Quantum Technologies in Space

Quantum Communication changes the paradigm with respect to current secure communication standards, making the transfer of information secure for long-term requirements and protecting against potential attacks, including those expected from quantum computers.

QKD consists of the establishment of secret keys in a way that is “information theoretically secure”. This means that the security of QKD protocols can be proven mathematically, based on direct information-theoretic argumentsRenner et al. (2005), eliminating the need for computational assumptions. There are two types of protocols for this, namely “prepare and measure” (PM-QKD) and “entanglement-based” (ENT-QKD).

QKD systems reached commercial-maturity level on terrestrial fibre links, and are based on PM-QKD, among which Decoy-State Bennett-Brassard (DS-BB84) is the most commonBennett and Brassard (1984); Lo et al. (2005). Key exchange exceeding a million secure bits per second was demonstrated on fibre links. PM-QKD is currently under development for space channels, with one project using a satellite in low-Earth orbitLiao et al. (2017). Such links, whose nodes – dubbed trusted – will allow the storage of keys for some time, will enable the realization of networks for key exchange of any size.

ENT-QKD is also possible with the use of a photon-pair source in between the two terminals where the key is generated. This kind of protocol is based on quantum entanglement distribution. It allows communication of arbitrary quantum states, and thus provides a more generic means of communication called quantum information network (or sometimes “quantum internet”), which can offer security and other applications beyond PM-QKD. As far as space is concerned, it allows establishing secret keys on the ground, for instance with the E91Ekert (1991) or the BBM92Bennett et al. (1992) protocols, with the satellite just distributing the entangled pairs to ground terminals. This was recently demonstrated using the Micius satelliteYin et al. (2020). Yet the implementation of entanglement-based protocols remains much more challenging, all the more when making use of long fibres or of high-altitude space-based optical links. For that reason, entanglement-based protocols might be a solution for the longer term eventually yielding a broader set of enabled applications.

Space-based QKD will be a crucial element to bootstrap the quantum information network at the planetary scale and beyond. Both types of QKD protocols shall be used, based on the needs of specific applications being considered, with a natural progression from PM-QKD – which is already well developed on the ground – to ENT-QKD. Space-based quantum communication is expected to unlock the implementation of quantum-communication applications beyond quantum cryptography, for instance in the distribution of time, metrology, and distributed quantum computation, in addition to fundamental investigations.

Although security can be quantified on the basis of QKD measurements, this is not sufficient for an operational system with mission-critical uses. In order to evaluate the security of a system, its use-case, boundary conditions and requirements need to be defined. The design and development of such a system shall then comply with rules derived from these requirements, and its compliance with them continuously assessed along the process.

In the case of quantum cryptography, an interdisciplinary approach is required between quantum physicists, cryptography/security experts, and engineers. Standards need to be defined and certification procedures developed by the relevant agencies on both national and international levels. Moreover, quantum and classical security concepts need to be combined and assessed in a single framework. These topics are currently being addressed by working groups within major standardization institutes, namely the European Telecommunications Standards Institute (ETSI), the International Standards Organization (ISO), and the International Telecommunication Union (ITU), for QKD as well as for longer-term quantum information networks.

The design of a secure communication infrastructure requires a complete system approach including space and terrestrial components. The system design has to be adapted to different use-cases. On ground, fibre-based solutions will offer short-range secure communication. Satellite-based quantum communication will provide a means for reaching global distances and secure space assets. The technology of terrestrial components (ground stations and management centers) and space components (satellite or satellite networks) need to be adapted to different use-cases.

The deployment of keys between receivers on ground to be used in pan-European as well as national secure communications will be the most immediate results of such Space QKD system. The development of compact receivers, suitable for the roof of a normal building, would allow the demonstration of QKD to all Member States regardless of their geographical location, much before the development of a continental repeater network based on fibres.

