Academic and industry researchers are increasingly eyeing the prospect of global communications networks that would take advantage of quantum technology. Long-distance quantum communication can be achieved by directly sending light through space using a train of orbiting satellites that function as optical lenses without using repeaters.

Some research groups are looking at satellite-based quantum communications, in which quantum information would ride on laser beams between spacecraft in low Earth orbit (LEO). However, the loss of photons in diffracting laser beams, as well as the curvature of the Earth itself, would likely limit realistic distances of high-efficiency quantum links between LEO satellites to less than 2000 km.

Image Credit: S. Goswami/University of Calgary

Recently, researchers Sumit Goswami of the University of Calgary, Canada, and Sayandip Dhara of the University of Central Florida, US, have laid out a proposal showing how those pitfalls could be overcome (Phys. Rev. Appl., doi: 10.1103/PhysRevApplied.20.024048). Their proposal involves relaying delicate quantum signals across a chain of relatively closely spaced, synchronously moving satellites. Those satellites, the pair suggests, could effectively act “like a set of lenses on an optical table,” focusing and bending beams along Earth’s curvature and preventing photon loss across distances as great as 20,000 km—without the need for quantum repeaters. Goswami said  that a chain of around 160 satellites would be needed to cover the full 20,000-km distance modeled in the paper. Such a single, geostationary chain, he noted, would cover most of the globe every three days as the Earth rotated beneath the satellite array—so, Goswami said, “even just one chain can be used for connecting many places at different times.” But a larger, 2D network, to enable uninterrupted worldwide quantum communications, would require tens of thousands of new satellites.

While Goswami and Dhara metaphorically refer to the nodes in their proposed all-satellite quantum network (ASQN) as satellite lenses, in reality the optical magic happens with mirrors, to keep absorption-related photon losses to an absolute minimum. In simplified terms, a given satellite in the chain sends a beam of light to the next one, perhaps 120 km away. That next satellite captures and refocuses the beam with a receiving mirror and bounces it off of two smaller mirrors to a final transmitting mirror, which relays the signal on to the next satellite in the chain.

In their modeling, Goswami and Dhara considered a chain of satellites, each separated from the next by 120 km; given expected beam divergence in Earth orbit, that implies a telescope diameter of 60 cm for each satellite. The team’s modeling suggests that such a relay setup, using vortex beams to pass the quantum signal from satellite to satellite, would virtually eliminate diffraction loss across distances of 20,000 km.

With diffraction loss taken care of, Goswami and Dhara methodically looked at other potential sources of loss in the satellite-lens system. One obvious one is reflection loss of some photons at the mirrors themselves, which the pair thinks could be kept manageable through a configuration combining large metal mirrors and small, ultrahigh-reflectivity Bragg mirrors. Another source of loss lies in tracking and positioning errors for the satellites in the chain; such hiccups would need to be held to a minimum to keep the satellites in sync with one another.

A final source of loss has nothing to do with the satellites. Depending on the quantum communication architecture, quantum information needs to be transmitted from and to stations on Earth’s surface. For free-space optical signals, that opens the prospect of data losses due to atmospheric turbulence, which can dramatically increase the beam size and spread.  Turbulence turns out to be a much bigger problem for data in the uplink (ground to satellite) than in the downlink (satellite to ground). That’s because in the uplink, the turbulence is doing its dirty work at the beginning of the communication chain rather than at its end; thus the turbulence-induced beam divergence and fragmentation is magnified across the large propagation distance of the satellite network as a whole.

Outperforming fiber—without repeaters:

 

Image Credit: S. Goswami and S. Dhara, Phys. Rev. Appl. 20, 024048 (2023), doi: 10.1103/PhysRevApplied.20.024048; copyright 2023 by the American Physical Society [Enlarge image]

For their proposed all-satellite quantum network (ASQN), Goswami and Dhara modeled two different quantum communication schemes. In one, qubit transmission (top), photons are transmitted from a ground-based source to a first satellite, relayed through space along a chain of reflector satellites, and beamed to another ground station, with beam diffraction controlled by focusing. In the other, entanglement distribution, an entanglement source is located either in a satellite (S1) or on the ground (S2), and entangled photons are distributed to widely separated ground stations, where they’re tested for quantum-secure communication.

Taking all of these sources of loss (and a few others) into account, Goswami and Dhara numerically simulated how such a chain of relay satellite lenses might work in transmitting quantum information under two scenarios. One is so-called entanglement distribution, the protocol demonstrated by researchers in China on the Micius satellite, in which photons are entangled in space and sent in different directions via the satellite lenses, ultimately to be transmitted down to widely separated stations on Earth and tested for quantum security.

The other is a simpler “qubit transmission” protocol, in which quantum bits (qubits) are simply sent from a ground station to the first satellite, transmitted across the chain and finally beamed down to a second, distant ground station. Such a system would require a different kind of optical design, to counteract the impact of turbulence on the satellite uplink. Goswami and Dhara think this approach may have certain advantages, however, as it keeps both the qubit source and detection in more controllable, better-outfitted ground stations.

Under both scenarios, the team found that the total signal loss across 20,000 km would come in at around 30 dB. That’s comparable to the loss experienced across only 200 km of a direct optical-fiber link (assuming a loss rate of 0.15 dB/km in the fiber). “Such a low-loss satellite-based optical-relay protocol,” Goswami and Dhara write, “would enable robust, multimode global quantum communication and would not require either quantum memories or repeater protocol.”

“What this proposal basically does,” Goswami observed in an email to OPN, “is that it shifts the task of creating quantum network from physics to engineering.” He added, however, that some of the engineering likely wouldn’t be trivial, particularly with respect to designing and developing the satellites in the fleet. Still, he and Dhara stress in the paper that recent developments in space technology—embodied in reusable launch vehicles from organizations such as SpaceX and the vast constellations of classical-communications satellites being lofted into LEO by a number of private companies—make a system such as their ASQN considerably more feasible than it would have been in the past.  Goswami and Dhara stress that recent developments in space technology make a system such as their ASQN considerably more feasible than it would have been in the past.

Goswami and Dhara believe that, by dispensing with the need for quantum repeaters or memory, the scheme they’ve proposed and modeled could open a range of possibilities implicit in a quantum network. Such prospects include secure communication via quantum key distribution, the linking of quantum computers, and precision long-distance quantum sensing.

The researchers admit, however, that a more complex network—that is, the long-term vision of a “quantum internet” now being fleshed out in a variety of research labs—would still require some sort of quantum memory to ensure completely lossless transmission.

This research could pave the way for the development of globally secure quantum communications networks, as the use of satellites would provide a high level of security against hacking and eavesdropping. The proposed system still needs further development and testing, but it presents a promising solution for enabling long-distance quantum communication without the need for repeaters.

References:

https://www.optica-opn.org/home/newsroom/2023/august/building_a_quantum_network_with_satellite_lenses/

https://physics.aps.org/articles/v16/s103

New Proposal for a Global Quantum Communications Network