A research team from the University of Calgary, Canada, and the University of Central Florida, USA, has modeled how spacecraft converging in low-Earth orbit equipped with low-loss signal-relay mirrors can serve as “satellite lenses” orbiting the Earth. Enabling globe-spanning quantum communication networks. (Photo: Courtesy of S. Goswami)

Researchers and industry are increasingly looking to the prospects of global communications networks that would take advantage of the security provided by quantum technology. One drawback, however, has been the lack of scalable “quantum repeaters” similar to the ones that keep optical signals alive in long-haul classical fiber networks.

As an alternative, some research groups are looking at satellite-based quantum communications, where quantum information is transmitted on laser beams between spacecraft in low Earth orbit (LEO). However, even satellite charts have their drawbacks. The loss of photons in the scattering lasers, combined with the curvature of the Earth itself, likely limits the realistic distances of highly efficient quantum links between LEO satellites to less than 2,000 kilometres.

Now, researchers Sumit Goswami of the University of Calgary, Canada, and Sayandeep Dara of the University of Central Florida, USA, have developed a proposal for how these pitfalls can be overcome (Phys. Rev. Appl., doi: 10.1103/PhysRevApplied).20.024048). Their proposal involves transmitting precise quantum signals via a series of relatively spaced, synchronously moving satellites. The pair suggest that these satellites could work effectively “like an array of lenses on an optical table”, focusing and bending rays along the Earth’s curvature and preventing photon loss over distances of up to 20,000 km — without the need for quantum repeaters.

It’s all done with mirrors

While Goswami and Dara figuratively refer to the nodes in the proposed all-satellite quantum network (ASQN) as satellite lenses, the optical magic actually happens with the mirrors, to keep the photon losses associated with absorption to an absolute minimum. In simple terms, one particular satellite in the chain sends a beam of light to the next moon, perhaps 120 kilometers away. The next satellite picks up the beam, refocuses it using a receiving mirror, and bounces it from two small mirrors to a final transmitting mirror, which transmits the signal to the next satellite in the chain.

Under their proposal, the researchers say, converging satellites effectively act “like an array of lenses on an optical table”, focusing and bending beams along the curvature of the Earth and preventing photon loss due to diffraction.

In their model, Goswami and Dara consider a series of satellites, each 120 kilometers apart; Given the expected beam deflection in Earth’s orbit, this means that the telescope diameter is 60 cm for each satellite. The team’s modeling indicates that this relay setup, using vortex beams to pass the quantum signal from one satellite to another, would virtually eliminate diffraction loss over distances of up to 20,000 km.

Other loss management

With diffraction loss concerned, Goswami and Dara systematically looked at other possible sources of loss in the satellite lens system. One obvious problem is the loss of reflection of some photons in the mirrors themselves, which the pair believe can be controlled by a combination of large metallic mirrors and superreflective small Bragg mirrors. Another source of loss lies in the tracking and positioning errors of the satellites in the chain; Such hiccups must be minimized to keep the satellites in sync with each other.

The ultimate source of loss has nothing to do with satellites. Depending on the architecture of quantum communications, quantum information must be transmitted to and from stations on Earth’s surface. For free-space optical signals, this opens the possibility of data loss due to atmospheric turbulence, which can greatly increase beam size and spread.

Disturbance turns out to be a much bigger problem for data in the uplink (from the ground to the satellite) than the downlink (from the satellite to the ground). This is because in the uplink, the disturbance does its dirty work at the beginning of the communication chain, not at the end of it; Thus the beam divergence and fragmentation caused by disturbance is amplified across the large propagation distance of the satellite network as a whole.

Superior Fiber – No Duplicates

Diagrams of quantum communication diagrams

For the proposed All-Satellite Quantum Network (ASQN), Goswami and Dara have designed two different quantum communication schemes. In one, qubit transmission (top), photons are transmitted from a ground source to a first satellite, relayed through space along a series of reflecting satellites, and sent to another ground station, with beam diffraction controlled by focusing. In the other method, entanglement distribution, the source of the entanglement is located either in a satellite (S1) or on Earth (S2), and the entangled photons are distributed to widely separated ground stations, where they are tested for secure quantum communication. (Photo: Reprinted with permission from S. Goswami and S. Dhara, Phys. Rev. Appl. 20, 024048 (2023), doi: 10.1103/PhysRevApplied.20.024048; Copyright 2023 American Physical Society (enlarged image)

Taking all these sources of loss (and a few others) into account, Goswami and Dara numerically simulated how a series of relay satellite lenses would transmit quantum information under two scenarios. One is something called entanglement distribution, a protocol demonstrated by researchers in China on the MESUS satellite, in which photons in space are entangled and sent in different directions through the satellite’s lenses, eventually to be transmitted to widely separate stations on Earth and tested for quantum security.

The other is the simpler “quantum bit transfer” protocol, in which qubits are simply sent from an earth station to the first satellite, transmitted up the chain, and finally transmitted to a second, more distant earth station. Such a system would require a different type of optical design, to counteract the impact of disturbance on the satellite uplink. Goswami and Dara think this approach may have certain advantages, because it keeps the qubit source and detection at more controllable and better-equipped ground stations.

Under both scenarios, the team found that the total signal loss over 20,000 km would be about 30 decibels. This is comparable to the loss experienced over just 200 km of a direct fiber optic link (assuming a loss rate of 0.15 dB/km in fibre). “Such a low-loss, satellite-based protocol would enable robust, multimode global quantum communications, and would not require quantum memories or a redundant protocol,” Goswami and Dara wrote.

Great engineering is needed

“What this proposal essentially does,” Goswami noted in an email to OPN, “is that it shifts the task of creating a quantum network from physics to engineering.” But he added that some of the engineering work would likely not be trivial, especially in terms of designing and developing the satellites in the fleet. However, he and Dara stress in their paper that recent advances in space technology — exemplified by reusable launch vehicles from organizations like SpaceX and the vast arrays of conventional communications satellites being launched into low Earth orbit by a number of private companies — make a system like this. Because their ASQN is more feasible than it was in the past.

Goswami and Dara stress that recent advances in space technology make a system like their ASQN much more feasible than it was in the past.

Goswami told OPN that a chain of about 160 satellites is needed to cover the full distance of 20,000 km as described in the paper. He noted that such a single geostationary chain would cover most of the globe every three days while the Earth rotates under the constellation of satellites, so Goswami said, “Even just one chain can be used to connect many places at different times.” But creating a larger two-dimensional network, to enable uninterrupted quantum communications around the world, will require tens of thousands of new satellites.

Quantum network Yes, maybe the quantum internet

Goswami and Dara believe that by eliminating the need for quantum repeaters or quantum memory, the scheme they proposed and designed could unlock the range of possibilities built into a quantum network. These prospects include secure communication via quantum key distribution, interconnecting quantum computers, and precise quantum remote sensing.

However, the researchers acknowledge that a more complex network — the long-term vision of a “quantum internet” now being fleshed out in a variety of research labs — will still require some kind of quantum memory to ensure completely lossless data transfer. . However, Goswami and Dara argue that by eliminating diffraction loss, their setup should mitigate some of the more stringent efficiency requirements for the required quantum memory. Thus, some of their ASQN configurations, they write, could not only serve to build an orbital quantum network, but could prove “another interesting candidate for implementing a quantum internet.”

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