No Memory Required! Global quantum communication by satellite

Long-distance quantum communications can be realized by using a train of orbiting satellites as optical lenses to send light directly through space.

Transmitting quantum information between widely separated locations is necessary to develop a global quantum network. The high photon losses inherent in long-distance fiber optic transmission, the default for optical quantum bits, have hindered this project. To address this issue, Chinese researchers have demonstrated the transmission of quantum signals via satellite.

A series of demonstrations on China's "Mozi" have laid the groundwork for a satellite-based quantum communications network.

Now, Sumit Goswami of the University of Calgary in Canada and Sayandip Dhara of the University of Central Florida have shown how quantum information can be relayed over long distances via such satellite networks.

The results were published Aug. 18 in Applied Physical Review under the title "Satellite-Relayed Global Quantum Communication without Quantum Memory."

The program involves an array of LEO satellites, each equipped with a pair of reflecting telescopes. A particular satellite uses one telescope to receive light quantum bits and then uses the other telescope to transmit the quantum bits. The satellites would effectively bend photons around the curvature of the Earth while controlling photon loss due to beam divergence. The team likened this scheme to a set of lenses on an optical bench.

Simulations of satellites 120 kilometers apart and equipped with 60-centimeter-diameter telescopes showed that beam divergence losses disappeared. At a distance of 20,000 km, the total loss (mainly reflection loss, but also alignment and focusing errors) can be reduced to several orders of magnitude less than that of a few hundred kilometers of optical fiber. Ultra-high reflectivity telescope mirrors can further reduce this loss.

Goswami and Dhara studied two protocols based on this setup. In one of the protocols, two entangled photons are transmitted in opposite directions from a satellite light source. In the other protocol, quantum bits are transmitted in one direction, with both the light source and the detector on the ground. The latter performs well, despite the effects of atmospheric turbulence, and has the advantage of keeping the necessary quantum hardware on Earth.

The schematic explains how a specific lensing device can completely control beam divergence due to diffraction indefinitely.

(a)-(d) illustrate various possible telescope setups applicable to a chain of satellite reflectors, showing beam focusing and bending. (a) and (b) show off-axis telescope setups, while (c) shows an on-axis setup. The on-axis setup (without the folding mirror) can be modeled with the screen-lens-screen system in (d). The diffraction loss through this on-axis system can be controlled by using vortex beams. (e) Simulation of entangled photon pairs propagating 20,000 km in a vortex beam profile by an on-axis system showing almost identical initial and final beams (after 10,000 km of propagation per photon). (f) Plot of the transmission probability of entangled photon pairs as a function of distance (not including ground link or other losses).

Reference links:

[1]https://physics.aps.org/articles/v15/172

[2]https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.20.024048

[3]https://www.science.org/doi/10.1126/science.aan3211?utm_source=general_public&utm_medium=youtube&utm_campaign=vid-quantum-sate- 13632

[4]https://physics.aps.org/articles/v16/s103