&ball; Physics 14, 7
A small prototype of a drone-based quantum network has successfully relayed a quantum signal over a kilometer of free space.
Airwaves are teeming with “classic” information from cell phones, radio stations, and Wi-Fi hubs, but one day these waves could carry quantum encrypted messages or data input for a quantum computer. A new experiment used a pair of hovering drones to distribute quantum information to two ground stations 1 km apart . This demonstration could lead to a drone-based quantum network that could be positioned – and easily repositioned – over a city or rural area.
Quantum communication promises fully secure message sharing. For example, two users could exchange encrypted messages using “entangled” photons, pairs of particles with a unique quantum-mechanical relationship. For each pair, a photon would be sent to each of the users, who would be alerted to any eavesdropping by a loss of entanglement between the photons. One of the most common methods of sending such quantum encrypted messages relies on optical fibers (see Viewpoint: Recording distance for quantum cryptography). But in fibers, a large part of the photons diffuse before reaching their destination. More photons can survive if quantum information is transmitted through the atmosphere, as in the quantum link established using a Chinese satellite in 2018 (see Focus: Intercontinental, quantum encrypted messaging and video). However, satellites are expensive and difficult to adapt to changing demands in the field.
Small drones carrying optical equipment could provide a flexible solution that could connect multiple users in a quantum network. “Drones can be deployed for a mobile quantum connection anytime and anywhere when needed,” says Zhenda Xie of Nanjing University in China. Unlike a fixed tower, drones can also move around to avoid pollution or fog.
Several teams around the world have worked on systems based on drones. Early last year, Xie and colleagues reported quantum binding using a single octocopter-style drone . The drone generated pairs of infrared photons whose polarization orientations were entangled. Using a high-speed tracking system, the drone directed one photon at a ground station labeled Alice and the other at a ground station labeled Bob. To collect the incoming photons, each station was equipped with a telescope with a 26 mm wide aperture and a single photon detector.
However, a major challenge for this form of optical communication comes from diffraction. As each photon propagates, its wavefront spreads out, like the beam of a flashlight. If this spreading makes the wavefront larger than the aperture of the telescope, the photon will be unlikely to be collected. The team selected a short station-to-station distance of 200m to ensure that diffraction effects were negligible.
To increase the separation of the stations, the team has now added a second drone that acts as a relay between the first drone and Bob. This drone collects the photons from the first drone and collimates them through an optical fiber. This process reshapes the wavefronts of photons – like a focusing lens does – so that photons have a better chance of reaching Bob’s telescope.
During a demonstration, the team positioned the two drones between Alice and Bob, with a drone-drone separation of 200 m and a drone-station separation of 400 m, i.e. a station-station distance of 1 km. The Alice detector recorded about 25% of the photons sent in its direction from the first drone, while the Bob detector registered about 4% of the photons sent towards it.
The team performed a version of the so-called Bell inequality test by comparing the polarizations of photons received at Alice and Bob. The results confirmed that the photons remained entangled, so the quantum information survived the trip. The team now plans to expand the size of the network with several drones that could provide quantum links across a city, for example.
Georg Harder, quantum engineer at the Parisian company Veriqloud, has experience in building photon entanglement systems on large optical tables. “It made me smile when I read that the authors managed to put everything in a drone,” he says. He adds that this demonstration opens up new options for quantum communication. “Until now, quantum networks require either dedicated fiber networks or very expensive satellite links. Drones complement these existing systems. “
One of the advantages of a drone system is that it can allow open space communication between partners who do not have a direct line of sight, explains Martin Bohmann, quantum information specialist at the Academy. Austrian Science Institute from Vienna. He points out that photon transmission losses must be reduced to make a multidrone system competitive with other quantum lattice technologies, but he believes such improvements are possible.
Michael Schirber is a correspondent editor for Physics based in Lyon, France.
- H.-Y. Liu et al., “Optical relay entanglement distribution using drones as mobile nodes”, Phys. Rev. Lett. 126, 020503 (2021).
- H.-Y. Liu et al., “Distribution of entanglement by drone to mobile quantum networks”, Natl. Sci. Tower. seven, 921 (2020).