Quantum communication should be implemented on a practical and reliable platform to provide world-wide secure communication for European assets. This is needed as soon as possible, as is the European-scale demonstration and the involvement of all Member States in the QKD network. As the time for implementation and in-orbit operation of the usual satellite-based systems goes well beyond a decade, and the threat of a future quantum computer has to be considered in the same time-frame (even sooner in case of retroactive decryption), the urgency of action is striking.

Space-based QKD can provide access to such networks sooner and easier in the case of Member States or their territories that are geographically separated from others by long distances or by sea. The pursuit of such goals defines a clear roadmap that includes the following key pillars, to be developed in parallel:

Operational systems. SAGA (Security And cryptoGrAphic mission)Lewis and Travagnin (2019) was originally an ESA internal study for ENT-QKD. In this context, we use SAGA as reference to the space component (encompassing both ground and space segments) of the future Quantum Communication Infrastructures (QCIs), comprising different types of implementation: · SAGA1: “Security & cryptography”: The aim is to develop satellite-based quantum cryptography for world-wide secure key-distribution using PM-QKD protocols and networks with trusted nodes. The system should include key management, secure symmetric encryption on classical channels, and options depending on user needs. Terrestrial terminals (ground receivers for the keys and interfaces to the fibre links) have to be developed as economically affordable, easily deployable and that can connect to ground-based fibre networks. (Demonstration mission launch: 2023/2024; Full Service starting: 2028/2029.) · SAGA2: “Further applications”: entanglement-based and more. The aim is to develop entanglement-based quantum communication using satellites. Depending on use-cases the satellite may use different types of orbits. In addition, quantum repeaters may concretely be implemented in space systems to enable further applications on a larger scale. As a vision, a full quantum information network can be implemented world-wide. (Demonstration mission launch: 2028; Entanglement Distribution Service: 2035.)

Standards/certification development. Standardization and certification efforts have to start in parallel to the development of the operational systems. First, the development of operational systems will serve as an input to the development of standards and certification, and later the standards and certification requirements will steer the development of the operational systems. (First draft standards: 2021; First certification: 2022; Advanced certification and standards developed: 2024; Certification for service: 2028.)

This vision is encompassed in Figure 1.

In addition to the concrete operational developments, research and development is needed in interdisciplinary topics. These include the development of high-rate QKD payloads, advanced pointing systems, low-loss space-to-ground links, also based on adaptive optical systems, and inter-satellite links.

Moreover, we aim at the exploitation of quantum functionalities where a provable advantage can be shown and that require current, near-term, or longer-term quantum technology. The most notable ones are:

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  Quantum random number generation, which is achievable with current or near-term technology.

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  Quantum communication complexity protocols (with the most prominent ones being quantum fingerprinting and hidden matching/sampling matching).

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  Quantum cryptographic primitives like position verification, oblivious transfer, coin flipping, anonymous message transmission, leader election, secret sharing.

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  Delegated and distributed quantum computing.

In our opinion, the EU effort in this regard is essential to complement that of the Space Agencies.

The exploration of QKD protocols using different photonic degrees-of-freedom (for example, frequency, time bins, polarization, orbital angular momentum) will require advanced security analysis and the development of frameworks enabling new, affordable and practical services, and the highest security applications as well as a variety of new applications involving time, position, navigation, and much more.

To develop the program described here, we envisage a joint effort of the European Space Agency and the European Commission. To establish the space part of the European institutional QKD infrastructure, we recommend that ESA shall act as project management for the European Commission for technological and program implementation, as is the case for Europe’s global navigation system, Galileo. In addition, we recommend that development of protocols and R&D continue being supported by the Commission’s Directorate-General for Defence Industry and Space (DEFIS) and the Directorate-General for Communications Networks, Content and Technology (DG Connect) directly.

Time standards and frequency transfer have a rich set of applications relevant to engineering, science and society. In high-speed communications, timekeeping makes use of atomic clocks synchronized by GNSS based time dissemination services to perform time stamping and information routing. These capabilities are fundamental to the high performance and availability of the internet and all its associated services. GNSS based services rely on high performance TFT capabilities. They have spawned many services and applications leading to multi-billion Euro turnover per year, and are key to defense and security today as well as enabling for future applications such as autonomous drivingTeunissen and Montenbruck (2017).

