Advances in space quantum communications

IET Quantum CommunicationEarly View REVIEWOpen Access Advances in space quantum communications Jasminder S. Sidhu, Corresponding Author jasminder.sidhu@strath.ac.uk orcid.org/0000-0002-6167-8224 SUPA Department of Physics, University of Strathclyde, Glasgow, UK Correspondence Jasminder S. Sidhu, SUPA Department of Physics, University of Strathclyde, 107 Rottenrow, Glasgow G4 0NG, UK. Email: jasminder.sidhu@strath.ac.ukSearch for more papers by this authorSiddarth K. Joshi, orcid.org/0000-0003-4987-457X Quantum Engineering Technology Labs, H. H. Wills Physics Laboratory & Department of Electrical and Electronic Engineering, University of Bristol, Bristol, UKSearch for more papers by this authorMustafa Gündoğan, orcid.org/0000-0002-0069-4386 Institüt für Physik, Humboldt-Universität zu Berlin, Berlin, GermanySearch for more papers by this authorThomas Brougham, SUPA Department of Physics, University of Strathclyde, Glasgow, UKSearch for more papers by this authorDavid Lowndes, Quantum Engineering Technology Labs, H. H. Wills Physics Laboratory & Department of Electrical and Electronic Engineering, University of Bristol, Bristol, UKSearch for more papers by this authorLuca Mazzarella, orcid.org/0000-0001-6328-1135 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USASearch for more papers by this authorMarkus Krutzik, Institüt für Physik, Humboldt-Universität zu Berlin, Berlin, GermanySearch for more papers by this authorSonali Mohapatra, SUPA Department of Physics, University of Strathclyde, Glasgow, UK Craft Prospect Ltd., Glasgow, UKSearch for more papers by this authorDaniele Dequal, Scientific Research Unit, Agenzia Spaziale Italiana, Matera, ItalySearch for more papers by this authorGiuseppe Vallone, orcid.org/0000-0003-4965-5801 Dipartimento di Ingegneria dell’Informazione, Università degli Studi di Padova, Padova, Italy Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Padova, ItalySearch for more papers by this authorPaolo Villoresi, Dipartimento di Ingegneria dell’Informazione, Università degli Studi di Padova, Padova, ItalySearch for more papers by this authorAlexander Ling, Centre for Quantum Technologies, National University of Singapore, Singapore, Singapore Department of Physics, National University of Singapore, Singapore, SingaporeSearch for more papers by this authorThomas Jennewein, Institute for Quantum Computing, University of Waterloo, Waterloo, Ontario, Canada Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario, CanadaSearch for more papers by this authorMakan Mohageg, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USASearch for more papers by this authorJohn G. Rarity, Quantum Engineering Technology Labs, H. H. Wills Physics Laboratory & Department of Electrical and Electronic Engineering, University of Bristol, Bristol, UKSearch for more papers by this authorIvette Fuentes, School of Physics and Astronomy, University of Southampton, Southampton, UKSearch for more papers by this authorStefano Pirandola, Department of Computer Science, University of York, York, UKSearch for more papers by this authorDaniel K. L. Oi, orcid.org/0000-0003-0965-9509 SUPA Department of Physics, University of Strathclyde, Glasgow, UKSearch for more papers by this author Jasminder S. Sidhu, Corresponding Author jasminder.sidhu@strath.ac.uk orcid.org/0000-0002-6167-8224 SUPA Department of Physics, University of Strathclyde, Glasgow, UK Correspondence Jasminder S. Sidhu, SUPA Department of Physics, University of Strathclyde, 107 Rottenrow, Glasgow G4 0NG, UK. Email: jasminder.sidhu@strath.ac.ukSearch for more papers by this authorSiddarth K. Joshi, orcid.org/0000-0003-4987-457X Quantum Engineering Technology Labs, H. H. Wills Physics Laboratory & Department of Electrical and Electronic Engineering, University of Bristol, Bristol, UKSearch for more papers by this authorMustafa Gündoğan, orcid.org/0000-0002-0069-4386 Institüt für Physik, Humboldt-Universität zu Berlin, Berlin, GermanySearch for more papers by this authorThomas Brougham, SUPA Department of Physics, University of Strathclyde, Glasgow, UKSearch for more papers by this authorDavid Lowndes, Quantum Engineering Technology Labs, H. H. Wills Physics Laboratory & Department of Electrical and Electronic Engineering, University of Bristol, Bristol, UKSearch for more papers by this authorLuca Mazzarella, orcid.org/0000-0001-6328-1135 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USASearch for more papers by this authorMarkus Krutzik, Institüt für Physik, Humboldt-Universität zu Berlin, Berlin, GermanySearch for more papers by this authorSonali Mohapatra, SUPA Department of Physics, University of Strathclyde, Glasgow, UK Craft Prospect Ltd., Glasgow, UKSearch for more papers by this authorDaniele Dequal, Scientific Research Unit, Agenzia Spaziale Italiana, Matera, ItalySearch