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.
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.
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.
Designing a laser communications payload integral to a nano-satellite, capable of fine pointing, is a difficult task. Integrating this with a quantum key distribution payload for inter-satellite secure communication adds significant complexity, as does ground based communication with the moving objects in low Earth orbit. The advantages of low path length, secure, global communication make this an opportunity not to be missed. We report on developments of such a system. Beginning with a satellite demonstrator with a Pointing, Acquisition-and Tracking system that involves fine precision star-tracker, plasma and aerodynamic force interactions for steering, and an inter-satellite laser link demonstration on orbit using LED beacon for acquisition, and laser for communications link closure. Combining design studies for a nano-satellite quantum payload for transferring entangled photon-pairs between the satellites at long-range in Low Earth Orbit, and incorporating ground to space and space to ground laser communications, are described in the context of quantum optical ground station efforts within Australia.
Space quantum communications is a potential means for establishing global secure communications and quantum networking. Despite pioneering demonstrations of satellite quantum key distribution, considerable challenges remain for wide deployment such as the local effects of the atmosphere on the transmission of single-photon level quantum signals. As part of Ireland's efforts to establish quantum links with the rest of Europe and further afield, we present a preliminary study of the feasibility of satellite quantum key distribution taking into account geographic and weather effects on the space-Earth channel. Weather data over 5 years covering 4 locations across Ireland were used to assess performance and the prospects of optical ground station (OGS) geographic diversity to improve service availability. Despite significant cloud cover that may reduce the performance of a single OGS location, the use of a 4-OGS network can provide up to 45% improvement for a single satellite exploiting anti-correlation in cloud cover, though most gains are achieved with 2 or 3 OGSs.
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.
Feature Issue on Optical Wireless Communications (OWC) We investigate the design and the accommodation of a quantum communication transceiver in an existing classical optical communication terminal on board a satellite. Operation from a low earth orbit (LEO) platform (e.g., the International Space Station) would allow transmission of single photons and pairs of entangled photons to ground stations and hence permit quantum communication applications such as quantum cryptography on a global scale. Integration of a source generating entangled photon pairs and single-photon detection into existing optical terminal designs is feasible. Even more, major subunits of the classical terminals such as those for pointing, acquisition, and tracking as well as those providing the required electronic, thermal, and structural backbone can be adapted so as to meet the quantum communication terminal needs.
Single-photon quantum communications (QC) offers the attractive feature of 'future proof', forward security rooted in the laws of quantum physics. Ground based quantum key distribution (QKD) experiments in optical fiber have attained transmission ranges in excess of 200km, but for larger distances we proposed a methodology for satellite-based QC. Over the past decade we have devised solutions to the technical challenges to satellite-to-ground QC, and we now have a clear concept for how space-based QC could be performed and potentially utilized within a trusted QKD network architecture. Functioning as a trusted QKD node, a QC satellite ('QC-sat') could deliver secret keys to the key stores of ground-based trusted QKD network nodes, to each of which multiple users are connected by optical fiber or free-space QC. A QC-sat could thereby extend quantum-secured connectivity to geographically disjoint domains, separated by continental or inter-continental distances. In this paper we describe our system concept that makes QC feasible with low-earth orbit (LEO) QC-sats (200-km-2,000-km altitude orbits), and the results of link modeling of expected performance. Using the architecture that we have developed, LEO satellite-to-ground QKD will be feasible with secret bit yields of several hundred 256-bit AES keys per contact. With multiple ground sites separated by {approx} 100km, mitigation of cloudiness over any single ground site would be possible, potentially allowing multiple contact opportunities each day. The essential next step is an experimental QC-sat. A number of LEO-platforms would be suitable, ranging from a dedicated, three-axis stabilized small satellite, to a secondary experiment on an imaging satellite. to the ISS. With one or more QC-sats, low-latency quantum-secured communications could then be provided to ground-based users on a global scale. Air-to-ground QC would also be possible.