station. We built an experiment that mimics a single photon source on a satellite, exploiting the telescope at the Matera Laser Ranging Observatory of the Italian Space Agency to detect the transmitted photons. Weak laser pulses, emitted by the ground-based station, are directed toward a satellite equipped with cube-corner retroreflectors. These reflect a small portion of the pulse, with an average of less-than-one photon per pulse directed to our receiver, as required for faint-pulse quantum communication. We were able to detect returns from satellite Ajisai, a low-Earth orbit geodetic satellite, whose orbit has a perigee height of 1485 km."
And here is a link to the full paper (PDF format, 12 pages, 948 KB). Here is an excerpt from the conclusions. "In this work, we have achieved significant experimental results toward the realization of a quantum communication channel from a source on a LEO [Low Earth Orbit] satellite to a receiver on Earth, as well as how to actually adapt an existing laser-ranging facility for quantum communication. The high level of losses in the experiment in the two-way path prevented us from implementing a quantum-key-distribution protocol. Nevertheless, the good agreement of the return data with the model for the satellite link and the effective detection of returns by using the present technology that result here, also corroborated by the findings of the entanglement-based communication in the very long terrestrial link of 144 km [10], attest clearly on an experimental base the feasibility of a satellite-based quantum channel in the near future."
The above figure has been extracted from this paper which is -- theoretically -- available without registration.
Sources: Institute of Physics news release, March 27, 2008; and various websites
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