Quantum technologies have the potential to significantly affect many areas of human activity. This is especially true for the defence sector. Quantum technologies can impact all the domains of modern warfare. The second quantum revolution will improve sensitivity and efficiency, and introduce new capabilities and sharpen modern warfare techniques rather than lead to new types of weapons.
The following text maps the conceivable quantum technology applications for military, security, space and intelligence in different aspects of modern warfare, as sketched in Fig. 1. It also mentions the industrial applications which may suggest quantum technologies’ capabilities and performances, especially when no public information on military applications is available.
It is important to notice that many applications are still more theoretical than realistic. The significant quantum advancement achieved in the laboratory does not always result in similar progress outside the laboratory. The transfer from laboratory to practical deployment involves other aspects too, such as portability, sensitivity, resolution, speed, robustness, low SWaP (size, weight and power) and cost, apart from a working laboratory prototype. The practicality and cost-effectiveness of quantum technologies will determine whether particular quantum technologies are manufactured and deployed.
The integration of quantum technology into a military platform is even more challenging. Apart from quantum computers that will mostly be located at data centres similarly as for civil use, the integration and deployment of quantum sensing, imaging and networks faces several challenges posed by the increased demands of military use (in comparison with civil/industry or scientific requirements). For example, the military level requirement of precise navigation necessitates fast measurement rates that can be quite limiting for the current quantum inertial sensors. There are more examples, and probably more are yet to come.
Moreover, this area is still very young, and new technological surprises, both in a bad and a good sense, could impose other quantum advantages or disadvantages.
5.1 Quantum cybersecurity
Key points:
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Necessity of quantum crypto-agility implementation.
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Operations that want to take advantage of Shor’s algorithm should start to collect the data of interest before the quantum-safe encryption is deployed.
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The implementation of QKD needs to be carefully considered.
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In QKD, the endpoints will be the weakest part of the system.
Quantum advantage in cyber warfare can provide new, but on the one hand very effective (with exponential speedup), vectors of attack on the current asymmetric encryptions (based on integer factorisation, the discrete logarithm or the elliptic-curve discrete logarithm problem) and, theoretically, on symmetric encryption [90, 156]. On the other side are new quantum-resilient encryption algorithms and approaches, as well as quantum key distribution. For an overview, see, for example, [157–160].
The current trend also is the development and employment of machine learning or artificial intelligence for cyber warfare [161]. For more details on the quantum opportunities, see Sect. 5.2.
5.1.1 Quantum defence capabilities
The post-quantum cryptography implementation is the ‘must-have’ technology that should be carried out as soon as possible. The risk that hostile intelligence is gathering encrypted data with the expectation of future decryption using the power of quantum computers is real, high and present [162]. This applies to military, intelligence and government sectors as well as to industry or academia where secrets and confidential data are exchanged or stored. The current trend is to start preparing the infrastructure for implementing quantum crypto-agility when the certified (standardised) post-quantum cryptography becomes ready to deploy [90, 156].
New quantum-resilient algorithms can offer not only a new mathematical approach difficult enough even for quantum computers, but also a new paradigm of working with encrypted data. For instance, fully homomorphic encryption (FHE) allows the data to never get decrypted—even if they are being processed [163]. Although the security applications, such as for genomic data, medical records or financial information, are the most mentioned, applications for intelligence, military or government are evident, too. As such, FHE is a good candidate for cloud-based quantum computing to ensure secure cloud quantum computation [164].
Note that post-quantum cryptography should be implemented in the Internet of Things (IoT), or the Internet of Military Things (IoMT) [165], as a rapidly growing sector with many potential security breaches. For an overview of post-quantum cryptography for IoT, see [166].
Quantum key distribution (QKD) [160, 167, 168] is another new capability that allows safe encryption key exchange where the security is mathematically proven. Although it is impossible to eavesdrop on the quantum carrier of the quantum data (key), the weaknesses can be found at the end nodes and trusted repeaters, due to imperfect hardware or software implementation. Another question is the cost, considering the quantum data throughput, security and non-quantum alternatives independently if the solution is optical fibre-based or utilising quantum satellites. The QKD solution seems to be preferred in EU [169], while the post-quantum encryption solution finds favour in US [170].
