Quantum Key Distribution (QKD) directly exploits the quantum phenomenon of entanglement to allow the secure sharing of a cryptographic key for information encoding. The current generation of QKD devices typically operate over dedicated and expensive private ‘dark fiber’ networks, where they are limited in transmission range to 200-300km due to the lack of quantum repeaters. This paper is concerned with an alternative approach that can lift this range limit by exploiting QKD over free-space links between satellites. Typically, commercial QKD systems rely on phase encoding of information on single photons, and more recently on continuously variable schemes with more powerful lasers. However, these protocols are not suitable for communications through atmosphere. On the other hand, QKD by polarization-entanglement holds great promise for satellite-based QKD encoded communications links if the entangledphoton source can be packaged in a compact, robust and commercially-viable form. This paper will describe the development and packaging of an entangled-photon source utilizing space-qualified telecoms packaging techniques, resulting in a compact device that targets satellite deployment. The key design choices that impact performance in a space environment will be discussed and the results of device characterization in the laboratory environment will be shared.
Quantum Key Distribution (QKD) directly exploits the quantum phenomenon of entanglement to allow the secure sharing of a cryptographic key for information encoding. The current generation of QKD devices typically operate over dedicated and expensive private ‘dark fiber’ networks, where they are limited in transmission range to 200-300km due to the lack of quantum repeaters. This paper is concerned with an alternative approach that can lift this range limit by exploiting QKD over free-space links between satellites. Typically, commercial QKD systems rely on phase encoding of information on single photons, and more recently on continuously variable schemes with more powerful lasers. However, these protocols are not suitable for communications through atmosphere. On the other hand, QKD by polarization-entanglement holds great promise for satellite-based QKD encoded communications links if the entangledphoton source can be packaged in a compact, robust and commercially-viable form. This paper will describe the development and packaging of an entangled-photon source utilizing space-qualified telecoms packaging techniques, resulting in a compact device that targets satellite deployment. The key design choices that impact performance in a space environment will be discussed and the results of device characterization in the laboratory environment will be shared.
Quantum key distribution (QKD) enables tap-proof exchange of cryptographic keys guaranteed by the very laws of physics. One of the last remaining roadblocks on the way towards widespread deployment of QKD is the high loss experienced during terrestrial distribution of photons, which limits the distance between the communicating parties. A viable solution to this problem is to avoid the terrestrial distribution of photons via optical fibers altogether and instead transmit them via satellite links, where the loss is dominated by diffraction instead of absorption and scattering. First dedicated satellite missions have demonstrated the feasibility of this approach, albeit with relatively low secure key rates. In order for QKD to become commercially viable, the design of future satellite missions must be focused on achieving higher key rates at lower system costs. Current satellite missions are already operating at almost optimal system parameters, which leaves little room for enhancing the key rates with currently deployed technology. Instead, fundamentally new techniques are required to drastically reduce the costs per secret bit shared between two distant parties. Entanglement-based protocols provide the highest level of security and offer several pathways for increasing the key rate by exploiting the underlying quantum correlations. In this contribution, we review the most relevant advances in entanglement-based QKD which are implementable over free-space links and thus enable distribution of secure keys from orbit. The development of satellite missions is notoriously lengthy. Possible candidates for a new generation of quantum payloads should therefore be scrutinized as early as possible in order to advance the development of quantum technologies for space applications.
