Quantum-encrypted communications have for a while been touted as the only form of communication that can ensure perfect security, ruling out the possibility of interception or eavesdropping completely.
However, free-space quantum communication experiments were, until recently, unable to be implemented for distances further than several tens of kilometres.
Large scale distribution of encrypted keys to enable secure communications on a global level, were deemed impossible due to the difficulties involved in long-distance propagation of the optical beam, which is affected by atmospheric and geographic hurdles. Wavefront distortion due to atmospheric turbulence and the absorption of radiation in the lower layers of the atmosphere include some of the critical issues that hamper extension of quantum communications to longer distances.
Now, Italian and Austrian researchers have come up with a novel idea to overcome the above challenges by attempting to establish a quantum communication link with orbiting satellites and ground-based stations. In the course of their experiment, they were able to perfectly identify individual photons fired from the ground station and reflected off a satellite in space, equipped with retroreflectors. Thus they were able to establish the viability of constructing a secure quantum communication channel between Earth and space.
This is the first necessary step in enabling space-ground and space-space communications based on coding the information bits in the quantum state of a photon - known as a qubit.
Making the crucial crossing from theoretical predictions to practical demonstration, the experiment required expertise ranging from classical optics to quantum optics and from advanced electronics to satellite laser ranging (SLR).
For the experiment, the researchers used a rapid sequence of weak laser pulses emitted by the ground-based station in the Matera Laser Ranging Observatory (MLRO) of the Italian Space Agency ASI, located in Matera, Italy. The pulses were directed toward a Japanese low-Earth orbiting (LEO) geodetic satellite, Ajisai, whose orbit has a perigee height of 1485 km and the retroreflected radiation was detected by a telescope back at MLRO. The main challenge was to detect the small number of reflected photons amongst a huge background noise, which mimics a real-life situation with an actual quantum communication system.
While this experiment clearly establishes the feasibility of a secure quantum communication channel through space, the next step would be to place an active source of single photons and entangled photons in space.
"Having a quantum communication source on a satellite will allow us to perform quantum physics experiments over much larger distances than possible on ground, and also allow us to perform quantum key distribution on a global scale," says Paolo Villoresi, one of the lead researchers involved in the experiment.
Besides the novel methods for information exchange - such as quantum teleportation - that become more viable, several new applications can also be expected to crop up in the future, such as atmospheric monitoring, based on the modification of the optical signal during downlink. Quantum communication through satellites can be expected to attract the attention of military establishments first, before civilian applications become widely available.
Villoresi is currently coordinating a study sponsored by the Italian Space Agency for having a dedicated satellite for quantum communications. These experiments, despite being oriented toward real-life operations, are still exploratory and commercially viable solutions are expected no sooner than 8-10 years from now.
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