Abstract
We study information theoretical security for space links between a satellite and a ground-station. Quantum key distribution (QKD) is a well established method for information theoretical secure communication, giving the eavesdropper unlimited access to the channel and technological resources only limited by the laws of quantum physics. But QKD for space links is extremely challenging, the achieved key rates are extremely low, and day-time operating impossible. However, eavesdropping on a channel in free-space without being noticed seems complicated, given the constraints imposed by orbital mechanics. If we also exclude eavesdropper's presence in a given area around the emitter and receiver, we can guarantee that he has only access to a fraction of the optical signal. In this setting, quantum keyless private (direct) communication based on the wiretap channel model is a valid alternative to provide information theoretical security. Like for QKD, we assume the legitimate users to be limited by state-of-the-art technology, while the potential eavesdropper is only limited by physical laws: physical measurement (Helstrom detector) and quantum electrodynamics (Holevo bound). Nevertheless, we demonstrate information theoretical secure communication rates (positive keyless private capacity) over a classical-quantum wiretap channel using on-off-keying of coherent states. We present numerical results for a setting equivalent to the recent experiments with the Micius satellite and compare them to the fundamental limit for the secret key rate of QKD. We obtain much higher rates compared with QKD with exclusion area of less than 13 meters for Low Earth Orbit (LEO) satellites. Moreover, we show that the wiretap channel quantum keyless privacy is much less sensitive to noise and signal dynamics and daytime operation is possible.
Highlights
Quantum key distribution (QKD) was proposed in 1984 by Bennet and Brassard [1]
III, we propose a simple yet accurate channel model using binary modulated coherent states [onoff keying (OOK)], and derive and analyze the private capacity for Bob using realistic, state-of-the-art photon counting detectors while the signal reception by Eve is limited by only the laws of physics
As a realistic physical scenario, we use as a reference the recent experiment of QKD with the Chinese low earth orbit (LEO) satellite Micius [4,5]
Summary
Quantum key distribution (QKD) was proposed in 1984 by Bennet and Brassard [1]. Today, commercial fiberbased systems are available with operational distances steadily increasing [2,3]. A potential eavesdropper, Eve, has complete control of the quantum channel She can intercept all quantum states and perform any measurement allowed by quantum mechanics, including entangling the states with some auxiliary system and storing them in perfect quantum memories. There is some initial work investigating the consequences of restricted power for Eve on the performance of QKD links [21,22,23,24] Given their additional, reasonable assumptions, we find exactly the scenario where keyless private communication is possible. We present numerical results for a realistic scenario taking the performance of the Micius system as a reference and compare it to the fundamental limit for the secret key rate of QKD [27]
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