Inflight demonstrator of quantum key distribution between CubeSats of Q-ANSER program
Angel Jimenez-Girela, Pablo Arteaga-Díaz, Daniel Merino-Perez, P. García Parejo, Santiago Rodríguez Bustabad, Tomás Belenguer, Verónica Fernández, A. Álvarez‐Herrero
In Quantum Key Distribution (QKD), the emitter and receiver need to share an optical quantum channel - which can be optical fibre, terrestrial free-space or space-based links- to exchange the quantum states. However, with the future aim to achieve a quantum global communication network, communications links between small satellites in constellations will be required. In this context, the experience of INTA in the ANSER (Advanced Nanosatellite Systems for Earth Observation Research) small satellite constellation program will be exploited. This program develops a set of missions that will include groupings of a minimum of three CubeSats (a leader and two or more followers) flying in formation and in coordinated operation for a common mission. Therefore, the only difference between ANSER and Q-ANSER program will be the payload of the satellite. In Q-ANSER, in which a prepare-and-measure B92 QKD protocol will be used to generate the secret key, two optical systems will be introduced. In the emitter this system will be capable of sending polarized weak coherent laser pulses, attenuated to single-photon level, to the receiver, which will also be an optical system capable of receiving and detecting these single photons. Prepare-and-measure QKD schemes with polarization encoding require the minimization of polarization degradation both in the transmitter and receiver designs. In particular, the polarization extinction ratio (PER) should be maintained as high as possible to reduce the quantum bit error rate (QBER) . This polarization control will be done with the polarization modulators based on liquid crystals developed by INTA.
In Quantum Key Distribution (QKD), the emitter and receiver need to share an optical quantum channel - which can be optical fibre, terrestrial free-space or space-based links- to exchange the quantum states. However, with the future aim to achieve a quantum global communication network, communications links between small satellites in constellations will be required. In this context, the experience of INTA in the ANSER (Advanced Nanosatellite Systems for Earth Observation Research) small satellite constellation program will be exploited. This program develops a set of missions that will include groupings of a minimum of three CubeSats (a leader and two or more followers) flying in formation and in coordinated operation for a common mission. Therefore, the only difference between ANSER and Q-ANSER program will be the payload of the satellite. In Q-ANSER, in which a prepare-and-measure B92 QKD protocol will be used to generate the secret key, two optical systems will be introduced. In the emitter this system will be capable of sending polarized weak coherent laser pulses, attenuated to single-photon level, to the receiver, which will also be an optical system capable of receiving and detecting these single photons. Prepare-and-measure QKD schemes with polarization encoding require the minimization of polarization degradation both in the transmitter and receiver designs. In particular, the polarization extinction ratio (PER) should be maintained as high as possible to reduce the quantum bit error rate (QBER) . This polarization control will be done with the polarization modulators based on liquid crystals developed by INTA.
Q-ANSER is a Spanish technological in-orbit demonstration mission specifically designed to progress on the technology necessary to perform a Quantum Key Distribution (QKD) link between small satellites in space. Two 16U CubeSats in formation based on the previous INTA developed platform called ANSER will be used for this purpose. The QKD protocol that is going to be implemented in this mission is a simplified BB84 polarization-based protocol that allows to minimize the number of required detectors to two. This is an important advantage taking into account the limited available resources in a CubeSat. The main link indicators (i.e.: Quantum Bit Error Rate and Secret Key Rate) have been calculated to determine the critical parameters (i.e.: transmitter aperture, receiver aperture, quantum channel wavelength, pointing error, satellites distance, minimum transmission time) to guarantee the success of the mission. Taking into account this, the conceptual design of the mission and, specifically, the payload, has been carried out and a summary is described here.
Abstract A satellite-constellation based global quantum network could allow secure quantum communication between remote users worldwide. Such a constellation could be formed of micro- or even nanosatellites, which have the advantage of being more cost-effective than larger expensive spacecrafts. At the same time, the features of quantum communication impose a number of technical requirements that are more difficult to meet when using small satellites. Full-fledged quantum communication has been demonstrated with neither a micro- nor a nanosatellite so far. The authors took up this challenge and have developed a 6U CubeSat weighting 9.5 kg. The satellite is to be launched in 2023 and has already successfully passed all the pre-flight tests. The mission is not yet intended for fully quantum communication. Nevertheless, the authors are testing such key functional elements as polarization reference-frame synchronization and acquisition, pointing and tracking system on it. Besides that, the payload accommodates a full-duplex telecommunication system operating at a bit rate of 50 Mbit/s: an up- and a downlink at wavelengths of 808 and 850 nm. After the satellite is launched, the main goal to be achieved is to demonstrate stable connection between it and an optical ground station and carry out multiple communication sessions. In quantum communication, generating secret keys from raw measurement data implies two-way exchange of significant amount of information and therefore availability of a classical communication channel with a high bandwidth is one of the crucial things. In the following mission, which envisages an overall quantum key distribution system, we plan to use the free-space optical link for such an exchange of data, whereas the RF link will only be used for telemetry and telecommand.
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.
Satellite based Quantum Key Distribution (QKD) in Low Earth Orbit (LEO) is currently the only viable technology to span thousands of kilometres. Since the typical overhead pass of a satellite lasts for a few minutes, it is crucial to increase the the signal rate to maximise the secret key length. For the QUARC CubeSat mission due to be launched within two years, we are designing a dual wavelength, weak-coherent-pulse decoy-state Bennett- Brassard ’84 (WCP DS BB84) QKD source. The optical payload is designed in a 12×9×5cm3 bespoke aluminium casing. The Discrete Variable QKD Source consists of two symmetric sources operating at 785 nm and 808 nm. The laser diodes are fixed to produce Horizontal,Vertical, Diagonal, and Anti-diagonal (H,V,D,A) polarisation respectively, which are combined and attenuated to a mean photon number of 0.3 and 0.5 photons/pulse. We ensure that the source is secure against most side channel attacks by spatially mode filtering the output beam and characterising their spectral and temporal characterstics. The extinction ratio of the source contributes to the intrinsic Qubit Error Rate(QBER) with 0.817±0.001%. This source operates at 200MHz, which is enough to provide secure key rates of a few kilo bits per second despite 40 dB of estimated loss in the free space channel.1
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.
Quantum Key Distribution (QKD) offers a fundamentally secure method for key exchange, grounded in the principles of quantum mechanics rather than computational complexity. As advancements in quantum computing threaten classical cryptographic systems (such as RSA) QKD presents itself as a crucial technology for future secure communications. Among the primary challenges for global-scale QKD is the limited range of optical fiber channels, making satellite-based free-space optical links an attractive alternative. QKD presents unique constraints related to the short and unreliable communication windows, due to atmospheric conditions and the presence of sunlight, which makes the real time operation of QKD constellations almost impossible. Traditional configurations offering instantaneous global coverage, such as Walker constellations, are not necessary for QKD networks and other constellation designs may be explored. In this work, we present as an alternative a two-height, single orbital plane constellation, which facilitates regular and extended inter-satellite visibility, enabling the constellation to function as a virtual trusted node and increasing network capacity and flexibility. The main technical contribution is the development of a simulation tool designed to evaluate and optimize satellite QKD constellations. The simulator allows researchers and mission planners to assess key generation rates and overall system performance, helping to reduce the risks, costs, and uncertainties involved in satellite deployment. Our approach provides a powerful framework for advancing the development of scalable and efficient global QKD networks.