The low-cost small aperture commercial optical ground stations (OGSs) will play an important role in the realization of a global scale satellite-to-ground quantum key distribution (QKD) networks. OGSs with < 60 cm diameter telescope and a small mount, can have stringent size and weight limits on the payload that can be accommodated. In order to ensure easy integration with an OGS, we have designed and produced a 40×30×10 cm receiver module which comprises polarization detection optics, fine point and track (PAT) subsystem and coarse wavelength division multiplexing optics for receiving optical signals over the visible, near-infrared and infrared spectrum. The tracking capability of the module to track multiple beams of different wavelength is evaluated in an emulated beam-wander induced free-space link. The receiver module is characterized for transmission loss and polarization extinction ratio (PER) by sending an 844 nm collimated beam to the module in a 3 m free-space link and measuring the power coupled into the four single mode fibers (SMF) for H, V, D and A polarized components. Furthermore, we have used the receiver module in our satellite-to-ground QKD emulator and have measured the secure key that can be generated within different maximum elevation satellite overpasses when the quantum signal is coupled into AR-coated SMF. Such modules can be incorporated into optical ground stations (OGS) to mitigate atmospheric beam-wandering and coarse misalignment while allowing wavelength division multiplexing of multiple visible, near-infrared and infrared beams to enable co-propagation of classical and quantum communication beams for real-time QKD.
The low-cost small aperture commercial optical ground stations (OGSs) will play an important role in the realization of a global scale satellite-to-ground quantum key distribution (QKD) networks. OGSs with < 60 cm diameter telescope and a small mount, can have stringent size and weight limits on the payload that can be accommodated. In order to ensure easy integration with an OGS, we have designed and produced a 40×30×10 cm receiver module which comprises polarization detection optics, fine point and track (PAT) subsystem and coarse wavelength division multiplexing optics for receiving optical signals over the visible, near-infrared and infrared spectrum. The tracking capability of the module to track multiple beams of different wavelength is evaluated in an emulated beam-wander induced free-space link. The receiver module is characterized for transmission loss and polarization extinction ratio (PER) by sending an 844 nm collimated beam to the module in a 3 m free-space link and measuring the power coupled into the four single mode fibers (SMF) for H, V, D and A polarized components. Furthermore, we have used the receiver module in our satellite-to-ground QKD emulator and have measured the secure key that can be generated within different maximum elevation satellite overpasses when the quantum signal is coupled into AR-coated SMF. Such modules can be incorporated into optical ground stations (OGS) to mitigate atmospheric beam-wandering and coarse misalignment while allowing wavelength division multiplexing of multiple visible, near-infrared and infrared beams to enable co-propagation of classical and quantum communication beams for real-time QKD.
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.
Quantum Key Distribution (QKD) systems will play an important role in future networks for secure data communication. In order to provide a global coverage of a future QKD service, satellites are needed to bridge large distances. These satellite systems must be cost efficient to facilitate deployment since many network nodes will be needed. The CubeSat standard is frequently used for New Space projects as a versatile platform for satellite payloads. It is also chosen as a baseline for the construction of the system described in this paper. The DLR Institute of Communications and Navigation develops optical free space communication systems for scientific research in classical and quantum communications. A previous development of a 1/3 U CubeSat communication terminal for up to 100 Mbit/s downlinks is adapted to be used for quantum communications tests with multiple transmitters. Since the original laser terminal was designed for C-band wavelengths, a redesign of the optical system is needed to achieve polychromatic performance for the three sources. The optical system consists of a fiber collimator, a beam steering system and an afocal telescope. Most important requirement of the latter is a similar magnification for all wavelengths to ensure concentric beams pointing to the OGS. Scenarios in which the system is optimized either for lowest divergences or a similar magnification of the telescope are demonstrated in Figure 1. As the afocal telescope is used bidirectionally, it also needs to be optimized for the incident wave-front of the beacon from the OGS with respect to the beam shape at the four quadrant detector. These parameters are important for a correct pointing control. A fiber-based wavelength division multiplexer (WDM) is used for combining signals from three sources into one output fiber. As shown in the block diagram in Figure 2, it is based on cascaded thin film interference filters which are coupled to the fibers. Especially the propagation of 850 nm and 1550 nm signals in one single mode fiber is critical. Therefore the types of optical fibers were selected with respect to the bend loss, single mode propagation behavior, polarization integrity and optimal cladding diameter for the production.
