This article summarizes the results of a system engineering approach to the design of a Transportable Optical Ground Station (TOGS) intended to be used for quantum key distribution in a number of scenarios. Key requirements are listed and a product breakdown is proposed, identifying parts, which are specific to a particular scenario and others, which are common to many of them, and thus will allow their implementation to be shared. We are proposing the use of Adaptive Optics (AO) for compensation of the effects of the atmospheric turbulence, in order to improve the efficiency of the signal coupling to a Single-Mode Fiber.
This article summarizes the results of a system engineering approach to the design of a Transportable Optical Ground Station (TOGS) intended to be used for quantum key distribution in a number of scenarios. Key requirements are listed and a product breakdown is proposed, identifying parts, which are specific to a particular scenario and others, which are common to many of them, and thus will allow their implementation to be shared. We are proposing the use of Adaptive Optics (AO) for compensation of the effects of the atmospheric turbulence, in order to improve the efficiency of the signal coupling to a Single-Mode Fiber.
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 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.
Quantum key distribution (QKD) is a promising field within telecommunication and cryptography. QKD allows two parties to generate a shared secret key, which only the two communicating parties know, and can later be used to encrypt and decrypt messages. This leads to unbreakable data transmission. QKD was experimentally already tested on a Chinese mission called MICIUS. Future QKD protocols may also benefit from coupling into an optical single-mode fiber. Adaptive optics is, from a quantum communication perspective, very interesting. In this paper, we present the overall system design, the optical and mechanical design, and the factory performance test results, of an adaptive optics module for post-compensation of the quantum signal in a QKD ground station. The set-up is integrated in a compact and transportable “AO-box”. The AO-box has been designed to be directly mounted on the flange of an 80 cm fork mount telescope and delivers the fiber-coupled quantum signal at its output interface.
Free space optical communications implementing QKD protocols demand sensors with very high signal to noiseratio, compatible with the reduced size of a single-mode fiber core. The use of adaptive optics allows highdata throughput links, by correcting the aberrations generated by atmospheric turbulence. In this paper wepresent the design of an adaptive optics (AO) system-box to correct for daytime and nightime atmosphericturbulence. The system will be small enough to be implemented at the Nasmyth focus of a transmitter/receiver70cm-aperture telescope used as a Transportable Optical Ground Station in urban sites or LEO links scenariosor in the OGS at Teide Observatory 1 meter-aperture telescope. The setup will correct for aberrations basedon a plenoptic wavefront sensor camera. The optical performance of the system will be analysed, together withsimulations of turbulence to estimate the increase on the coupling from free-space to SMF
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 Institute of Astrophysics of the Canary Islands, IAC, and specifically its technology transfer division IACTEC, has a long history of participation in free-space optical communications projects (FSOC). These activities are nowadays being pushed forward to achieve practical applications of quantum key distribution, leveraging its facilities and experience in the FSOC field and its “know-how” in Adaptive Optics. A Free-Space Communications Laboratory has been installed at IACTEC clean rooms, intended to be used as a permanent facility where specific equipment for optical communications, quantum and classical, is available to our research and development group and its collaborators. Single photon detectors and sources, including both avalanche and superconducting nanowires(SNSPD), 16 GHz oscilloscope, FPGA-based time to digital converter, and many optical and electronic general-purpose building blocks are examples of the items existing at the facility, which should become a complement of the existing OGS telescope located at the Observatorio del Teide. The main objective of the group is to make extensive use of the well-known adaptive optics techniques for the compensation of the aberrations introduced in the communications channel by the atmospheric turbulence. Specifically, a Transportable Optical Ground Station (TOGS) with adaptive optics will be presented, designed with the aim to provide efficient transmission of quantum keys between islands or from a LEO satellite. The efficiency of coupling the received signal to a Single Mode Fibre is vital when using a SNSPD because they are commonly used as input to the cryostat, in order to minimize dark counts. When the communication optical link is expected to suffer atmospheric turbulence during its free-space path, being able to compensate them using adaptive optics could improve the coupling efficiency by a factor of 25 for a 1-meter telescope, as has been obtained in simulations carried out by our group. Our group has also found in simulations that an adaptive optics system based on the plenoptic camera will behave especially well in the situation of high turbulence, as it can be expected for sea-level communications. An optical design based in the use of the plenoptic camera as wavefront sensor for the TOGS will be presented.