• Physics 14, 7
A small prototype of a drone-based quantum network has successfully transmitted a quantum signal over a kilometer of free space.
X.-H. Tian, H.-Y. Liu, & Z. Xie / Nanjing Univ.
The airwaves are chock-full of “classic” information from cell phones, radio stations, and Wi-Fi hubs, but one day the waves may carry quantum-encrypted messages or data input for a quantum computer. A new experiment used some drones to distribute quantum information to two ground stations separated by 1 km [1]. This demonstration could lead to a drone-based quantum network that can be positioned over an urban or rural area and easily repositioned.
Quantum communication promises to share secure messages. For example, two users can exchange encrypted messages using ‘tangled’ photons, particle pairs with a unique quantum-mechanical relationship. For each pair, one photon will be sent to each of the users, who will 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 is based on optical fibers (see Viewpoint: Record distance for quantum cryptography). But in fibers, a large fraction of the photons disperse before they reach their destination. More photons can survive if quantum information is transmitted through the atmosphere, as in the quantum link established by a Chinese satellite in 2018 (see Focus: intercontinental, quantum-encrypted messages, and video). However, satellites are expensive and difficult to adapt to changing demands on the ground.
Small drones with optical equipment can provide a flexible solution that can connect multiple users in a quantum network. “Drones can be used at any given time and place for a mobile quantum connection where needed,” says Zhenda Xie of Nanjing University in China. Unlike a fixed tower, drones can also move around to prevent pollution or fog.
X.-H. Tian, H.-Y. Liu, & Z. Xie / Nanjing Univ.
Several teams around the world have been working on drone-based systems. Early last year, Xie and colleagues reported a quantum link using a single octocopter-style drone [2]. The drone generated pairs of infrared photons whose polarization orientations were entangled. Using a speed tracking system, the drone aimed one photon at a ground station named Alice and the other at a ground station named 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 through diffraction. As each photon propagates, its wavefront propagates like the ray of a flashlight. If this distribution makes the wavefront larger than the telescope aperture, the photon will have little chance of being captured. The team chose a short station-to-station distance of 200 m to ensure that the demolition effects were negligible.
To increase station separation, the team has now added a second drone that serves as a relay between the first drone and Bob. This drone collects photons from the first drone and collimates them through an optical fiber. This process reshapes the photon wave fronts – similar to what a focus lens does – so that the photons have a greater chance of reaching Bob’s telescope.
In a demonstration, the team positioned the two drones between Alice and Bob, with drone-to-drone separation of 200 m and drone-to-station separation of 400 m – which is a station-to-station distance of 1 km. The Alice detector recorded about 25% of the photons sent its direction from the first drone, while the Bob detector recorded about 4% of the photons sent there.
The team performed a version of the so-called Bell inequality test by comparing the photon polarizations received from Alice and Bob. The results confirmed that the photons remained entangled so that the quantum information survived the journey. The team now plans to increase the size of the network with several drones that can provide quantum links in a city, for example.
Georg Harder, a quantum engineer at the Paris company Veriqloud, has experience building photon entanglement systems on large optical tables. “It made me smile when I read that the writers could put it all in a drone,” he says. He adds that this demonstration offers new options for quantum communication. “So far, quantum networks require dedicated fiber networks or very expensive satellite links. The drones complement these existing systems. ”
One advantage of a drone system is that it can allow free spatial communication between partners who do not have a direct line of sight, says Martin Bohmann, a quantum information specialist at the Austrian Academy of Sciences in Vienna. He points out that photon transfer losses need to be reduced to make a multidron system competitive with other quantum network technologies, but he believes such improvements are possible.
–Michael Schirber
Michael Schirber is a corresponding editor for Physics based in Lyon, France.
References
- HE. Liu et al., “Options-divert entanglement distribution using drones as mobile nodes,” Fis. Ds Lett. 126, 020503 (2021).
- HE. Liu et al., “Drone-based entanglement distribution to mobile quantum networks,” Natl. Sci. Ds. 7, 921 (2020).