Precision time standards are at the root of the aforementioned applications and modern day metrology. Precision time metrology is at the heart of the international standard definition and an enabling factor in a trade-based economy. Recently, in radio astronomy real-time imaging of a black hole has been demonstrated using synthetic aperture imaging for radio frequency signalsCollaboration et al. (2019a, b). This was enabled by TFT technologies.

Fundamentally TFT is based on two technological elements: precision time standards (clocks) and the ability to transfer frequency (more precisely the phase of the clock) over a large distance with high precision. State-of-the-art commercial atomic clocks operate at radio frequencies (several GHz) and achieve accuracies down to $10^{-15}$. Current time standards are defined on improved clocks of the same type with accuracies of about $10^{-16}$Cartlidge (2018). The limitation of these clocks is given by the fundamental frequency of operation (GHz) and the ability to measure these. Phase (and frequency) transfers are standard methods in high frequency RF communications and have been demonstrated on satellite links with a performance of $10^{-15}$Riehle (2017) – with the same fundamental limitations imposed by the frequency used for the transfer. These technologies are about to be demonstrated at increased accuracy in space in ESA’s International Space Station-based Atomic Clock Ensemble in Space (ACES) project, which contains both frequency standards and transfer capabilities to groundSavalle et al. (2019). ACES is currently scheduled to launch in mid-2021Sun et al. (2020).

The fundamental improvement enabled by QT is given by an increase in fundamental frequency from the radio frequency domain into the optical domain by a factor of $10^{5}$Ludlow et al. (2015). This step is possible based on the evolution of laser technology (stable optical oscillators and frequency combs) and optical clock development (quantum state control). Optical clocks have been demonstrated in the laboratory environment to have a fractional uncertainty of less than $10^{-18}$Brewer et al. (2019) with further improvements expected (optical lattice clock and single ion clock). Frequency transfers have been demonstrated on a ground-based optical fibre link with a length of $920\,$km at an accuracy of better than $4\times 10^{-19}$Droste et al. (2012).

ESA has studied the implementation of a sequel mission to ACES based on an optical atomic clock and optical links concept, Space Optical Clock on the International Space Station (ISS, I-SOC) with a possible implementation in the early 2020sOriglia et al. (2016). These experiments clearly show the superior performance of the QT-based optical TFT which have improved the performance at this early stage of development already by three orders of magnitude (two in stability, one in precision)Origlia et al. (2018).

Making use of the technological advancement will bring improvements to the quality of existing applications or allowing building more efficient system architectures. Time dissemination services and metrology will have a direct improvement of three orders of magnitude. High precision clocks in space will provide a secure (i.e. hard to jam) and independent time base for global time keeping. Combined with space-space and space-ground optical links they will allow global TFT. Such a space-based time standard and time distribution system will provide an efficient globally available infrastructure (compared to fibre networks which are only available in densely populated areas) for all of the above applications. For Global Navigation Satellite (GNS) Systems, the system architecture is suggested to be improved with a more efficient implementation than is currently possible and a higher accuracy for specific use-cases.

With the accuracy of optical clocks and the capability to compare these at large distances, new applications now become feasible. As the relative gravitational red shift is $10^{-18}$ per cm of geopotential height, clocks and optical transfers can be used to measure the geopotential difference of two locationsMcGrew et al. (2018). If one location is, for example, in orbit, the absolute geopotential on Earth’s surface can be characterized (geodesy application). Similar to this application gravitational waves can be detected by comparing two optical clocks on two distant satellitesEl-Neaj et al. (2020); Kolkowitz et al. (2016); Tino et al. (2019). This concept augments the ESA/NASA Laser Interferometer Space Antenna (LISA) system in the fact that it has very high sensitivity to gravitational waves at low frequency, i.e., it is complementary to LISA. Finally, analogous to the radio-frequency synthetic-aperture observation concept, an optical synthetic aperture telescope can be envisaged by the fact that a set of fully synchronized optical clocks enables a phase measurement of the impinging light wave at several locations with large separation. This allows “synthesizing by analysis” a telescope with an aperture size comparable to the separation of the locations (potentially thousands of kilometres) leading to the capability of direct observation of extrasolar planets.

