The paper provides an overview of the current advances of space quantum communications, including the most viable applications. While traditional computers operate on linear processing, quantum computers use parallel processing, thus enabling performing very complex computations very easily and fast. This is particularly important in regards to cryptography, as quantum computing might easily break some of the strongest defences we currently utilise.

Quantum communications mainly uses to quantum effects – the superposition of the qubit and quantum entaglement (particles remain connected even across large distances; measuring one particle will affect its entangled particle).

• The main network application of quantum is QKD, Quantum Key Distribution, a protocol for secure communication that utilises quantum mechanics properties to exchange cryptographic keys. Classical encryption protocols rely on prime factoring (e.g RSA). Advances in quantum computing (e.g Shor's algorithm) show that quantum computers could factor primes, rendering current encryption protocols useless. QKD relies on quantum properites to always detect the presence of a third party, therefore rendering eavesdropping and MTM attacks void.
• Quantum entanglement is also useful for errorless clock synchronisation (atomic clocks can error up to 10 nanoseconds per 24 hours), which is crucial in some applications that rely on exactly the same time parameters (e.g timestamping). 
• Quantum entanglement distribution would mean that multiple ground stations would cooperate with multiple satellites – OGS in a city connected to a satellite delivering QKD links to users. Ultimately, quantum computers would be deployed on satellites.
• Quantum detection methods can increase efficiencies for LR optical communications. For transmitting, high-powered lasers and high diameters are necessary; for reception, large diameter optics receivers.

Some current developments include the Chinese QUESS, which achieved three milestones: secure satellite-to-ground exchange of cryptographic keys, capability of two-photon entanglement distribution to ground stations with 1200km distance, demonstrating feasibility of a global network and quantum teleportation of independent single-photon qubits. The total quantum communication distance achieved was 4600km.

CubeSats also show great promise, as they allow for rapid and low-cost development. 6U cubesats are normally utilised, with several missions underway by Canada, Singapore, UK and others.

NASA's Deep Space Quantum Link is pioneering experiments for relativistic effects on quantum systems. They are considering Lunar Gateway as a potential orbit – minimum 100MHz with very low jitter needed for Lunar-Earth link.

Challenges:

• restricted transmission times
• physical limitations
• very high losses
• atmospheric and environmental impact
• etc

The paper proposes further research and developing of free-space optical links. For success, we need enhanced capability of exchanging large volumes of classical data; the question of sources for photons needs to be solved; better detection capabilities, cryogenic systems and clock synchronisation is also needed. The main takeaway is to increase the robustness of quantum signals, as noise is a problem in free-space channels; also components should be engineered for space-preparedness.

• Quantum comms allow us to overcome the limitations of ground based networks
• Integrating space-based systems with terrestrial optical networks helps us to realise a global quantum internet
• From its nascence in 1990s to the successful in-orbit demonstration of the Micius satellite, space-based quantum communications is quickly gaining traction
• Quantum internet will impact everything from fundamental science to commercial ventures to security

Source:

Sidhu, J. S., Joshi, S. K., Gundogan, M., Brougham, T., Lowndes, D., Mazzarella, L., Krutzik, M., Mohapatra, S., Dequal, D., Vallone, G., Villoresi, P., Ling, A., Jennewein, T., Mohageg, M., Rarity, J., Fuentes, I., Pirandola, S., & Oi, D. K. L. (2021). Advances in Space Quantum Communications. IET Quantum Communication, 2(4), 182–217. https://doi.org/10.1049/qtc2.12015