Enhancing the optical communication telescope laboratory to support quantum communications between earth and space
Alexander Lohrmann, Ioana Craiciu, Kittrin T. Matthews, Gerardo G. Ortiz, Lewis C. Roberts, W. T. Roberts, Ryan Rogalin, Dmitrios Antsos, Matthew D. Shaw, Makan Mohageg
We report on the design and development of a quantum backend for an optical ground station for space-based quantum communication and science experiments. The quantum backend will enable the Optical Communication Telescope Laboratory (OCTL) to establish links with quantum satellites in the future. We aim to test this quantum enabled ground station with upcoming satellite Quantum Key Distribution (QKD) missions. We present measurements of the ground station properties that are relevant for future quantum links. Specifically, we discuss the polarization disturbance imposed by the optical communication telescope and present mitigation strategies in the form of polarization control systems. In addition to the optical design, we also present an end-to-end QKD model that is used to guide the development.
We report on the design and development of a quantum backend for an optical ground station for space-based quantum communication and science experiments. The quantum backend will enable the Optical Communication Telescope Laboratory (OCTL) to establish links with quantum satellites in the future. We aim to test this quantum enabled ground station with upcoming satellite Quantum Key Distribution (QKD) missions. We present measurements of the ground station properties that are relevant for future quantum links. Specifically, we discuss the polarization disturbance imposed by the optical communication telescope and present mitigation strategies in the form of polarization control systems. In addition to the optical design, we also present an end-to-end QKD model that is used to guide the development.
Optical quantum communication utilizing satellite platforms has the potential to extend the reach of quantum key distribution (QKD) from terrestrial limits of ∼200 km to global scales. We have developed a thorough numerical simulation using realistic simulated orbits and incorporating the effects of pointing error, diffraction, atmosphere and telescope design, to obtain estimates of the loss and background noise which a satellite-based system would experience. Combining with quantum optics simulations of sources and detection, we determine the length of secure key for QKD, as well as entanglement visibility and achievable distances for fundamental experiments. We analyse the performance of a low Earth orbit satellite for downlink and uplink scenarios of the quantum optical signals. We argue that the advantages of locating the quantum source on the ground justify a greater scientific interest in an uplink as compared to a downlink. An uplink with a ground transmitter of at least 25 cm diameter and a 30 cm receiver telescope on the satellite could be used to successfully perform QKD multiple times per week with either an entangled photon source or with a weak coherent pulse source, as well as perform long-distance Bell tests and quantum teleportation. Our model helps to resolve important design considerations such as operating wavelength, type and specifications of sources and detectors, telescope designs, specific orbits and ground station locations, in view of anticipated overall system performance.
The development of space-based quantum communication is a critical step toward establishing global-scale secure networks. Quantum communication via satellites enables long-distance key exchange beyond the limitations of terrestrial fiber networks. To support such missions, robust optical ground stations are essential for maintaining high-fidelity quantum links with orbiting platforms. Commissioned in early 2024, the Singapore Optical Ground Station is the first dedicated facility of its kind located near the equator. Its equatorial position enables frequent access to satellites in a wide range of orbital inclinations, making it uniquely suited for supporting diverse global quantum networks. Its receiving optics can accommodate quantum links across a broad range of wavelengths and support various encoding schemes, including polarization and phase. In this work, detailed results from preliminary testing of the pointing, acquisition, and tracking system for low Earth orbit satellites are presented. For LEO satellites, a tracking accuracy reaching 50 microradians, approaching the local atmospheric seeing limit was achieved.Tracking stability is maintained using a hybrid feedback control loop designed to accommodate sporadic link losses caused by cloud cover. These results demonstrate the readiness of the SOGS to support current and future missions towards satellite quantum networks.
Recent advances in pointing and tracking capabilities of small satellite platforms have enabled adoption of capabilities such as high-resolution Earth Observation (EO), inter-satellite laser communications and, more recently, quantum communications. Quantum communications requires unusually narrow optical beams and tight pointing performance (on the order of ten microradians) to close an inherently brightness-limited quantum link. This limit is due to quantum communication protocols such as quantum key distribution and teleportation requiring individual quantum states to be transmitted with photon number restrictions. We examine an opportunity to combine quantum communications with laser communications in sharing an optical link. We discuss a combined quantum and laser communication terminal capable of performing space-to-ground entanglement-distribution and high data rate communications on a 12U CubeSat with a 95mm beam expander and an 60 cm aperture optical ground telescope. Photon pairs produced by the quantum terminal are entangled in polarization so the polarization must be maintained throughout the optical link. We discuss active and passive compensation methods in space and polarization reference frame correction using a polarized reference beacon at the ground station. The combined quantum and laser communication terminal approach enables secure communications over an optical channel with rates of 100 Mbps and sub-nanosecond time transfer.