for more papers by this authorGiuseppe Vallone, orcid.org/0000-0003-4965-5801 Dipartimento di Ingegneria dell’Informazione, Università degli Studi di Padova, Padova, Italy Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Padova, ItalySearch for more papers by this authorPaolo Villoresi, Dipartimento di Ingegneria dell’Informazione, Università degli Studi di Padova, Padova, ItalySearch for more papers by this authorAlexander Ling, Centre for Quantum Technologies, National University of Singapore, Singapore, Singapore Department of Physics, National University of Singapore, Singapore, SingaporeSearch for more papers by this authorThomas Jennewein, Institute for Quantum Computing, University of Waterloo, Waterloo, Ontario, Canada Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario, CanadaSearch for more papers by this authorMakan Mohageg, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USASearch for more papers by this authorJohn G. Rarity, Quantum Engineering Technology Labs, H. H. Wills Physics Laboratory & Department of Electrical and Electronic Engineering, University of Bristol, Bristol, UKSearch for more papers by this authorIvette Fuentes, School of Physics and Astronomy, University of Southampton, Southampton, UKSearch for more papers by this authorStefano Pirandola, Department of Computer Science, University of York, York, UKSearch for more papers by this authorDaniel K. L. Oi, orcid.org/0000-0003-0965-9509 SUPA Department of Physics, University of Strathclyde, Glasgow, UKSearch for more papers by this author First published: 19 July 2021 https://doi.org/10.1049/qtc2.12015AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat Abstract Concerted efforts are underway to establish an infrastructure for a global quantum Internet to realise a spectrum of quantum technologies. This will enable more precise sensors, secure communications, and faster data processing. Quantum communications are a front-runner with quantum networks already implemented in several metropolitan areas. A number of recent proposals have modelled the use of space segments to overcome range limitations of purely terrestrial networks. Rapid progress in the design of quantum devices have enabled their deployment in space for in-orbit demonstrations. We review developments in this emerging area of space-based quantum technologies and provide a roadmap of key milestones towards a complete, global quantum networked landscape. Small satellites hold increasing promise to provide a cost effective coverage required to realise the quantum Internet. The state of art in small satellite missions is reviewed and the most current in-field demonstrations of quantum cryptography are collated. The important challenges in space quantum technologies that must be overcome and recent efforts to mitigate their effects are summarised. A perspective on future developments that would improve the performance of space quantum communications is included. The authors conclude with a discussion on fundamental physics experiments that could take advantage of a global, space-based quantum network. 1 INTRODUCTION The second quantum revolution promises a paradigm shift in quantum technologies that will deliver new capabilities, and performance enhancements in security, accuracy and precision [1]. The working principles that permit these performance improvements are deep-rooted phenomena in quantum mechanics, such as entanglement [2], teleportation [3-5], the Heisenberg uncertainty principle [6] and the no-cloning theorem [7]. With no classical analogues, these effects illustrate a key departure from classical physics and are at the heart of delivering improvements to technologies that underpin modern society. Fundamentally, quantum theory governs how information is instantiated and processed in the configuration of matter and energy. This realisation led to an explosion in the development of quantum technologies. The race to develop mature quantum technologies has become a discipline in its own right with major renewed international efforts, both governmental and commercial. A number of technologies can inherit enhanced performances and securities when availed to quantum mechanical resources. These technologies include sensing [8-10], metrology [11-13], navigation and timing [14], state discrimination [15-17], communication [18, 19] and computation [20]. Today, these technologies have long evolved from theoretical curiosities to significant developments and even field realisations. Notably, enhanced quantum sensors are now in the vanguard of early practical application of quantum technologies. Fundamental precision bounds to measurements of physical signals can be attained by exploiting specific quantum correlations in probe states [21]. This was recently applied in the Laser Interferometer Gravitational-Wave Observatory, which