The following roadmap for the maturation of QT enhanced TFT technologies is suggested based on the maturity and complexity of the individual elements.

Given the extreme effects of global warming that humankind is facing, Earth observation is perhaps the most important scientific endeavour of our times. Already today, the study of global mass transport phenomena via satellite gravimetry provides important insights for the evolution of our planet and climate change, by improving our understanding of the distribution of water and its changes. Recently, the NASA gravity mission, Grace, found that the temperature of the water in the deep ocean rifts has not changed in recent decadesWouters et al. (2014). Gravimetric satellite data are now used as an early warning system for floods as well as droughts both in the agriculturally important Midwest of the USThomas et al. (2016) and in the rainforest in CongoCrowley et al. (2006). ESA’s gravity mission (GOCE)Cesare et al. (2010) produced detailed maps of deep-sea ocean currents, which are very important drivers of Earth’s climate. It revealed the remnants of lost continents hidden deep under the ice sheet of Antarctica, and it also detected a post-glacial rebound. The changes of Earth’s gravity scarred by earthquakes gives valuable data for predicting future earthquakes. GOCE’s data are also being used to improve models of Earth’s geology, indicating the potential locations of subsurface energy sources. Just like gravitational waves heralded a new age in astronomy, mapping the earth’s gravitational field is proving to be an immensely valuable tool in understanding EarthLoomis et al. (2020).

ESA and NASA are working on future versions of their gravity missions, which will have much improved resolution and sensitivity. However, it has become clear that the classical measurements cannot be pushed much further. Classical accelerometers used so far in gravity missions exhibit increased noise at low frequency and have large long-term drifts. This severely limits the ability of faithfully reconstructing the earth gravity field at low degrees and precisely modelling its temporal fluctuations. The NASA future geodesy mission, Mass Change (MC), is in preparation for launch by 2026Caltech (2019).

Today, the study of global mass transport phenomena via satellite gravimetry provides important insights for the evolution of our planet and climate changeKornfeld et al. (2019). Atom interferometry will play an instrumental role to improve satellite-based measurements for space-geodesy. They promise smaller low-frequency noise and long-term drifts, which will be beneficial for faithfully reconstructing the Earth’s gravity field at low degrees and even more for precisely modelling its temporal fluctuations.

Atom-interferometric quantum sensors offer far superior long-term stability and higher sensitivity. For this reason, quantum sensors are considered by ESA as a potential instrumentMüller et al. (2020), or a demonstratorBidel et al. (2020). ESA’s future geodesy mission, Next Generation Gravity MissionHaagmans et al. (2020), classified as a Mission of Opportunity, will include laser ranging but consider quantum sensors as a candidate for the following mission if the technology is ready at that time.

Since 2000, missions exploiting inertial quantum sensors (IQS) have been proposed to ESA. In 2010, an ESA road map highlighted space-borne IQS for fundamental physics in space, such as gravitational wave detection. IQS were selected for studies by ESA for satellite gravimetry using cold-atom interferometry (CAI)Bresson et al. (2006), the Lense-Thirring effect with the hyper-precision cold-atom interferometry in space proposal (HYPER)Jentsch et al. (2004), as transportable devices in the space atom interferometer proposal (SAI) and, most recently, was among the three selected candidate missions for a quantum test of the equivalence principle (STE-QUEST: Space-Time Explorer and QUantum Equivalence principle Space Test)Hechenblaikner et al. (2014). Concerns of technological immaturity prevented selection and motivated support for some critical payload sub-components.

