Quantum-limited coherent measurements of optical signals were sent from a satellite in Earth’s orbit to an optical ground station over a distance of 38,600 kilometers (almost 24,000 miles). Excess noise was bound. The precise Earth-based measurement of optical signals from a satellite demonstrates the potential for a satellite-based quantum encryption network using equipment that is already in space. From the ground, researchers measured laser signals that originated from a satellite and traveled through Earth's gravitational potential and the turbulent atmosphere. The successful characterization of quantum features under such conditions is a precondition for the implementation of a global quantum communication network using satellites that would link metropolitan area quantum networks on the ground. Picture of the Earth courtesy of Google. Picture of the satellite courtesy of ESA. Although methods for quantum encryption have been in development for more than a decade, the technology has been unable to work over long distances because residual light losses in the optical fibers used for telecommunications networks on the ground degrade the quantum signals. According to researchers, encryption techniques such as quantum key distribution will be of increasing importance as current encryption codes based on mathematical algorithms become easier to crack. A team from the Max Planck Institute for the Science of Light worked with satellite telecommunications company Tesat-Spacecom GmbH and the German Space Administration to conduct the experiments. The researchers wanted to see if it was possible to measure quantum states encoded in a laser beam sent from one of the satellites already in space. In 2015 and 2016, the team took measurements from a ground-based station in Tenerife, Spain. They created quantum states within a range in which the satellite normally did not operate and were able to make quantum-limited measurements from the ground. “From our measurements, we could deduce that the light traveling down to Earth is very well-suited to be operated as a quantum key distribution network,” Max Planck researcher Christoph Marquardt said. “We were surprised because the system was not built for this.” A satellite-based quantum encryption network would provide an extremely secure way to encrypt data sent over long distances. “We were quite surprised by how well the quantum states survived traveling through the atmospheric turbulence to a ground station,” said Marquardt. “The paper demonstrates that technology on satellites, already space-proof against severe environmental tests, can be used to achieve quantum-limited measurements, thus making a satellite quantum communication network possible. This greatly cuts down on development time, meaning it could be possible to have such a system as soon as five years from now.” Developing such a system in just five years is an extremely fast timeline since most satellites require around 10 years of development. The researchers are now working with Tesat-Spacecom and others in the space industry to design an upgraded system based on the hardware already used in space. This will require upgrading the laser communication design, incorporating a quantum-based random number generator to create the random keys, and integrating post-processing of the keys. The results of initial experiments indicate that quantum communication using satellites in space is feasible and could open the possibility of a global quantum key distribution network for secure communication. “There is serious interest from the space industry and other organizations to implement our scientific findings,” said Marquardt. “We, as fundamental scientists, are now working with engineers to create the best system and ensure no detail is overlooked.” The research was published in Optica, a publication of OSA, The Optical Society (doi:10.1364/OPTICA.4.000611).