In the absence of technically mature quantum repeaters, losses in optical fibers limit the distance for ground-bound quantum key distribution. One way to overcome these losses is via optical links to satellites, which has been demonstrated in course of the Chinese-Austrian QUESS mission. Though its findings were impressive, such a large-scale project requires massive financial and time resources. We propose a 32x10x10cm³ nanosatellite orders of magnitude cheaper which is able to perform quantum key distribution (QKD) in a trusted-node scenario, using only commercially available components. We have performed a detailed analysis of such a CubeSat mission (“Q³Sat”), finding that cost and complexity can be reduced by sending the photons from ground to satellite, i.e. using an uplink. Calculations have been done for a prepare-and-send protocol (BB84 with decoy pulses) and for a protocol exploiting quantum entanglement (E91), both using polarization as information carrier. We specified the minimum requirements for the sender stations for these two different protocols. Possible orbits have been assessed, regarding both height and ellipticity to maximize link time and minimize losses. Using long-term weather data, we developed a beam model taking into account absorption, turbulence-induced beam divergence and pointing stability of sender and receiver telescope. Using light pollution measurements from space and their spectra, we arrive at maximum expectable noise count rates. We also specify the requirements for clock stability, classical communication speed and computing requirements. Incorporating all these parameters into our model, we arrive at a link budget which we can use to calculate the expected quantum secure key rates. We have also created a preliminary design of such a 3U CubeSat including a detailed size, weight and power budget and a CAD to account for the assembly of the components. Deploying a 10 cm long mirror telescope covering the small surface of the satellite leaves enough space for a polarization analysis module and housekeeping, communication and computing electronics. Polarization analysis can be done via a polarizing beam splitter and single-photon detectors with a cross section small enough to rule out radiation damage. Pointing stability and detumbling is crucial especially for such a small satellite and can be achieved via spinning wheels, achieving a precision in the tilt and yaw axis of 40 mrad. For one such CubeSat, we estimate the quantum secure key to be acquired between two ground stations during one year to be about 13 Mbit when deploying a decoy protocol. A Bell test between ground and satellite would also be feasible. The uplink design allows to keep the more sensitive, computation-intensive and expensive devices on ground. The experiment proposed by us therefore poses a comparably low-threshold quantum space mission. For a two-year lifetime of the satellite, the price per kilobit would amount to about 20 Euro. In large constellations, Q³Sats could be used to establish a global quantum network, which would further lower the cost. Summarizing, our detailed design and feasibility study can be readily used as a template for global-scale quantum communication.
This study investigates the design and performance of a transmitter optical system optimized for Low-Earth-Orbit (LEO) satellite-based Free-Space Quantum Key Distribution(FS-QKD). The architecture integrates a 120 mm aperture telescope, a beam expander, and an optical-axis alignment module, complemented by a pointing control unit to maintain link stability. Performance was evaluated across 660 nm, 810 nm, and 1,550 nm wavelengths. Link budget analysis for a 10 km channel with a 200 mm receiver aperture demonstrated that a 660 nm beacon laser achieves a diffraction loss of 4.71 dB and a received power of 15.5 dB, with the beam expanding to approximately 440 mm. Notably, the integration of a Fast Steering Mirror(FSM) enhanced the average Signal-to-Noise Ratio(SNR) by nearly ninefold. These results validate the system's efficacy in mitigating pointing errors and optimizing optical coupling for robust inter-satellite quantum communication.
This paper presents an in-depth satellite-to-ground Quantum Key Distribution (QKD) feasibility analysis between a Low Earth Orbit (LEO) satellite and a 0.28 m small-sized Optical Ground Stations (OGSs) located in urban areas, over nighttime. Both Prepare & Measure (P&M) and Entanglement (ENT)- based QKD protocols are employed and compared in terms of distilled key bits. The performance of various detection setups and environmental conditions including different wavelengths (1550 nm and 810 nm) as well as different types of detectors (Avalanche and Superconducting Nanowires) is simulated. The results are compared to those obtained when OGSs with much larger receiver apertures are placed in rural environment. Our analysis delivers a framework examining the parameters affecting the QKD satellite downlink, and also provides a thorough theoretical framework for the employment of small-sized OGSs in urban environments.
In the absence of technically mature quantum repeaters, losses in optical fibers limit the distance for ground-bound quantum key distribution (QKD). One way to overcome these losses is via optical links to satellites, which has most prominently been demonstrated in course of the Chinese-Austrian QUESS mission. Though its findings were impressive, such a large-scale project requires massive financial and time resources. We propose a 34x10x10cm³ nanosatellite orders of magnitude cheaper which is able to perform QKD in a trusted-node scenario, using only commercially available components. We have performed a detailed analysis of such a CubeSat mission (“Q3Sat”), finding that cost and complexity can be reduced by sending the photons from ground to satellite, i.e. using an uplink. Calculations have been done for a prepare-and- send protocol (BB84 with decoy pulses) using polarization as information carrier. We have created a preliminary design of a 3U CubeSat including a detailed size, weight and power budget and a CAD to account for the assembly of the components. Deploying a 10 cm long mirror telescope covering the small surface of the satellite leaves enough space for a polarization analysis module and housekeeping, communication and computing electronics. For one such CubeSat, we estimate the quantum secure key to be acquired between two ground stations during one year to be about 13 Mbit. A Bell test between ground and satellite would also be feasible. The uplink design allows to keep the more sensitive, computation-intensive and expensive devices on ground. The experiment proposed by us therefore poses a comparably low-threshold quantum space mission. For a two-year lifetime of the satellite, the price per kilobit would amount to about 20 Euro. In large constellations, Q3Sats could be used to establish a global quantum network, which would further lower the cost. Summarizing, our detailed design and feasibility study can be readily used as a template for global-scale quantum communication.