Quantum Key Distribution Takes Flight

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Thanks to new research from two separate, global teams, QKDs may head up toward the sky and stars. [Image: iStock]

Three research teams—in Canada, in China, and in Germany—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 satellites 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). And researchers at the Max Planck Institute for the Science of Light, Germany, were able to demonstrate ground-based measurements of quantum states sent by a laser from a satellite 38,000 kilometers above Earth’s surface—using components not even designed for quantum communication (Optica, doi:10.1364/OPTICA.4.000611).

It's a bird, it's a plane, it's QKD

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.

Passing through a satellite

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.

Leveraging existing tools

A third set of experiments—conducted by a team led by OSA Member Christoph Marquardt, working in the research group of OSA Fellow Gerd Leuchs at the Max Planck Institute in Erlangen, Germany—built off of efforts toward satellite-to-earth optical communications by the German government, operating in partnership with the firm Tesat-Spacecom GmbH. And, notably, the experiments leveraged components not originally built for quantum communications.

In the German experiments, coherent beams from a 1065-nm  Nd:YAG laser communications terminal on the geostationary Earth orbiting satellite Alphasat I-XL, originally lofted into space in July 2013, were received at a transportable optical terminal then located at the Teide Observatory in Tenerife, Spain. The terminal was equipped with an adaptive-optics setup that corrected for phase distortions and piped the signal into a single-mode fiber, and used homodyne detection to pull out the quantum signature.

To show that a true quantum link between satellite and ground, through the turbulent atmosphere, was possible, the Max Planck team used a phase modulator in the satellite equipment to encode a number of binary phase-modulated coherent states on the light field—states known to be compatible with quantum communication. With amplification and processing of the signal, the researchers were able to reliably pick up those quantum states at the ground station, from a beam that had “propagated 38,600 km through Earth’s gravitational potential, as well as its turbulent atmosphere.”

“We were quite surprised by how well the quantum states survived traveling through the atmospheric turbulence to a ground station,” Marquardt noted in a press release. And, he said, the experiments suggested that the light beamed from a satellite to Earth could be “very well suited to be operated as a quantum key distribution network”—a surprising finding, he says, because the system was not built for quantum communication. In light of the work, he predicted that such a network “could be possible” in as little as five years.

[Note: This article was revised on 6/16/2017 to add the work by the Max Planck group. Stewart Wills contributed to this reporting.]

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