We present QuBUS, a mobile optical ground station designed for free-space optical communication (FSOC) and quantum key distribution (QKD). Built into a transportable lab container, QuBUS includes key subsystems such as an optical antenna with uplink lasers, a freeform telescope, acquisition and tracking hardware, relay optics, and an adaptive optics system for single-mode fiber coupling. The station is being prepared for an air-to-ground quantum communication experiment involving an aircraft-based transmitter. Initial 7-km horizontal link tests validate core functionalities, demonstrating QuBUS’s potential as a compact and deployable platform for future secure quantum communication networks.
A worldwide growing interest in fast and secure data communications pushes technology development along two lines. While fast communications can be realized using laser communications in fiber and free-space, inherently secure communications can be achieved using quantum key distribution (QKD). By combining both technologies in a single device, many synergies can be exploited, therefore reducing size, weight and power of future systems. In recent experiments we demonstrated quantum communications over large distances as well as between an aircraft and a ground station which proved the feasibility of QKD between moving partners. Satellites thus may be used as trusted nodes in combination with QKD receiver stations on ground, thereby enabling fast and secure communications on a global scale. We discuss the previous experiment with emphasis on necessary developments to be done and corresponding ongoing research work of German Aerospace Center (DLR) and Ludwig Maximilians University Munich (LMU). DLR is performing research on satellite and ground terminals for the high-rate laser communication component, which are enabling technologies for the QKD link. We describe the concept and hardware of three generations of OSIRIS (Optical High Speed Infrared Link System) laser communication terminals for low Earth orbiting satellites. The first type applies laser beam pointing solely based on classical satellite control, the second uses an optical feedback to the satellite bus and the third, currently being in design phase, comprises of a special coarse pointing assembly to control beam direction independent of satellite orientation. Ongoing work also targets optical terminals for CubeSats. A further increase of beam pointing accuracy can be achieved with a fine pointing assembly. Two ground stations will be available for future testing, an advanced stationary ground station and a transportable ground station. In parallel the LMU QKD source size will be reduced by more than an order of magnitude thereby simplifying its integration into future free-space optical communication links with CubeSats.
Quantum Key Distribution (QKD) is a promising method to guarantee future-proof, information theoretic security. Since optical fibers have an exponential loss with distance, satellite-based QKD solutions are being developed in order to realize long-distance links. Therefore, Optical Ground Stations for QKD (QKD-OGS) need to be designed to enable quantum communication with satellites. Different link configurations will result in different integration options of the QKD-OGS in the terrestrial fiber network and therefore impact its performance. Applicable integration options must be identified and trade-off analysis conducted at the architecture, system and sub-system level. The reference scenario is a Low Earth Orbit (LEO) downlink configuration employing a decoy-state BB84 protocol with polarization encoded qubits at 1550 nm wavelength. The satellite acts as trusted node for implementation of the key distribution between two parties on ground. Furthermore, only direct connections from the satellite to the end-users are considered, i.e. the QKD-OGS does not serve as a trusted-node relay. This results in the selection of three main integration concepts: first, a free-space, end-user QKD-OGS which is located directly at the end-user where the quantum signal is measured directly behind the receiver telescope in free-space; second, a fiber-coupled, end-user QKD-OGS where the quantum signal is coupled into a single-mode fiber and guided to a co-located server room hosting the QKD Receiver system; third, a fiber-coupled, provider QKD-OGS, which reaches several end-users by using a local fiber network. In order to evaluate these three concepts, the comparison parameter is defined to be an estimated factor of suppliable end-users. The estimated comparison factor shows that the fiber-coupled, enduser QKD-OGS can supply 6.7 times more end users than the free-space, end-user QKD-OGS. The fiber-coupled, end-user QKD-OGS can supply 129.7 times more end-users than the free-space, end-user QKD-OGS.