The last note refers to quantum random number generators. QRNG increases security [171] and denies attacks on pseudorandom number generators [172].
5.1.2 Quantum attack capabilities
With Shor’s algorithm-based quantum cryptoanalysis of Public key encryption (PKE)—for instance, RSA, DH, ECC—the attacker can decrypt the encrypted data collected earlier. There is no precise forecast when the so-called ‘Q-Day’, the day when a quantum computer breaks the 2048-bit RSA encryption, will happen. However, the general opinion is it will take about 10–15 years (based on a survey in 2017) [173]. A similar threat applies to most message authentication codes (MAC) and authenticated encryption with associated data (AEAD), such as HMAC-CBC and AES-GCM, because of Simon’s algorithm and superposition queries.
One has to assume that such offensive operations already exist or that intense research is being done. In 10 years, most sensitive communication or subjects of interest will be using the post-quantum cryptography or QKD implemented in the next six years. That means by the time a quantum computer able to crack PKE becomes available, most of the security-sensitive data will be using a quantum-safe solution.
In theory, Grover’s algorithm weakens the symmetric key encryption algorithms; for example, DES and AES. However, the quantum computing, and in particular quantum memory, requirements are so huge that it seems to be unfeasible in the next few decades [174].
Another vector of attack uses the classical hacking methods of classical computers that will remain behind quantum technologies. In general, quantum technology is a technologically young sector where plenty of new quantum system control software is being developed. The new software and the hardware tend to have more bugs and security breaches. For example, the current QKD quantum satellites working as trusted repeaters controlled by a classical computer can be an ideal target for a cyber attack. Moreover, specific physical-based vectors of attack against quantum networks (e.g. QKD) are the subject of active research [175], such as photon-number-splitting [81] or the Trojan-horse attack [82], and future surprises cannot be excluded. For an overview of quantum hacking, see, e.g. [157].
5.2 Quantum computing capabilities
Key points:
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Quantum computing capabilities will increase with the number of logical qubits.
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Most likely, quantum computing will be used as part of a hybrid cloud.
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Small, embedded quantum computing systems are desirable for direct quantum data processing.
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General use for quantum optimisations, ML/AI enhancements and faster numerical simulations.
Quantum computing will introduce new capabilities to the current classical computing services, helping with computational problems of high complexity. Further, besides the quantum simulations described above, quantum computing covers quantum optimisations, machine learning and artificial intelligence (ML/AI) improvement, quantum data analysis, and faster numerical modelling [11, 24]. The military problems that could be solved with near-term quantum computers were presented in [10]. They are: Battlefield or war simulations; Analysis of radio frequency spectrum; Logistics management; Supply chain optimisation; Energy management; and Predictive maintenance.
To get the most effective results, future quantum computing implementation will be in computing farms along with classical computers, which will create a hybrid system. A hybrid quantum-classical operating system will analyse the tasks to be computed using ML/AI, and split individual computations into resources such as CPU, GPU, FPGA,Footnote 10 or quantum processor (QPU), where the best and fastest result can be obtained.
A small, embedded quantum computer that could be placed, for example, in an autonomous vehicle or mobile command centre is questionable. The current most advanced qubit designs need cryogenic cooling. Therefore, more efforts should be focused on the other qubit designs as photonic, spin or NV centres that can work at room temperature. The embedded quantum chip could perform simple analytical tasks or serve for simple operations related to quantum network applications where a straightforward quantum data process is desired. Nevertheless, the machine learning and model optimisation of autonomous systems and robotics can also benefit from ‘large’ quantum computers.
Quantum computing is likely to be efficient in optimisation problems [10, 176, 177]. In the military sector, examples of quantum optimisations could be logistics for overseas operations and deployment, mission planning, war games, systems validation and verification, new vehicles’ design and their attributes such as stealth or agility. At the top will be an application for enhanced decision making, supporting military operations and functions through quantum information science, including predictive analytics and ML/AI [178]. Specifically, quantum annealers have proven themselves in verifying and validating complex systems’ software code [179, 180].
