Satellite Quantum Network Optimisation

Quantum networks will undoubtedly play crucial role in advancing quantum technologies including quantum communication, distributed quantum computing, and quantum metrology. Currently a nascent topic, the fundamental understanding of how to properly and efficiently simulate usable quantum networks will have a profound impact on how to design and implement this technology on a global scale. In one application, such networks are used in quantum communication, wherein provably-secure quantum encryption protocols face important challenges. Reaching high key transmission rates over long distances through fiber or atmosphere is difficult due to exponential photon loss, and can be ameliorated by incorporating satellite links into the quantum communication network. Within this project, an optimized theoretical framework for a global network of satellites will be developed.

State of the Art

Over the last couple of decades, research progress on quantum technologies has seen them migrate out of the lab and into a plethora of real-world applications - from quantum computing and sensing to the focus of this project, quantum communication. Establishing a secure and reliable quantum communication network is currently of great interest, as this form of network could be used to securely transmit information over large distances, with important benefits to, for example, secure banking and government data processing [1].

At the heart of quantum communication lies Quantum Key Distribution (QKD). There are several approaches to QKD, such as BB84, with all being provably secure [2] and not just computationally difficult, as is the case for today’s globally implemented classical protocols. The security of QKD arises from the fact that if a malicious eavesdropper tries to intercept and alter a quantum state, it will be detected by the legitimate communicating partners. Furthermore, the no-cloning theorem forbids an eavesdropper from copying the state of a quantum system, which would make for easy interception and replacement. These two statements form the basis of provably-secure QKD.

The principal problem faced in entanglement distribution over long distances stems from transmission losses of single photons along the channel, which grow exponentially with distance through both fiber and atmosphere. In order to mitigate this loss, signal amplification, as in classical communication, does not work in the quantum context due to the no-cloning theorem. One solution to this problem is to establish ground to space satellite links for the quantum channel, which benefit from the exponential drop off in atmospheric density with increasing altitude. Thus, creating a satellite-based QKD network is an important next step towards a global quantum-enabled internet.

Theoretical frameworks to model single satellite-to-ground-station systems have been studied [3,4]. However, these channels are difficult to optimize due to the many parts involved in building such an infrastructure, such as QKD sources, the links, and detectors: Different sources operate at different wavelengths, and have varied efficiencies for different QKD schemes [5, 6, 7]; The optical links involve considering uplink versus downlink infrastructures, and thoroughly analyzing the effect of the atmosphere [8]; Pointing, acquisition, and tracking also play a role in the channel analysis, as these affect the type and size of the transmitters and receivers [9]; Finally, the type of detectors used also plays a role in the fidelity of the quantum system. Detector dark count and thermal management are important to consider [8]. Figure 1 summarizes all of the noise and loss contributions in the system, which is dependent on many factors including the wavelength chosen, the diameter of the receivers and transmitters, and the kind of detectors among others.

Project Goals

The goal of this project is to develop a modular channel model which includes all relevant sources of noise and loss in such a system, and to create an optimization tool which selects the best parameters for a given infrastructure. This will also take into account satellite orbits and their effects on communication windows. Once a single link system has been established, we will be able to simulate a dynamic multi-satellite network with several ground stations, with room for further optimization for parameters such as the minimum number of satellites. Ultimately, we will provide a modular approach for simulating a global network for various cryptographic schemes and satellite constellations, paving the way for a global quantum internet.

References:

1. S. Wehner, D. Elkouss, and R. Hanson, “Quantum internet: A vision for the road ahead,” Science, vol. 362, oct 2018.

2. C. H. Bennett and G. Brassard, “Quantum cryptography: Public key distribution and coin tossing,” Theoretical Computer Science, vol. 560, pp. 7–11, mar 2020.

3. A. Scriminich et al., “Optimal design and performance evaluation of free-space Quantum Key Distribution systems.” 2022.

4. E. Kerstel, A. Gardelein, M. Barthelemy, S. K. Joshi, and R. Ursin, “Nanobob: A Cubesat Mission Concept For Quantum Communication Experiments In An Uplink Configuration,” p. 34.

5. Rarity, J., Tapster, P., Gorman, P. & Knight, P. Ground to satellite secure key exchange using quantum cryptography. New. J. Phys. 4, 82 (2002).

6. Yin, J. et al. Satellite-based entanglement distribution over 1200 kilometers. Science 356, 1140–1144 (2017)

7. Durak, K. et al. The next iteration of the small photon entangling quantum system (SPEQS-2.0). Adv. Photonics Quantum Comput. Mem. Commun. IX, 976209 (SPIE, 2016).

8. R. Bedington, J. M. Arrazola, and A. Ling, “Progress in satellite quantum key distribution,” npj Quantum Inf, vol. 3, no. 1, pp. 1–13, Aug. 2017, doi: 10.1038/s41534-017-0031-5.

9. Elser, D. et al. Network architectures for space-optical quantum cryptography service. in Proceedings International Conference on Space Optical Systems and Applications (ICSOS) 2012, Post-1, Ajaccio, Corsica, France, October 9–12, 12 (2012).