Superconducting nanowire detectors fed by single mode fibres are essential to minimize dark counts in a quantum communication receiver. Efficient coupling of the received light to the single mode fibre is fundamental for the system performance, but it gets deeply affected by the atmospheric turbulence. Adaptive Optics measure and correct for the atmosphere effect on the light optimising the fibre coupling. An Adaptive Optics system is proposed to be installed at the ground station that will receive quantum key distribution from a geostationary satellite. Numerical simulations have been performed for this scenario; the improvement in the fibre coupling have been assessed with and without atmospheric turbulence correction. The present paper encapsulates the simulation framework and main findings, showing the potential for this technology in the quantum communications niche.
Superconducting nanowire detectors fed by single mode fibres are essential to minimize dark counts in a quantum communication receiver. Efficient coupling of the received light to the single mode fibre is fundamental for the system performance, but it gets deeply affected by the atmospheric turbulence. Adaptive Optics measure and correct for the atmosphere effect on the light optimising the fibre coupling. An Adaptive Optics system is proposed to be installed at the ground station that will receive quantum key distribution from a geostationary satellite. Numerical simulations have been performed for this scenario; the improvement in the fibre coupling have been assessed with and without atmospheric turbulence correction. The present paper encapsulates the simulation framework and main findings, showing the potential for this technology in the quantum communications niche.
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.
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.
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.
IACTEC, as the technology transfer division of the Instituto de Astrofísica de Canarias (IAC), is initiating a program towards the practical applications of optical communications and specifically the quantum key distribution, taking advantage of its facilities and experience in free-space optical communications and its know-how in Adaptive Optics. The main objective of the optical communication group is related to the use of the atmospheric turbulence compensation to improve the performance of a Transportable Optical Ground Station (TOGS). This compensation is especially challenging when daytime operations are considered, and also if the TOGS is expected to work in virtually any location, including ordinary city buildings, where atmospheric turbulence is much worse than that found in astrophysical observatories. The role of Adaptive Optics could be especially relevant when using superconducting nanowires detectors, which can achieve very low dark counts when fed from a single mode fiber. In this situation, the ability to couple the light received at the telescope to the fiber is vital, and if the wavefront aberration caused by the atmospheric turbulence is corrected before the coupling to the fiber, the total loss of the quantum channel will be lowered and the key rate improved. This improvement will be estimated through simulations of the atmospheric turbulence characteristics of the different scenarios.
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.
The efficient coupling of photons from a free-space quantum channel into a single-mode optical fiber (SMF) has important implications to 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 a transmitter in low-Earth orbit to a terrestrial ground station and evaluates the efficiency with which photons couple either through a circular field stop or into a 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.