Quantum computers are expected to play a significant role in Command and Control (C2) systems. The role of C2 systems is to analyse and present situational awareness or assist with planning and monitoring, including simulation of various possible scenarios to provide the best conditions for the best decision. Quantum computers can improve and speed up the scenario simulations or process and analyse the Big Data from ISR (Intelligence, Surveillance and Reconnaissance) for enhanced situational awareness. This also includes the involvement of quantum-enhanced machine learning and quantum sensors and imaging.
Quantum information processing will probably be essential for Intelligence, Surveillance, and Reconnaissance (ISR) or situational awareness. ISR will benefit from quantum computing, which offers a considerable boost to the ability to filter, decode, correlate and identify features in signals and images captured by ISR. Quantum image processing in particular is an area of extensive interest and development. It is expected that in the near term situational awareness and understanding can benefit from quantum image analysis and pattern detection utilising neural networks [13].
Quantum computing will enhance classical machine learning and artificial intelligence [54], including for defence applications [178]. Here, quantum computing will surely not be practical to carry out the complete machine learning process. Nevertheless, quantum computing can improve ML/AI machinery (e.g. quantum sampling, linear algebra, quantum neural networks). A recent study [181] shows that quantum ML provides an advantage just for some kernels fitting particular problems. Quantum computing can possibly enhance, in principle, most classical ML/AI applications in defence; for example, automating cyber operations, algorithmic targeting, situation awareness and understanding and automated mission planning [182, 183]. The most immediate application of quantum ML/AI is probably quantum data; for instance, data produced by quantum sensing or measuring apparatus [55]. Actual applicability will grow with quantum computer resources, and in eight years, quantum ML/AI can be one of the important quantum computing applications [184]. Such applicability can be accelerated by hybrid classical-quantum machine learning where tensor network models could be implemented on small near-term quantum devices [185].
Quantum computers, through quantum neural networks, can be expected to provide superior pattern recognition and higher speed. This may be essential, for instance, in bio-mimetic cyber defence systems that protect networks, analogously to the immune systems of biological organisms [13].
Besides, through faster linear algebra (see 3.2.5), quantum computing has the potential to improve the current numerical linear equation-based numerical modelling in the defence sector, such as war games simulations, radar cross section calculations, stealth design modelling, etc.
In the long term, the quantum systems can enable Network Quantum Enabled Capability (NQEC) [13]. NQEC is a futuristic system that allows communication and sharing information across the network between individual units and the commander to respond quickly to battlefield developments and for coordination. Quantum enhancement can bring secured communication, enhanced situational awareness and understanding, remote quantum sensor output fusing and processing, and improved C2.
5.3 Quantum communication network
Key points:
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Various security applications (e.g. QKD, identification and authentication, digital signatures).
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The adoption of security applications will happen as quickly as all new technology security aspects are explored, carefully.
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Quantum clock synchronisation allows utilising higher precision quantum clocks.
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Quantum internet is the most effective way of communication between quantum computers and/or quantum clouds.
Quantum internet stands for a quantum network with various services [186] which have significant, and not only security, implications. However, many progressive quantum communication network applications require quantum entanglement; that is, they require quantum repeater and quantum switch. Recall that the trusted repeaters can be used for QKD only (see Sect. 3.3.1). Future combinations of optical fibre and free-space channels will interconnect various end nodes such as drones, planes, ships, vehicles, soldiers, command centres, etc.
5.3.1 Security applications
Quantum key distribution is one of the most matured quantum network applications. This technology is going to be interesting for the defence sector later, when long-distance communication using MDI-QKD or quantum repeaters becomes possible. Currently, basic commercial technology that uses trusted repeaters is available. These pioneers can serve as a model of how quantum technologies can be employed. Here, QKD companies promote the technology as the most secure, and more and more use cases appear, especially in the financial and healthcare sectors. On the other hand, the numerous recommendation reports and authorities are more circumspect; for example, the UK National Cyber Security Centre [187] that does not endorse QKD for any government or military applications in its current state.
