Preparation of free-space quantum key distribution using adaptive optics for long-distance communication

Sheng‐Kai Liao, Hai-Lin Yong, Chang Liu +22 more

Francis Bennet, Ping Koy Lam, Lyle E. Roberts +4 more

Lukas Scarfe, Felix Hufnagel, Manuel F. Ferrer-Garcia +3 more

Quantum Key Distribution (QKD) promises a provably secure method to transmit information from one party to another. Free-space QKD allows for this information to be sent over great distances and in places where fibre-based communications cannot be implemented, such as ground-satellite. The primary limiting factor for free-space links is the effect of atmospheric turbulence, which can result in significant error rates and increased losses in QKD channels. Here, we employ the use of a high-speed Adaptive Optics (AO) system to make real-time corrections to the wavefront distortions on spatial modes that are used for high-dimensional QKD in our turbulent channel. First, we demonstrate the effectiveness of the AO system in improving the coupling efficiency of a Gaussian mode that has propagated through turbulence. Through process tomography, we show that our system is capable of significantly reducing the crosstalk of spatial modes in the channel. Finally, we show that employing AO reduces the quantum dit error rate for a high-dimensional orbital angular momentum-based QKD protocol, allowing for secure communication in a channel where it would otherwise be impossible. These results are promising for establishing long-distance free-space QKD systems. High-dimensional quantum key distribution will allow for higher information density and greater error tolerance in future quantum networks. This work experimentally demonstrates how implementing an adaptive optics system in a spatial-mode free-space optical link can allow for quantum communications where it would otherwise be impossible.

Yijun Wang, Xiaodong Wu, Ling Zhang +3 more

Edoardo Rossi, Francesco Picciariello, Ilektra Karakosta-Amarantidou +4 more

Quantum Key Distribution (QKD) is a cryptography technique that allows two parties to share a secret common key that can be used for a secure exchange of information. To meet the requirements and interests of different final users, the technological development of this technique has rapidly led toward commercial systems for a global quantum infrastructure, namely a quantum network. Communication among parties requires an optical channel, which for short distances is typically provided by optical fiber. However, the limitation of optical losses on longer fiber links, or the unavailability of an exploitable deployed fiber, requires the use of a different QKD medium, i.e. free-space. At the University of Padova we are investigating, both theoretically and experimentally, how Adaptive Optics (AO) can be used to improve the performance of free-space quantum communication, with a particular focus on QKD. Following our theoretical studies on the optimal design for an intermodal free-space/ fiber link, we were able to realize different QKD field trials along a 600m-long free-space link exploiting polarization encoding. Besides the metropolitan area of Padova, we are now working on the development of an extra-urban free-space link, which is more prone to atmospheric turbulence effects. For this link, we equipped a radar tower located on a hill in Teolo with a remotised optical transmitter, and we are now developing the AO system for the receiving telescope, located at the Department of Information Engineering in Padova, with which we plan to realize QKD during day and night time.

Naveed Mahmud, Andrew MacGillivray, Apurva Rai +5 more