significantly improves the maturity of gravitational wave astronomy [22-24]. Further, implementation of secure quantum communications have gained momentum owing to sustained improvements in quantum key distribution (QKD) protocols [18]. These improvements have boosted the appetite for early adoption of dedicated quantum technologies. While focussed works have demonstrated performance improvements across the spectrum of quantum technologies, they do not yet satisfy the concomitant technological demand. In addition, most practical applications require the joint operation of multiple quantum technologies. Recent works have established a path towards practical realisation and readiness, by requiring a networked infrastructure of dedicated quantum technologies to upscale capabilities. The essential resource in networked quantum technologies is quantum entanglement, which can be distributed directly [25-28] or through swapping [3, 29] and purification [30, 31] operations across an arbitrary topology of quantum nodes [32-41]. Heuristically, two attributes of quantum entanglement motivate a networked approach to quantum technologies. First, quantum correlations between entangled nodes grants maximum coordination among distant processors. This provides a natural approach to quantum clock synchronisation [14, 42]. It also heralds great potential for applications that employ distributed systems, such as distributed sensing [43-48], astronomical long-baseline interferometry [49], enhanced positioning [50], and consensus and agreement tasks [51, 52]. Second, the monogamy of entanglement provides an inherent privacy that cannot be breached [19]. This is particularly suited to quantum cryptography, where QKD enables secure communications between multiple remote network nodes [25]. The impetus to develop quantum communication is primitive and born from the growing threat of quantum computers on the classical cryptosystems. A networked infrastructure is also intuitively reasoned in quantum computing, where computationally intensive tasks are delegated across shared systems [53, 54]. In direct analogy with classical computing, the capability of quantum computers significantly improves with remote access to ubiquitous quantum processors. Improvements to this delegated approach to quantum computing is conditioned on the existence of high-speed global quantum communications networks. By viewing it in this way, QKD has become the precursor to early applications of quantum communications. The parallel advancements in quantum communications and networked quantum computing progressively evolves the maturity of an emerging quantum Internet [55-57]. This is anticipated to impart a similar revolutionary impact on our world as the classical Internet. To date, the distance-limited span of quantum networks restricts a concrete implementation of this vision. Overcoming this limitation captures the essence of an increasing number of recent works in quantum communications. Ultimately, photonic losses dissolve the hope for a quantum Internet based on optical fibre networks alone. Quantum repeaters [58-62] can extend this capability by providing efficient routing of entanglement [32-35]. Quantum networks have emerged in and between major metropolis areas [25, 63-65]. Despite significant progress on quantum repeaters, global quantum networks remain a hopeful endeavour. The inability to place quantum repeaters along geographically inimical locations rules that option alone as insufficient. The emergence of the quantum Internet necessarily requires the combination of both terrestrial and space-based segments. In this review, we focus on capturing the fast progress in the emerging area of space-based quantum communications. We detail a roadmap to a future implementation of the quantum Internet that unifies both terrestrial and space networks. We start by first exploring applications of a fully operational quantum network in Section 2. The state of art for each application is provided. In Section 3, we review the progress to date in the emerging area of satellite-based quantum communications. This section highlights key milestones in field-demonstrations. We distinguish these demonstrations in terms of the size of the satellite(s) involved in the quantum network. In Section 4, we explore outstanding challenges that currently impede the maturity of space quantum communications. We draw attention to current efforts to mitigate these challenges. In section 5, we review key enabling technologies. Section 6 provides an outlook to the open questions in quantum mechanics that can be addressed with access to a mature realisation of space-based quantum communication network. Throughout this review, we aim to capture the rapid progress in the race to realise global quantum communications. To achieve this, we draw attention to progress in different academic, governmental and commercial efforts, in addition to ongoing difficulties in space technologies. 