Apart from QKD, which distributes the key only, the quantum network could be used for quantum-secure direct communication (QSDC) [188–191] between space, special forces, air, navy and land assets. Here, the direct messages encrypted in quantum data take advantage of security similar to QKD. One obstacle could be a low qubit rate, which will only allow sending simple messages and not audiovisual and complex telemetry data. In that case, the network switch to the QKD protocol for distributing the key and the encrypted data will be distributed over classical channels. Other protocols such as quantum dialogue [192] and quantum direct secret sharing [193] aim to use the quantum network for provable secure communications as QSDC. Note that QKD and QSDC are considered to be a native part of 6G wireless communication networks and discussed accordingly in [194].
Another significant contribution of the quantum approach to security is the quantum digital signature (QDS) [195]. It is the quantum mechanical equivalent of a classical digital signature. QDS provides security against tampering of a message after a sender has signed the message.
Next, quantum secure identification exploits quantum features allowing identification without revealing authentication credentials [72]. Non-quantum identification is based on the exchange of login and password or cryptographic keys, which allows intruders to at least guess who has tried to authenticate.
The other application is position-based quantum cryptography [196, 197]. Position-based quantum cryptography can offer more secure communication, where the accessed information will be available only from a particular geographical position, such as communication with military satellites only from particular military bases. Position-based quantum cryptography can also provide secure communication when the geographical position of a party is its only credential.
5.3.2 Technical applications
Quantum network will perform network clock synchronisation [71, 198] that is already a major topic in classical digital networks. Clock synchronisation aims to coordinate otherwise independent clocks, especially atomic clocks (e.g. in GPS) and local digital clocks (e.g. in digital computers). A quantum network that uses quantum entanglement will reach even more accurate synchronisation, especially when quantum clocks come to be deployed (for Time standards and frequency transfer see Sect. 5.4). Otherwise, the high precision of quantum clocks would be utilised locally only. Precise clock synchronisation is essential for the cooperation of C4ISR (Command, Control, Communications, Computers, Intelligence, Surveillance and Reconnaissance) systems for accurate synchronisation of various data and actions across radar, electronic warfare, command centres, weapon systems, etc.
A short note is dedicated to blind quantum computing [69, 70]. This class of quantum protocols allows for a quantum program to run on a remote quantum computer or quantum computing cloud and retrieve results without the owner knowing what the algorithm or result was. This is valuable when secret computation is needed (e.g. military operation planning or new weapon technology design) and no own quantum computer capability is available.
Distributed quantum computing via the quantum network—see Sect. 3.3.1—will be important for the military and governmental actors owning quantum computers, to build high-performance quantum computing services or quantum cloud.
A quantum network capable of distributing entanglement can integrate and entangle quantum sensors [77] for the purpose of improving the sensitivity of the sensors, reducing errors, and most importantly to perform a global measurement. That provides an advantage in cases where the parameters of interest are global properties of the entire network; for example, when a signal’s angle of arrival needs measurement from three sensors, where each measures a signal with a certain amplitude and phase. Afterwards, each sensor’s output can be used to estimate the angle of arrival of the signal. Quantum entangle sensors can evaluate this globally. This process can then be improved by machine learning [78].
Quantum protocols for distributed computing agreement [76] can have advantageous military application for a swarm of drones, or in general for a herd of autonomous vehicles (AVs). Here, quantum protocols can help achieve agreement between all AVs at the same time scale, independent of their quantity. Nevertheless, open space quantum communication between all rapidly moving AVs will be a challenge that has to be solved first. Note that the first experiment of quantum entanglement distribution from a drone was successfully carried out, recently [64].
5.4 Quantum PNT
Key points:
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All quantum PNT technologies have in common the demand for a highly accurate quantum clock.
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Quantum inertial navigation could bring few orders of magnitudes higher precision than its classical counterpart.
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Quantum inertial navigation can be extended by the quantum augmented navigation using quantum magnetic or gravity mapping.
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Promising quantum navigation based on Earth’s magnetic anomalies.
Quantum technologies are expected to significantly improve positioning, navigation and timing (PNT) systems, especially inertial navigation. Time standards and frequency transfer (TFT) is a fundamental service that provides precise timing for communication, metrology, but also global navigation satellite system (GNSS). Although present TFT systems are well established, the performance of optical atomic or quantum clocks in combination with TFT utilizing quantum networks [199, 200] will keep pace with the increasing demands of the present applications (communication, GNSS, financial sector, radars, electronic warfare systems) and enables new applications (quantum sensing and imaging).
