New Proposal for a Global Quantum Communications Network

A team of researchers from the University of Calgary in Canada and the University of Central Florida in the USA have put forward a proposal for a global quantum communications network using a chain of closely spaced, synchronously moving satellites. This network would overcome the limitations of current satellite-based quantum communications, which are restricted by the loss of photons in diffracting laser beams and the curvature of the Earth.

In their proposal, the researchers suggest using mirrors to relay quantum signals across the satellite chain, acting as “satellite lenses” that focus and bend beams along the Earth’s curvature. By keeping absorption-related photon losses to a minimum, this relay setup would eliminate diffraction loss across distances of up to 20,000 km without the need for quantum repeaters.

The team’s modeling considered a chain of satellites separated by 120 km, each equipped with mirrors to capture, refocus, and transmit the quantum signals. The use of vortex beams would further enhance the efficiency of the relay system.

To address other potential sources of loss, the researchers proposed a combination of large metal mirrors and small, ultrahigh-reflectivity Bragg mirrors to minimize reflection loss. They also highlighted the importance of minimizing tracking and positioning errors to keep the satellites in sync. Atmospheric turbulence, which can increase beam size and spread, was identified as a significant source of loss in the uplink from ground to satellite.

The researchers modeled two different quantum communication schemes: qubit transmission and entanglement distribution. Under both scenarios, they found that the total signal loss across 20,000 km would be comparable to the loss experienced across only 200 km of a direct optical-fiber link.

This proposal offers a promising approach to creating a global quantum communications network that surpasses the limitations of traditional fiber networks. It has the potential to enable secure quantum communication across vast distances without the need for frequent quantum repeaters.

Definitions:
– Quantum repeaters: devices that extend the range of quantum communication by preserving and relaying quantum information.
– Diffraction loss: the loss of photons due to the bending and spreading of a laser beam.
– Vortex beams: beams of light that carry orbital angular momentum and have helical wavefronts.
– Qubit: the basic unit of quantum information, analogous to a classical bit.

Sources:
– Phys. Rev. Appl., doi: 10.1103/PhysRevApplied.20.024048