2 APPLICATIONS The quantum Internet is set to deliver a profound impact across a range of technological frontiers, including quantum communications, computation and metrology. The working infrastructure to this is a globally interconnected network of quantum information processors that deliver enhanced capabilities over the use of purely classical information. Attaining such a connectivity over global scale requires the maturity of both ground and satellite nodes that allows efficient distribution and routing of quantum entanglement across the network, and teleportation of quantum states between nodes. A roadmap for the development of satellite-based quantum networks is presented in Figure 1. In this section, we review specific applications of the quantum Internet. FIGURE 1Open in figure viewerPowerPoint Stages to achieve a global satellite-based quantum Internet. Demonstrating each state reflects an increase to the functionality of the network at the expense of increased technological difficulty. For each stage, we note example applications and protocols that can be demonstrated. See Ref. [57] for a terrestrial-focussed roadmap. QKD, quantum key distribution 2.1 Quantum key distribution Quantum computation [66] has the potential for a paradigm change for solving problems in simulation and optimisation [67, 68]. However, these new computational capabilities also threaten the security of near-ubiquitous public key cryptosystems such as the Rivest–Shamir–Adleman (RSA) protocol [69] or Elliptic Curve Cryptography [70] that underlie much of the Internet infrastructure. While purely classical advances in number theory have posed ongoing threats to RSA [71], the possibility of implementing Shor's algorithm [72] on a large scale quantum computer in the near term poses a risk to both RSA and ECC that cannot be ignored, with major advances in scaling up quantum processors by companies such as Google [73], IBM [74] and Honeywell [75] highlighting a shrinking horizon until the cryptographic quantum apocalypse. Alternative methods for private communication that are quantum secure need to be developed. Progress along this endeavour has primarily proceeded along two distinct routes. First is post-quantum cryptography (PQC), which replaces RSA with alternative classical cryptosystems that are robust to factorisation and quantum algorithms [76]. However, PQC schemes provide a partial solution since they are not information-theoretically secure. The second one is a more promising candidate for quantum-safe encryption called QKD, which guarantees the privacy, authentication, and confidentiality of secure communications. Specifically, signals encoded in optical quantum systems are processed to provide a secure encryption key, which comes with a computable assessment of the knowledge of an eavesdropper. QKD systems are operationally different to classical encryption systems. They take advantage of fundamental properties in quantum mechanics, such as the uncertainty principle or the monogamy of entanglement [19, 77] to safeguard against adversaries with access to arbitrary computational power. There are three distinct families of different QKD protocols [18]. First are discrete-variable (DV) protocols that use discrete quantum degree of freedom to encode information, such as polarisation for free-space applications and phase coding in fibre-based approaches [78-82]. A second, natural alternative approach to QKD protocols is to use continuous-variable (CV) systems [83-95]. In general, thanks to the use of an infinite-dimensional Hilbert space, CV systems can transfer more information per signal compared with qubit-based approaches and rely on cheaper technological implementations; this advantage can be achieved assuming some favourable working conditions and good data-processing performance. A final class of QKD protocols, referred to as distributed-phase-reference coding, implements a hybrid approach [96, 97]. These protocols differ in the detection scheme used. DV and distributed-phase-reference protocols use photon counters and postselect the events in which a detection has effectively taken place. For some wavelengths, photon counters can suffer from low quantum detector efficiencies (DE) and are susceptible to high dark count rates (DCRs) and long dead times. CV protocols overcome these by using homodyne detection. However, a tradeoff between key rate and noise must be addressed for CV approaches. QKD devices have continuously increased their key generation rate and have started approaching maturity, ready for implementation in realistic settings [18, 19]. Secure communications then follow through applications of the one-time pad encryption [98, 99]. This encryption scheme is provably