Since the first decade of this century, national agencies have supported space-borne quantum-sensor development. Important milestones were achieved by (ICE (Interférométrie atomique à sources Cohérentes pour l’Espace, France) on parabolic flight studies for dual-species interferometryVaroquaux et al. (2009), by QUANTUS (Quantum Gases in Weightlessness, Germany) and MAIUS (Matter-Wave Interferometry in Weightlessness, Germany) establishing interferometry with Bose-Einstein condensates in space based on drop-towervan Zoest et al. (2010) and sounding-rocket experimentsBecker et al. (2018), and by the cold-atom laboratory (CAL, USA)Elliott et al. (2018), where US teams including German scientists explore physics with Bose-Einstein condensates in orbit in the Bose Einstein Condensates and Cold Atoms Lab (BECCAL)Frye et al. (2019). Benefiting from the space activities, novel terrestrial sensors and commercial spin-offs were created and will continue to emerge as industry starts to engage in the development of cold atom payloads.

Galileo exemplifies Europe’s ambition to establish its independence regarding key technologies for space. In the recent past, stepping-stones were placed in developing methods for space-borne high-precision gravity sensing. These sensors promise to improve Earth observation by their long-term stability and low drift. The grip of gravity is compromising the analysis of the performance of these sensors. Microgravity facilities provide only limited access to extend the free-fall and space-like environment for raising the TRL of key components and method development; only a pathfinder can sound out the potential performance in the relevant environment. The European competences have to be firmly bundled to master this challenge and to establish space-hardware for such a sensor. Mission design and exploitation plans need a close interdisciplinary cooperation between the quantum sensor community, high-tech industry, and geodesists. At this point, firm commitments between partners and stakeholders are required to jointly prepare such a mission.

1. (a)

   This requires a strong coordination in prototyping and performance tests. This could, for example, be performed in a joint research coordination as has been proposed by ESA technical officers for some time in the “C-COOL” laboratory initiative. Such a joint laboratory could harness European ground-based microgravity facilities and expertise and exploit European heritage and synergies. Parallel to this, the initiatives of ESA, CNES and DLR to elaborate the most efficient mission concept for satellite gravimetry with quantum sensors should be consolidated, especially with respect to improving ultimate performances (enhanced sensitivity, AOCS, size, mass and power reduction). All these efforts must be supported with the objective of constructing an Engineering Qualification Model, e.g., a space prototype that will also be used as the payload on a small pathfinder satellite to allow for a rapid launch (5 years).

2. (b)

   A model of an IQS for space needs to be developed and engineered. An Earth-Venture-like mission should be designed as pathfinder for in-orbit validation. This will start the crucial process of transfer of know-how to industry, the development of space-qualified hardware, and of an elegant breadboard for prototyping and performance tests in microgravity (10 years).

3. (c)

   The final goal is to perform a geodesy mission using one or multiple space-borne quantum sensors; exploit quantum sensors for other applications such as navigation, exploration and planetology (Moon, Mars) (¿ 10 years).

The Chinese Micius satellite, US-lead CAL on the ISS and German MAIUS rocket exploring Bose-Einstein condensates showed impressive QT-based results. In 2010, ESA published with community support a road map for fundamental physics highlighting the role of quantum technologiesESA . Prominent fundamental-physics mission proposals such as STE-QUESTHechenblaikner et al. (2014) and Macroscopic Quantum Resonators (MAQRO) Kaltenbaek et al. (2012, 2016) were investigated by ESA with no clear elected candidate. HYPERJentsch et al. (2004) and, recently the Quantum Physics Platform (QPPF)QPP (2019), were selected for a pilot study by the concurrent design facility (CDF). STE-QUEST was selected as a candidate for a medium-sized missions within Cosmic Vision in the context of an ESA call for proposals in 2010. No proposed or pre-selected mission exploiting QT was finally nominated due to major concerns about technology immaturity and feasibility.

