We present the activities of the research School of Astronomy and Astrophysics at the Australian National University to implement adaptive optics techniques for quantum communications through a free-space channel. We have developed a portable horizontal quantum key distribution system with adaptive optics and demonstrated improved secret key rates when using AO in a turbulent channel. We also report on development to equip the Mt Stromlo optical communications ground station with an adaptive optics system to enable quantum communication links between the ground and space.
We present the activities of the research School of Astronomy and Astrophysics at the Australian National University to implement adaptive optics techniques for quantum communications through a free-space channel. We have developed a portable horizontal quantum key distribution system with adaptive optics and demonstrated improved secret key rates when using AO in a turbulent channel. We also report on development to equip the Mt Stromlo optical communications ground station with an adaptive optics system to enable quantum communication links between the ground and space.
A collaborative research effort led by the Australian National University aims to develop an AO-assisted, quantum-encrypted free-space laser \ncommunications platform. It has been designed to correct strong turbulence effects whilst acquiring and maintaining links in unpredictable conditions. We \nadopt the continuous variable quantum key distribution scheme, making it much easier to integrate with out AO system and achieve secure, reliable, and \nhigh bandwidth communications at a relatively low cost. We present the development and results of a demonstrator platform designed to characterise the \nperformance of our systems over a ground-to-ground link of several kilometers.
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.
Satellites have the potential to support quantum communication over much longer distances than fibre optics networks and, as a result, there is a large amount of investment globally in quantum communication satellites. The Australian National University Optical Ground Station aims to provide ground services support for quantum communication satellites, and enhance the quantum link performance by the use of Adaptive Optics techniques. Numerical simulations have been carried out to assess the advantage of correcting the atmospheric turbulence on quantum downlinks with Adaptive Optics. Fibre coupling efficiency of the communications light onto a single mode fibre has been chosen as the performance metric. We pay special attention to the focusing optics and optimise them for the Single Mode Fibre coupling. We present the minimum requirements for an AO system to produce a significant improvement on the quantum link using off-the-shelf components. This paper gathers the main outcomes of the end-to-end numerical simulation and describes a potential pathway for future atmospheric corrected quantum communications.
Abstract For quantum communications, the use of Earth-orbiting satellites to extend distances has gained significant attention in recent years, exemplified in particular by the launch of the Micius satellite in 2016. The performance of applied protocols such as quantum key distribution (QKD) depends significantly on the transmission efficiency through the turbulent atmosphere, which is especially challenging for ground-to-satellite uplink scenarios. Adaptive optics (AO) techniques have been used in astronomical, communication, and other applications to reduce the detrimental effects of turbulence for many years, but their applicability to quantum protocols, and their requirements specifically in the uplink scenario, is not well established. Here, we model the effect of the atmosphere on link efficiency between an Earth station and a satellite using an optical uplink and how AO can help recover from loss due to turbulence. Examining both low Earth orbit and geostationary uplink scenarios, we find that a modest link transmissivity improvement of about 3 dB can be obtained in the case of a coaligned downward beacon, while the link can be dramatically improved, up to 7 dB, using an offset beacon, such as a laser guide star. AO coupled with a laser guide star would thus deliver a significant increase in the secret key generation rate of the QKD ground-to-space uplink system, especially as reductions of channel loss have a favourably nonlinear key-rate response within this high-loss regime.
This article offers a solution to the fragmentation problem through a unified model and simulation showing how Adaptive Optics (AO) improves both classical and quantum communications. Free-space optical links can carry Quantum Key Distribution (QKD) over distances that cannot be reached with fiber; however, atmospheric turbulence corrupts the wavefront. For a single-mode quantum receiver, this drives the quantum bit error rate above the security threshold before detector noise can even be considered. AO addresses this by restoring the wavefront and recovering the coupling loss. The theory, mitigation techniques, and quantum security analysis needed to understand this are spread across three research communities that rarely cite one another: atmospheric turbulence and AO theory, classical modulation over fading channels, and BB84 quantum key distribution. We consolidate and develops all three topics in order to trace the chain from wavefront error to bit error rate and secret key rate. We survey six families of turbulence mitigation and provide an assessment of their suitability for single-mode quantum links. In addition, we survey the free-space QKD experimental record, from terrestrial hops to satellite downlinks. A wave optics case study of a representative 500-km Low Earth Orbit (LEO) downlink, corrected by a closed-loop Shack–Hartmann and deformable-mirror receiver at 1.55 μm ties the review together. Across a turbulence sweep, the correction stabilizes single-mode coupling near its physical ceiling, while the uncorrected channel varies by more than an order of magnitude. When carried into the downlink budget, this lifts the quantum link across the 11% security bound at 12 of 15 turbulence conditions where the uncorrected link yields no key, thereby restoring a secure quantum bit error rate in a narrow 4.2–4.9% band throughout and a secret key rate of 7.6–11.2 kbit/s while simultaneously widening the margin of the co-propagating classical channel above its forward error correction threshold.
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.