A technology that carries a quantum light source into space may be able to preserve the quantum properties of light as it travels through an optical link from space to Earth. It could provide the basis for establishing a secure global quantum network in space that can reach from one end of Earth to the other.
Researchers at the National University of Singapore and University of Strathclyde, U.K, built a space-qualified light source that could fit on a nanosatellite by redesigning a table-top quantum setup to be small and robust enough to fly inside a satellite the size of a shoebox and weighing just 1.65 kg.
Researchers at the National University of Singapore and University of Strathclyde, U.K., have launched a satellite that is testing technology for a global quantum network. This image combines a photograph of the quantum device with an artist's illustration of nanosatellites establishing a space-based quantum network. Courtesy of the Center for Quantum Technologies, National University of Singapore.
The team then demonstrated the in-orbit operation of a photon pair source aboard the nanosatellite, laying the groundwork for future entangled-photon experiments. The device inside the satellite, known as Small Photon-Entangling Quantum System (SPEQS) and comprising a laser diode, crystals, mirrors and photon detectors, takes photons from a BluRay laser and splits them in two, then measures the pair's properties. The research team demonstrated photon pair generation and polarization correlation under space conditions. The in-orbit photon correlations exhibited a contrast of 97 ±2 percent, matching ground-based tests.
Further testing and refinement may lead to a way to use entangled photons beamed from satellites to connect points on opposite sides of the planet. A fleet of nanosatellites carrying sources of entangled photons would be used to enable private encryption keys between any two points on Earth. The team's plans call for a series of launches, with the next space-bound SPEQS slated to produce entangled photons.
Ph.D. students Zhongkan Tang (left) and Rakhitha Chandrasekara (right) at the Centre for Quantum Technologies, National University of Singapore, are pictured working on a SPEQS unit. Courtesy of the Center for Quantum Technologies, National University of Singapore.
With subsequent satellites, the researchers will try sending entangled photons to Earth and to other satellites. The team is working with standard CubeSat nanosatellites, which can be sent into space as rocket ballast. Ultimately, completing a global network would mean having a fleet of satellites in orbit and an array of ground stations.