In the QPPF study, a payload was designed and optimized, based on earlier work for MAQROKaltenbaek et al. (2016) and a related proposal for near-field high-mass interferometry on groundBateman et al. (2014); Delić et al. (2020) in order to address three science objectives: (1) testing quantum physics in a regime where alternative theoretical models predict noticeable deviations from quantum physics, (2) quantifying any such deviations, and (3) the tested parameter regime should have an overlap with experiments possible on ground. The QPPF study showed that such experiments should be possible by using appropriate cryogenic cooling and thermal shielding of the instrument. Given dedicated technology development, the QPPF study concluded it will be possible to perform MAQRO-type experiments in space with test masses of at least up to $\sim 5\times 10^{9}$ atomic mass units. However, the QPPF study identified three critical issues that first need to be addressed: (1) realizing a reliable method to load the test particles into an optical trap in vacuum, (2) achieving a sufficiently high vacuum (lower than $10^{-13}\,$mbar, ideally lower than $10^{-15}\,$mbar), (3) the originally proposed method for preparing high-mass superpositions could lead to decoherence due to light scattering. Efforts are on-going to address these critical issues. A prominent example of the progress being made is the first ground-state preparation of optically trapped sub-micron particlesDelić et al. (2020).

Nearly 30 years ago, LISAAmaro-Seoane et al. (2017), a long-standing L-class ESA mission to detect gravitational waves, faced similar scepticism for technological and even fundamental reasons. Starting with a low TRL of the employed sensing method, a dedicated pathfinder mission was created to demonstrate the appropriateness and performance of the proposed measurement concept. The LISA Pathfinder mission successfully completed in 2017Armano et al. (2016, 2017). In this sense, LISA could be seen as a successful role model for the design of high-risk experiments to host QT in space where Europe has demonstrated leadership.

Clearly, space is the only environment to enable some of the ambitious scientific goals such as the generation of macroscopic quantum superposition and entangled states – the ultimate test of quantum mechanics.

Available QTs for a fully-fletched FP mission include: classical and non-classical optical interferometry, the generation of non-local superpositions and entangled states, and the demonstration of robust-in-the-field quantum information protocols with photons. Atomic clocks and interferometers resembling the most precise meters and clocks available that build on quantum superposition states are becoming ever more robust and compact.

Optomechanical systems and matter-wave interferometers are pushing the boundaries of macroscopic quantum states in laboratory environments.

The interplay of relativity with quantum phenomena shall be studied in the framework of the observation of the strength of quantum correlations using photon states of different types, entangled or not. Space allows for experimental schemes that are impossible on the ground, in terms of gravitational potential variations, length scales and relative velocities, which widens the domain in which we are testing our current understanding of nature.

Budgetary constraints for M/L-class missions require collaboration – across the scientific communities, industry, agencies, on an international level – and the joint decision to select only a few topical FP missions and on their execution on a single or multi-platform approach. Europe has to be at the forefront in QT in order to launch a mission with ESA alone or in cooperation with strong worldwide partners.

For the first 5 years, we propose to define a roadmap for various selected scientific objectives and to develop key QT methods for the payload in a joint European effort. The advance of QTs to the required TRLs in micro-g environments remains the major technical challenge.

In the medium term (5-10 years), the FP community needs support to develop demonstrators for tests in microgravity environment and has to define pathfinders for validation of QT in orbit. An important objective is to explore platforms for multiple pathfinder activities.

Clearly, the long-term goal ($>$ 10 years) is to exploit QT for the scientific objective to test quantum mechanical states in an ESA mission with/without worldwide partners.

We recommend to implement a dedicated sustainable European Union program (5-10 years) to develop QT for FP in space in the coming years for the purpose of funding European collaborative projects, in order to perform proof-of-principle tests (5 years) and for increasing required TRLs (10 years) in order to be competitive in ESA’s calls. It is clear, that we need to exploit ESA’s mission heritage to define and develop together with ESA sustainable (5-10 year) programs for the transfer of technical innovations to space industries.

We further propose the establishment of joint, international laboratories for research teams to assemble core skills and knowledge to develop and test QTs for space towards the required TRLs and prepare the FP space missions.