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Quantum Communications and Quantum Imaging

Prospects of scalable quantum key distribution using nano satellites

Lukas Knips, Michael Auer, Adomas Baliuka, Moritz Birkhold, Michael Steinberger, Harald Weinfurter

Published Oct 4, 2024• 3 citations
Publisher

Abstract

Quantum Key Distribution (QKD) enables the secure distribution of secret keys for cryptographic purposes between two trusted parties over a quantum channel. For reaching global-scale quantum key distribution, satellite-based approaches are most promising as already demonstrated by the Chinese MICIUS mission. Here, we summarize our efforts in building a very compact QKD sender unit featuring polarization-encoded BB84 with weak coherent pulses. The unit is suitable for deployment in cube satellites such as our missions QUBE and QUBE-II which aim to demonstrate that affordable and scalable key distribution is possible in the near future.

DOI
10.1117/12.3027336

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Quantum Key Distribution (QKD) enables the secure distribution of secret keys for cryptographic purposes between two trusted parties over a quantum channel. For reaching global-scale quantum key distribution, satellite-based approaches are most promising as already demonstrated by the Chinese MICIUS mission. Here, we summarize our efforts in building a very compact QKD sender unit featuring polarization-encoded BB84 with weak coherent pulses. The unit is suitable for deployment in cube satellites such as our missions QUBE and QUBE-II which aim to demonstrate that affordable and scalable key distribution is possible in the near future.
Quantum Key Distribution (QKD) is a method to securely share secret keys between two parties via a quantum channel. Our QUBE mission [1], launched in summer 2024, is a pathfinder mission to test the suitability of components for economic global-scale QKD based on CubeSats. QUBE and its successor QUBE-II [1]–[3] are joint research projects by the German Aerospace Center Institute of Communications and Navigation (DLR-IKN), the Friedrich Alexander University Erlangen-Nuremberg (FAU), the Max Planck Institute for the Science of Light (MPL), Zentrum fur Telematik (ZfT), OHB System AG, and the Ludwig Maximilian University Munich (LMU).
Quantum key distribution (QKD) is a method for establishing secure cryptographic keys between two parties who share an optical, “quantum” channel and an authenticated classical channel. To share such keys across the globe, space-based links are required and in the near term these will take the form of trusted node, key management satellites. We consider such channels between two nanosatellite spacecraft for polarization entanglement-based QKD, and the optical channel is described in detail. Quantum channels between satellites are useful for balancing keys within constellations of trusted node QKD satellites and, in the future, may have applications in long-distance qubit exchange between quantum computers and in fundamental physics experiments. The nanosatellite mission proposed uses an optical link with 80-mm diameter optical terminals. If such a link could be maintained with 10-μrad pointing accuracy, then this would allow QKD to be performed for satellite separations up to around 400 km. A potential pointing and tracking system is also described although currently this design would likely limit the satellite separation to 100 to 150 km.
In 2017, the Micius mission [1] showed that satellite quantum key distribution (QKD) can surpass the range limit of fiber-based QKD [2]. Due to the high launch cost of satellites the cost of these systems would be reduced significantly if the size, weight and power of the satellites could be reduced. This requires the use of new integration technologies, like the miniaturized modules based on photonic integrated circuits (PIC) tested in the QUBE mission [3], [4]. Using the knowledge gained there, the QUBE-II mission aims to launch an 8U CubeSat(10×20×40cm3)(10\times 20\times 40\text{cm}^{3})(10×20×40cm3)with the goal of performing QKD between the cubesat and an optical ground station [5].
In the absence of technically mature quantum repeaters, losses in optical fibers limit the distance for ground-bound quantum key distribution (QKD). One way to overcome these losses is via optical links to satellites, which has most prominently been demonstrated in course of the Chinese-Austrian QUESS mission. Though its findings were impressive, such a large-scale project requires massive financial and time resources. We propose a 34x10x10cm³ nanosatellite orders of magnitude cheaper which is able to perform QKD in a trusted-node scenario, using only commercially available components. We have performed a detailed analysis of such a CubeSat mission (“Q3Sat”), finding that cost and complexity can be reduced by sending the photons from ground to satellite, i.e. using an uplink. Calculations have been done for a prepare-and- send protocol (BB84 with decoy pulses) using polarization as information carrier. We have created a preliminary design of a 3U CubeSat including a detailed size, weight and power budget and a CAD to account for the assembly of the components. Deploying a 10 cm long mirror telescope covering the small surface of the satellite leaves enough space for a polarization analysis module and housekeeping, communication and computing electronics. For one such CubeSat, we estimate the quantum secure key to be acquired between two ground stations during one year to be about 13 Mbit. A Bell test between ground and satellite would also be feasible. The uplink design allows to keep the more sensitive, computation-intensive and expensive devices on ground. The experiment proposed by us therefore poses a comparably low-threshold quantum space mission. For a two-year lifetime of the satellite, the price per kilobit would amount to about 20 Euro. In large constellations, Q3Sats could be used to establish a global quantum network, which would further lower the cost. Summarizing, our detailed design and feasibility study can be readily used as a template for global-scale quantum communication.