secure provided the keys are secure and not reused, and they are as long as the text to be encrypted [100]. Prominent QKD protocols include the original Bennett–Brassard 1984 (BB84) protocol [78], the two-state Bennett 1992, (B92) protocol [101] and the six-state protocol [102]. For entanglement-based protocols, the common QKD protocols are the Ekert 91 (E91) [80] and the BBM92 [79] protocols. Each protocol has their unique advantages and limitations. We direct the reader to Ref. [18] for a comprehensive review on different DV and CV protocols. The choice for the quantum degree of freedom used to encode information depends on the type of protocols used. A common DV approach is to use the polarisation of light [78]. Polarisation states are simple to prepare, control and measure. Furthermore, it is possible to produce high fidelity polarisation entangled photons [103]. Since there are only two linearly independent polarisation states, only one bit per photon can be encoded using this approach. There are alternative DV encodings that include the orbital angular momentum (OAM) [104, 105] and time-bin encodings [106, 107]. Both of these approaches allow multiple encoded bits on each photon. These high-dimensional encodings can also be more robust to certain types of errors [108, 109]. However, these encodings are more difficult to realise experimentally. To illustrate this, consider the example of time-bin encodings. Entangled photon pairs that are in a superposition of many different discrete time-bins can be prepared using a mode-locked laser with beam-shaping techniques. It is possible to encode 10 bits per photon with practical laboratory conditions [110]. However, the greater the number of time-bins, the more difficult it becomes to perform the mutually unbiased measurement needed to detect an eavesdropper. For this reason, it is common to adopt a hybrid CV–DV encoding and use the so-called energy-time entangled photons. These are photons that are entangled both in time-of-arrival and frequency. Discrete time-bins can be imposed on the continuous time-of-arrival. An eavesdropper is then detected by a suitable interferometeric measurement [111]. QKD has been demonstrated reliably on the ground, and the process of being deployed across ground networks is well under way. To attain better key rates, practical implementations of QKD differ from theoretical proposals primarily in the components used in the system. Unfortunately, most security proofs are very sensitive to small differences between the physical devices used by the protocol and the theoretical model used to describe them. This introduces side channels to QKD systems that can be exploited for vulnerabilities in practical devices [112-119], therefore introducing an important trade-off between security and rate [120]. There is another crucial trade-off that must be considered. The foremost that hinders the maturity of QKD across global scales is the trade-off between rate and distance [121-123], typically induced by scattering effects in optical fibres [124, 125]. Specifically, the maximum number of secret bits that can be distributed over a lossy channel with transmissivity η is upper bounded by the repeaterless PLOB bound − log 2 (1 − η) [123], which approximately amounts to 1.44η bits per channel use at low transmissivity. This corresponds to the secret-key capacity of the lossy channel, an optimal key rate that cannot be overcome by any point-to-point protocol [18]. A similar limitation affects the free-space fading channel between two ground station, so that the optimal key rate cannot exceed the bound [126] −Δ log 2 (1 − η), where η is the maximum free-space line-of-sight transmissivity and Δ accounts for the non-trivial turbulence effects on the ground. Quantum repeaters [58-62] and, more generally, multi-hop quantum networks [32-35] can provide better rates over long distances. However, these strategies provide only limited remission and have their own limitations in achievable key rates. In particular, for a chain of quantum repeaters connected by fibre-links with transmissivities ηi, the maximum secret key rate achievable by the two end-users (i.e., Alice and Bob at the two opposite ends of the chain) cannot exceed −log 2 (1 − miniηi) bits per use of the chain [33]. The previous formula implies that, for Alice and Bob being able to generate 1 secret bit per chain use, we may tolerate at most 3 dB of loss in each individual link, which means that we need to insert a repeater every 15 km (at the standard fibre loss of 0.2 dB/km). Therefore, despite significant progress, all ground-based QKD approaches remain distance-limited due to loss. These limitations make purely ground-based systems very challenging or even impractical for a global distribution network. Besides QKD entanglement distribution also remains an open