As more aspects of modern society depend on digital communication, we increasingly rely on infrastructure that ensures the privacy and security of this communication. Classically, this has been provided by cryptographic protocols such as public-key encryption, in which secrets called keys are exchanged between different parties to enable secure communication. The rapid development of quantum algorithms which violate the assumptions of these protocols, however, poses a security challenge to modern cryptography. \n \nQuantum resources can also be used to strengthen cryptographic security, particularly the security of key exchange protocols. This approach, QKD, can be implemented by encoding in quantum systems such as single photons sent through free-space or a fiber. Fiber based QKD devices are already commercially available, but are fundamentally limited to distributing keys over a few hundred kilometers. To address this distance limitation, research QKD systems are being developed to exchange keys through free-space to satellites. This work considers practical challenges to building and testing both types of QKD devices. \n \nFirstly, we consider modeling and mission analysis for airborne demonstrations of QKD to stratospheric balloons and aircraft to simulate a satellite. Based on the mission parameters available for both platforms, we found aircraft platforms were more promising for testing prototype QKD satellite systems. We developed a mission planning tool to help design the flight geometries for testing the device. \n \nNext, we developed three new components for a QKD satellite prototype. The requirements for electro-optical devices in orbit are very different from lab environments, mandating new approaches to designing QKD devices. We developed a quad single photon detector package to meet the requirements for free-space links to low earth orbit. Moreover, we designed and built optical systems for analyzing the polarization of photons and an adaptive optics unit to increase the efficiency of collecting the encoded photons. All three devices were tested in conditions that simulated the time and loss of a satellite pass. \n \nFinally, we demonstrated a laser damage attack on a live commercial QKD system. Our attack injected additional optical power into the sender device to modify security-critical components. Specifically, our attack damaged the PIN diodes which monitor the encoded photon number, reducing their sensitivity or completely blinding them. Our damage could compromise the entire key, and was performed during system operation while raising no alarms. \n \nIn summary, this work shows the trade-offs of testing QKD payloads on different airborne platforms, develops components for a satellite QKD payload, and demonstrates a security vulnerability in a commercial QKD system that can fully compromise the key. These results help address practical challenges to building QKD devices, improving the security of modern cryptography.
Quantum key distribution (QKD), the technology to generate distant secure keys for later usage in secure communication, has become a mature topic since the 1980s. While terrestrial QKD over fiber networks is suitable for short-distance urban networks, QKD via satellites promises to gap the distance requirement on a global scale that limits terrestrial QKD. Satellite QKD requires the exchange of quantum states between a satellite and multiple ground stations. The goal is then to facilitate shared secret keys between the ground stations, which can then be relayed to users near the ground stations. The satellite usually carries the quantum sources as its payload and sends them to the ground station via a downlink. The QKD protocol is heavily influenced by the choice of the quantum source, with typically one of two types: approximate single photon source (such as a weak laser) or an entangled source. Similarly, the ground station must be equipped to receive, measure and process the quantum signals sent by the satellite. Although there are also reverse schemes, where the satellite acts as the receiver and the ground station as the sender, these schemes are less explored due to higher noise in the uplink and less engineering experience. As with any system containing a high heterogeneity, many different implementation schemes lead to a variety of optimized use cases based on different criteria. Satellite QKD carries the further complication that it must be adapted to international standards. In this paper we present different satellite and ground station architectures that lean on the developments in the projects EAGLE-1, QUBE, QuNET, RoGloQuaN, addressing different protocols, sources, satellite sizes and telescope sizes. We also discuss the need for agility and configuration of components in a heterogeneous environment.
Quantum Key Distribution (QKD) is a technology that can distribute private encryption keys between two parties with strong security assurances underpinned by quantum mechanics. Entanglement-based QKD is one of the strongest forms of QKD. Performing QKD using satellites can overcome the range constraints of ground-based QKD systems imposed by atmospheric losses or attenuation in optical fibers. In 2017, satellite-based QKD has been demonstrated by the Chinese Academy of Sciences’ 630kg Micius satellite. Independently in Singapore we are developing similar, but greatly more compact QKD technologies targeted at CubeSats. Our first on-orbit demonstration of a simple quantum light source was in 2016 on board the Galassia 2U CubeSat. Next on the pad, our SpooQy-1 CubeSat is a 3U GomSpace platform built at CQT that aims to validate the space-worthiness of our next generation entangled photon light source and demonstrate a radio beacon of quantum random numbers. In future missions our miniaturized entangled photon light sources can be combined with high precision Pointing, Acquisition and Tracking System (PATS) and optical communication links to enable a global QKD network with small satellites.
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.
Quantum Key Distribution (QKD) is a potential mitigation against quantum computation advances that threaten currently deployed public key cryptosystems. QKD is also a precursor to large-scale quantum communications and the quantum internet. Terrestrial direct transmission of quantum photonic signals is restricted by optical fibre exponential absorption, hence space-based systems have been proposed for intercontinental distribution of quantum keys with lower-loss free-space transmission through vacuum. However, challenges remain for the practical deployment of satellite QKD (SatQKD) such as weather, background light, and terminal deployment. In particular, the short transmission time inherent in low Earth orbit satellite QKD and finite statistical uncertainty can limit secure key generation. Here we illustrate the analysis of these effects and some practical engineering design and operational constraints on key length using the Satellite Quantum Modelling and Analysis (SatQuMA) software package being developed for satellite quantum communications research.