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International Conference on Space Optics (ICSO)

AO-enhanced quantum communications with the ANU Optical Ground Station

Noelia Martínez Rey, Sebastian P. Kish, Francis Bennet, Marcus Birch, Ping Koy Lam, Kate Ferguson

Published Jul 12, 2023• 2 citations
Publisher

Abstract

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.

DOI
10.1117/12.2691350

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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.
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Abstract Future quantum communication infrastructures will rely on both terrestrial and space-based links integrating high-performance optical systems engineered for this purpose. In space-based downlinks in particular, the loss budget and the variations in the signal propagation due to atmospheric turbulence effects impose a careful optimization of the coupling of light in single-mode fibers required for interfacing with the receiving stations and the ground networks. In this work, we perform a comprehensive study of the role of adaptive optics (AO) in this optimization, focusing on realistic baseline configurations of prepare-and-measure quantum key distribution, with both discrete and continuous-variable encoding, and including finite-size effects. Our analysis uses existing experimental turbulence datasets at both day and night time to model the coupled signal statistics following a wavefront distortion correction with AO, and allows us to estimate the secret key rate for a range of critical parameters, such as turbulence strength, satellite altitude and ground telescope diameter. The results we derive illustrate the interest of adopting advanced AO techniques in several practical configurations.
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.
Quantum key distribution (QKD) enables remote users to share cryptographic keys with information theoretic security. Satelliteto-ground QKD offers a route to extend the distances between users that is otherwise limited by losses through optical fibre. However, satellite QKD is typically limited to operation at night due to the background photons from sunlight. A proposed solution to reduce the background counts is tight spatial filtering via single-mode fibre coupling, but this comes at the cost of losses because of imperfect coupling efficiency due to wavefront distortions from atmospheric turbulence. To maximise the number of photons entering the fibre, adaptive optics (AO) can be used to correct for the atmospheric effects. We present results of end-to-end link simulations which show the trade-off between AO complexity and fibre coupling efficiency. These results show that for representative turbulence characteristics, a relatively small deformable mirror is required to obtain the required coupling efficiency for a reasonable key rate. This paves the way for affordable quantum communication in daylight conditions.
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.
The efficient coupling of photons from a free-space quantum channel into a single-mode optical fiber (SMF) has important implications for quantum network concepts involving SMF interfaces to quantum detectors, atomic systems, integrated photonics, and direct coupling to a fiber network. Propagation through atmospheric turbulence, however, leads to wavefront errors that degrade mode matching with SMFs. In a free-space quantum channel, this leads to photon losses in proportion to the severity of the aberration. This is particularly problematic for satellite-Earth quantum channels, where atmospheric turbulence can lead to significant wavefront errors. This report considers propagation from low-Earth orbit to a terrestrial ground station and evaluates the efficiency with which photons couple either through a circular field stop or into an SMF situated in the focal plane of the optical receiver. The effects of atmospheric turbulence on the quantum channel are calculated numerically and quantified through the quantum bit error rate and secure key generation rates in a decoy-state BB84 protocol. Numerical simulations include the statistical nature of Kolmogorov turbulence, sky radiance, and an adaptive-optics system under closed-loop control.