challenge over global transmission lengths. This latter aspect, together with practical types of quantum repeaters, will be discussed in Section 2.4. The range of communication may be extended by employing satellites equipped with high-quality optical links. Figure 2 illustrates different configurations of satellite-based quantum communications. The use of satellites reduces the demand on the number of ground quantum repeaters, since a single trusted-node satellite may physically travel from the proximity of Alice to the proximity of Bob, with the loss affecting only the uplink and downlink channels. It is also due to the fact that inter-satellite communications would be affected by less noise and loss with respect to ground links. The use of both ground and satellite-based quantum repeater networks provide the most promising solution to extend quantum communications to global scales. This provides a practical roadmap towards the implementation of a full quantum Internet [55-57]. It also paves the way to a promising realisation of a global networked infrastructure for global communication [133, 134], imaging [135], and enhanced sensing [8, 9, 11, 12, 10]. FIGURE 2Open in figure viewerPowerPoint Some satellite-based quantum communication configurations. There are many possible different ways of utilising satellites to mediate quantum communication links. Each red line represents a quantum channel from a transmitter (Tx) to a receiver (Rx). (a) An optical ground station transmits quantum signal states to an orbiting satellite where they are measured [127]. (b) This topology can be reversed such that the satellite carries the quantum source and sends signals down to the Earth [128]. (c) A satellite distributes entanglement to two grounds stations by transmitting entangled photon pairs [129, 130]. For MDI-QKD, a double uplink configuration could be used [131, 132]. (d) Inter-satellite links with ground links. In general, more complex network topologies can be created from multiple channels [133]. MDI, measurement device independent; QKD, quantum key distribution Transmitting information via satellites comes with its own difficulties [136-140]. Besides loss effects induced by free-space diffraction and atmospheric extinction (due to absorption and scattering), we also need to account for the inevitable beam wandering caused by pointing and tracking errors (specifically important for downlink) and atmospheric turbulence (relevant for uplink). Besides these effects, there is also the presence of background thermal noise affecting the receiver. Depending on the configuration, this may come from the sky or from the planetary albedos of the Earth and the Moon. Accounting for all these effects, one may compute the ultimate information-theoretic performances achievable by satellite quantum communications [136] together with practical key rates achievable by current technology [136, 139, 140]. In particular, one can prove that the number of secret bits that can be distributed between two remote ground stations by means of a trusted satellite can largely exceed the performance achievable by a chain of perfect quantum repeaters on the ground [136]. This is illustrated in Figure 3. Further studies have shown the utility of satellite-based QKD with respect to global quantum networks on the ground, even when the more powerful strategy of multi-path routing is used [41]. FIGURE 3Open in figure viewerPowerPoint Comparison between ground-based and satellite-based QKD. We plot the number of secret bits per day versus ground distance that are distributed between two stations, assuming a clock (repetition rate) of 5 MHz. The black lines represent the optimal key rates achievable by a repeaterless fibre-connection (PLOB bound [123]) and by repeater-based fibre connections assisted by 1, 5, 20 and 50 ideal quantum repeater nodes [33]. The red line represents the constant key rate that is achievable by using a near-polar sun-synchronous satellite crossing the zenith of the two ground stations operating in trusted node. This is for a mean altitude of 530 km in downlink at 800 nm. The performance accounts for all the effects of diffraction, atmospheric extinction, (weak) turbulence, pointing error and background thermal noise. Ground-based PLOB bound is beaten at ≃215 km of standard optical fibre. See Ref. [136] for details about the parameters. QKD, quantum key distribution 2.2 Remote clock synchronisation Remote clock synchronisation is an important requirement to the future of quantum telecommunication, precise positioning and navigation [12], and applications in fundamental science [141]. Conventional time synchronisation techniques rely on measuring the time of arrival of electromagnetic pulses [142, 143]. These classical methods are both susceptible to malicious intervention [144] and are limited in accuracy b