The practical transmission of single photons over a record-breaking distance of 20,000 km has been demonstrated between satellites in Earth orbit and a ground station in Italy. This was achieved by researchers led by Paolo Villoresi at the University of Padua, who say that the exchange confirms that satellite quantum communications are feasible on a global scale.

Global Navigation Satellite Systems (GNSSs) consist of constellations of satellites that broadcast timing signals to a ground-based receiver such as a mobile phone or vehicle navigation system. Several nations have constructed their own GNSSs, each with their own satellite constellations orbiting Earth at distances of 19,000-36,000 km.

These systems operate using microwaves at about 1 GHz, which means that a GNSS signal can easily be disrupted or intercepted by malicious third parties. This is a problem because GNSS’s are of crucial importance to the economic and military security of the nations that deploy them.

Quantum keys

Quantum communications uses single photons and offers a way of achieving secure and robust communications – both between satellites and between satellites and receivers on the ground. A well-established technique called quantum key distribution (QKD), for example, could be used to encrypt GNSS and other signals.

Quantum techniques involve the exchange of single photons, which has already been demonstrated for satellites that are relatively close to Earth. In 2016, Villoresi and colleagues achieved a transmission distance of 7000 km. The following year, physicists in China and Austria used a satellite to achieve QKD over a distance of 7400 km between Beijing and Vienna.

Longer communication

GNSS and other satellites at 19,000-36,000 km move slower relative to Earth’s surface than the satellites at lower orbits that were used in previous quantum demonstrations. As a result, such satellites can communicate for longer with individual grounds stations and are therefore better suited for creating quantum communications networks. However, exchanging single photons over these distances remains a challenge.

Beijing and Vienna have a quantum conversation

Villoresi’s team did their demonstration using the arrays of retroreflectors that are mounted on Russia’s GLONASS GNSS satellites. Working at the Italian Space Agency’s Matera Laser Ranging Observatory (MLRO), the researchers fired a train of laser pulses towards two GLONASS satellites. The light was then reflected back to the MLRO and collected by a single-photon detector.

By estimating the losses in the communication channel, the researchers were able to confirm that existing technology could deployed on satellites to generate key-carrying single photons. In future studies, they will develop active sources of QKD photons to be placed on-board GNSS satellites.