New quantum-based technologies and approaches support the development of sensitive precision instruments for PNT. The quantum advantage will be manifested for GPS denied or challenging operational environments, enabling precise operations. Examples of such environments are underwater and underground, or environments under GPS jamming.
Current GNSS (GPS, GLONASS, Galileo, BeiDou, …) rely on precise timing provided through multiple atomic clocks in individual satellites that are corrected by the more stable atomic clocks on the ground. The higher precision of the quantum clock will increase the accuracy of positioning and navigation as well. Over the long term, the GNSS satellites should be connected to the quantum internet for timing distribution and clock synchronisation. Chip-size precise mobile clocks could help discover GNSS deception and spoofing [201].
Some quantum GNSS (not only quantum clock) have been considered and investigated; for instance, interferometric quantum positioned system (QPS) [199, 202, 203]. One of the schemes of QPS [202, 203] has a structure similar to the traditional GNSS where there are three baselines, each consisting of two low-orbiting satellites, with the baselines are perpendicular to each other. However, although theoretically the accuracy of positioning is astonishing, significant engineering must be done to design a realistic QPS.
Most of the current navigation relies on GPS, or in general GNSS, which is the most precise available technology for navigation. GNSS technology is prone to jamming, deception, spoofing or GPS-deprived environments such as densely populated areas with high electromagnetic spectrum use. Moreover, for underground or underwater environments, GNSS technology is not available at all. The solution is inertial navigation. The problem with classical inertial navigation is its drifting, a loss of precision over time. For example, the marine-grade inertial navigation (for ships, submarines and spacecraft) has a drift 1.8 km/day and navigation grade (for military aircraft) has a drift 1.5 km/hour [204]. In 2014, DARPA started a MTO-PTN project with a goal to reach drift 20 m and 1 ms/hour [205]. Even so, some expectations are very high, that quantum inertial navigation will offer error of only approximately hundreds of meters per month [5, 206].
The full quantum inertial navigation system consists of a quantum gyroscope, accelerometer and atomic/quantum clocks. Although the individual sensors required for quantum inertial navigation are tested out of laboratories, it is still challenging to create a complete quantum inertial measurement unit. For navigation for highly mobile platforms, sensors need fast measurement rates of several 100 Hz, or to improve the measurement bandwidth of quantum sensors [204, 207]. The key component that needs the most improvement is the low-drift rotation sensor. The classical inertial sensors are based on various principles [208]. One common chip-size technology is the MEMS (Micro Electro Mechanical Systems) technology, where MEMS gyroscopes have demonstrated instabilities at level \(\sim 10^{-7}\text{ rad}\cdot \text{ s}^{-1}\) that is suitable for military applications [99]. The instability limit for the best current cold-atom gyroscopes is about \(\sim 10^{-9}-10^{-10}\text{ rad}\cdot \text{ s}^{-1}\) (at integration time 1000 s) [209]. The uncertainty is in the precision of the in field-deployable quantum sensors in comparison to the presented laboratory experiments’ precision. The intermediate step between classical and quantum inertial navigation can be a hybrid system fusing the outputs of classical and quantum accelerometers [210]. With the size of the quantum inertial navigation device decreasing to chip size, its deployment can be expected on smaller vehicles, especially unmanned autonomous vehicles or missiles. However, the miniaturisation we can reach is unknown. There are many doubts about chip-sized quantum inertial navigation. It is certainly a next-generation technology, although a very big challenge.
Currently, the individual elements, such as gyroscope or accelerometer, are also tested on various platforms; for instance, on board an aircraft [211], or more recently a [212].
For many years, the US National Oceanic and Atmospheric Administration (NOAA) were mapping the Earth’s magnetic anomaly and creating a magnetic anomalies map. Using sensitive quantum magnetometers in combination with Earth’s magnetic anomaly map is another way to realise quantum non-GNSS navigation [213, 214].
