Secure communications and data protection are fundamental in an information-based society, with a wide range of applications such as financial transactions, ensuring personal privacy, and maintaining the integrity of critical infrastructure in the Internet of things. Quantum key distribution (QKD) is a pioneering quantum communication technology on the brink of widespread deployment as an ultimately secure encryption method. QKD consists in the distribution of a secure key between two distant parties, which here we will call Alice and Bob, which is subsequently used for the symmetric encryption of their private messages. Classical encryption methods, used nowadays for instance for bank transfers and messages, are based on the exchange of a key that works for a limited time, the security of which is only guaranteed by the fact that current computers are unable to decrypt it during this time. In QKD instead, the quantum feature of entanglement allows the key exchange with privacy guaranteed by the very laws of physics.
However, if the two parts are more than a few hundred kilometers apart, the distribution of the secret key at practical rates of data exchange remains a major challenge, due to high losses in the transmission of the entangled photon pairs carrying the key. One approach to circumvent this is the deployment of optical satellite links to transmit the key. Consequently, in what has been called the quantum space race, international research groups are pursuing first missions involving space links, with first dedicated satellite transmitter payloads successfully launched into space.
Optimising these non-static satellite quantum links to yield the highest possible key exchange rate is challenging, but essential for their successful operation. In their new study, our researchers present an optimisation for high-loss free-space links with state-of-the-art hardware. They developed a high-brightness polarization-entangled photon pair source and a quantum ground receiver for polarization-based QKD protocols, which is compatible with most existing optical ground stations with satellite tracking capabilities. They also developed a model-based method to dynamically optimise the link parameters in orbit based on the current channel and receiver conditions.
They then performed an experimental field trial, where the two communicating parties were located on the Canary islands of La Palma (Alice) and Tenerife (Bob). The scientists were able to distribute photons between the two parties over a terrestrial free-space link at a distance of 143 km. This distance constitutes a worst-case scenario for satellite-based QKD via low Earth orbit (LEO) satellites in terms of channel attenuation, astronomical seeing and sky noise. Nevertheless, they were able to obtain the highest key rates to date over a terrestrial free-space link. "The employment of state-of-the-art quantum sources and receivers together with the dynamical prediction of optimal parameters enable high key rates and pave the way towards QKD on a global scale", says Sebastian Ecker, PhD student at IQOQI-Vienna and first author of the study. "These results will therefore prove helpful in the design and operation of future satellite missions, advancing the distribution of secret keys on a global scale for a future secure quantum internet."