Two teams, one in Canada and another in China, have lifted the message-encryption technique known as quantum key distribution (QKD) out of optical fibers and into literal new heights: an airplane in flight and a satellite orbiting Earth.

Preparing for a proposed Canadian quantum-communications spacecraft, researchers from the University of Waterloo, Ontario, uplinked secure quantum keys from a ground-based transmitter to a receiver that was mounted on an aircraft passing overhead (Quantum Sci. Technol., doi:10.1088/2058-9565/aa701f). Across the globe, a team from the Chinese Academy of Sciences sent entangled photon pairs from the country’s quantum-technology satellite to two different ground stations (Science, doi:10.1126/science.aan3211).

Scientists have been investigating QKD as an unbreakable encryption scheme for more than three decades, but transmitting the keys over optical fiber doesn’t work for distances greater than a few hundred kilometers, due to exponentially scaling losses. Short-range QKD has been demonstrated for a prototype handheld device, as well as key transmissions from aircraft to ground bases. However, until the Waterloo experiments, no one had sent quantum keys from a terrestrial transmitter to a moving aircraft, even though the uplink mode requires simpler airborne equipment than the downlink scheme.

The team from the University of Waterloo’s Institute for Quantum Computing, led by professor Thomas Jennewein and doctoral student Christopher Pugh, used many space-rated electronic components for its QKD receiver in anticipation of use in future satellites. Its ground transmitter, which was situated near a general-aviation airport in southern Ontario, employed two infrared lasers and the standard BB84 photon-polarization protocol (the technique of QKD was proposed by Charles H. Bennett and Gilles Brassard in 1984). The receiver, carried aboard a research aircraft, consisted of a 10-cm-aperture refractive telescope hitched to custom-designed sensors and controllers, including a dichroic mirror that separated the quantum and beacon signals. Both the transmitter and receiver used beacon lasers and tracking mechanisms to help find each other.

The aircraft made 14 passes at approximately 1.6-km above sea level, with line-of-sight distances to the transmitter of 3 to 10 km and the plane flying up to 259 km/h. The team registered a signal on seven of the 14 passes and extracted a secret key, up to 868 kilobits long, from six of those seven. According to the Canadian team, the equipment maintained milli-degree pointing precision while the receiver was moving at an angular speed simulating that of a low-Earth-orbit spacecraft. The experiments lay a foundation for Canada’s future Quantum Encryption and Science Satellite mission.

Last August, China launched the world’s first satellite for quantum optics experiments. Now researchers from multiple Chinese academic institutions have transmitted entangled photons from two widely separated ground stations via the orbiting satellite, officially named Quantum Experiment at Space Scale (QUESS) but informally dubbed Micius or Mozi after an ancient Chinese philosopher.

The team sent the transmission between two ground stations separated by 1203 km; the path lengths between QUESS and the stations, Lijiang in southwestern China and Delingha in the northern province of Qinghai, varied from 500 to 2000 km. One of the corresponding authors, Jian-Wei Pan of the University of Science and Technology of China, Shanghai, likens the satellite-borne message exchange to seeing a single human hair at a distance of 300 m, or detecting from Earth a single photon that came from a match’s flame on the moon.

Most of the photon loss and turbulence effects that plague free-space QKD occurs in the lower 10 km of the atmosphere, as the majority of the photons’ path is through a near vacuum. The Chinese researchers developed stable, bright two-photon entanglement sources with advanced pointing and tracking for both the satellite and the ground. Analysis of the received signals showed that the photons remained entangled and violated the Bell inequality. The researchers estimated that the link was 12 to 17 orders of magnitude more efficient than an equivalent long-distance connection along optical fibers.

Pan had wanted to experiment with space-borne quantum communications since 2003, when quantum-optics experiments usually happened on a well-shielded optical table. The following year, he participated in a distribution of entangled photon pairs through a noisy, ground atmosphere of 13-km path length. In 2010 and 2012, the group extended the ground-based teleportation range to 16 km and 100 km. “Through these ground-based feasibility studies, we gradually developed the necessary tool box for the quantum science satellite, for example, high-precision and high-bandwidth acquiring, pointing, and tracking,” Pan says.

And, according to Pan, the Chinese team will continue its quantum optical experiments at longer distances and also plan preliminary tests of quantum behavior under zero-gravity conditions.

Optics & Photonics News, 2017-06-15, Source: https://www.osa-opn.org/home/newsroom/2017/june/quantum_key_distribution_takes_flight/