Gravitational map matching [215] works on a similar principle, and one can expect improved performance using the quantum gravimeter. Together, quantum gravimeter and magnetometer could be a basis for a submarine quantum augmented navigation, especially in undersea canyons, wrinkled seabeds, or littoral environments.
In general, quantum inertial navigation or augmented navigation has vast potential, since there is no need for GPS, infra or radar navigation and it is not susceptible to jamming, or in general to electronic warfare attacks. However, the claim of ‘no need for GPS’ is not quite accurate. These systems will always need some external input on their initial position, most probably from GNSS.
5.5 Quantum ISTAR
Key points:
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Intense involvement of quantum computing to gather and process information.
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Desired deployment on low-orbit satellites, but the resolution is questionable.
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Vast applications for undersea operations.
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Expected advanced underground surveillance with uncertain resolution.
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New type of 3D, low-light or low-SNR quantum vision devices.
ISTAR (intelligence, surveillance, target acquisition and reconnaissance) is a crucial capability of a modern army for precise operations. Quantum technologies have the potential to dramatically improve situational awareness of multi-domain battlefields.
In general, a large impact can be expected from quantum computing that will help with acquiring new intelligence data, processing Big Data from surveillance and reconnaissance and identifying targets using quantum ML/AI [178, 183].
Apart from the processing part of ISTAR, dramatic advancement can be expected from quantum sensing placed on individual land/sea/aerial vehicles and low-orbit satellites.
Quantum gravimeters and gravitational gradiometers promise high accuracy that can improve or introduce new applications: geophysics study, seismology, archaeology, minerals (fissile material or precious metals) and oil detection, underground scanning and precise georeferencing and topographical mappings (e.g. of the seabed for underwater navigation) [7].
Another significant type of sensing is quantum magnetometry. The applications of quantum magnetometry are partially overlapped by applications for quantum gravimetry, thus introducing new applications: Earth’s magnetic field including magnetic anomalies, local magnetic anomalies due to the presence, such as metallic objects (submarines, mines, etc.), or weak biological magnetic signals (applications mainly for medical purposes) [7].
The third field interesting for ISTAR is quantum imaging. Quantum imaging offers plenty of diverse applications; for example, quantum radar (see Sect. 5.7), imaging devices for medicine, 3D camera, stealth rangefinder, etc.
The potential quantum computing applications in ISR and situational awareness are described in Sect. 5.2.
5.5.1 Quantum Earth’s surface and underground surveillance
Quantum sensing based on magnetometry, gravimetry and gravity gradiometry at the first level helps with the study of continents and sea surface, including underground changes of natural origin. Both magnetic anomaly and gravity-based sensing provide a different picture of the Earth’s surface. The Earth is very inhomogeneous (ocean, rocks, caves, metallic minerals, …), including the massive constructions or vehicles made by people which generate a unique gravitational (depending on the mass) and magnetic (depending on metallic composition) footprint.
The discussed quantum sensing technologies—magnetometry, gravimetry and gravity gradiometry—can reach very high precision, at least in the laboratory. For example, the precision of absolute gravimetry out of the laboratory is about \(1~\mu\text{Gal}\) (\(10\text{ nm}\cdot \text{ s}^{-2}\)) [216]. Note that the sensitivity of \(3.1~\mu \)Gal corresponds to a sensitivity per centimetre of height above the Earth’s surface. However, the problem is the spatial resolution that usually is anti-correlated with the sensitivity (higher sensitivity is at the cost of lower spatial resolution and vice versa). Spatial resolution and sensitivity are the critical attributes that define what you will recognise (large-scale natural changes or small underground structures) and from what distance (from the ground, drone or satellite-based measurement). Examples of the current spatial resolution are about 100 km [217] for satellite-borne gravity gradiometer or 16 km [218] additional width using radar satellite altimetry (for sea areas), or 5 km [219] for airborne gravimetry. For more information, see e.g. [5].
For many quantum sensing applications, it would be essential to place sensors on low Earth orbit (LEO) satellites [220]. However, the current sensitivity and spatial resolution allow only the applications for Earth monitoring (mapping resources such as water or oil, earthquake or tsunami detection).
Apart from low-orbit satellites, the mentioned quantum sensors are considered for deployment on airborne, sea or ground vehicle platforms. Nowadays, quantum sensing experiments are performed outside the laboratory environment, such as in a truck [221], on drones and aeroplanes [222, 223] or aboard ships [217]. For example, the quantum gravimeter could be mounted on drones to search for human-made structures such as tunnels used to smuggle drugs [223]. Placing quantum sensing devices on a drone (this may be an unmanned aerial vehicle (UAV), Unmanned Surface Vessel (USV), Remotely Operated Vehicle (ROV) or unmanned underwater vessel (UUV)) needs more engineering to reach the best sensitivity, resolution and operability simultaneously.
Low-resolution quantum sensing could be used for precise georeferencing and topographical mappings to help with underwater navigation or mission planning in rugged terrain. Also, the detection of new minerals and oil fields can become a new centre of interest, especially under the seabed [224]. This can be a source of international friction, despite the fact that borders are clear in most cases.
High-resolution quantum magnetic and gravity sensing [217, 225–227] is considered in numerous reports and articles [7, 225, 228–231] to be able to: detect camouflaged vehicles or aircraft; effectively search for a fleet of ships or individual ships from LEO; detect underground structures such as caves, tunnels, underground bunkers, research facilities and missile silos; localise buried unexploded objects (landmines, underwater mines and improvised explosive devices); achieve through-wall detection of rotating machinery.
However, note again that it is highly uncertain where the technical limits are and whether the mentioned quantum gravimetry and magnetometry applications will reach such sensitivity and resolution (especially for using from LEO) as to realise all the aforementioned ideas. Quantum sensors will be delivered to the market in many generations, each with better sensitivity and resolution and lower SWaP, allowing more extensive deployment and application.
5.5.2 Quantum imaging systems
Besides quantum radar and lidar (see Sect. 5.7), there are other military-related applications of quantum imaging. In general, all-weather, day-night tactical sensing for ISTAR for long/short-range, active/passive regime, invisible/stealth using EO/IR/THz/RF frequencies features and advantages are considered. Quantum imaging systems can use various techniques and quantum protocols; for example, SPAD, quantum ghost imaging, sub-shot-noise imaging, or quantum illumination as was described in Sect. 3.4.4. In general, it is not a problem to construct quantum imaging systems of small sizes. The critical parameters are the flux of the single-photon/entangled photon emitter or the single-photon detection resolution and sensitivity. Moreover, a large-scale deployment of a quantum imaging system with high photon flux will require powerful processing that can limit the system deployability and performance.
Quantum 3D cameras exploiting quantum entanglement and photon-number correlations will introduce fast 3D imaging with unprecedented depth of focus with low noise aiming at sub-shot noise or long-range performance. This capability can be used to inspect and detect deviation or structural cracks on jets, satellites and other sensitive military technology. Long-range 3D imaging from UAV can be used for reconnaissance and to explore mission destination or hostile facilities and equipment.
Another commercially available technology is quantum gas sensors [232]. Technically it is a single-photon quantum lidar calibrated to detect methane leakage. The next prepared product is a multiple gas detector able to also detect carbon dioxide (CO2). With proper improvement and calibration, it could serve for human presence detection, too.
A specific feature at short range is the possibility of behind-the-corner or out of the line-of-sight visibility, [126]. These methods can help to locate and recover trapped people, people in hostage situations or to improve automated driving by detecting incoming vehicles from around a corner.
Quantum imaging can serve as a low-light or low-SNR vision device; for example, in an environment such as cloudy water, fog, dust, smoke, jungle foliage or in the nighttime, leading to an advantage. Low-SNR quantum imaging could help in target detection, classification and identification with low signal-to-noise ratios or concealed visible signatures and potentially counter adversaries’ camouflage or other target-deception techniques. Quantum imaging will be very useful for helicopter pilots when landing in dusty, foggy or smoky environments [9].
One significant product will be a quantum rangefinder [233, 234]. Conventional rangefinders use a bright laser and can be easily detected by the target. A quantum rangefinder will be indistinguishable from the background both temporally and spectrally when viewed from the target. In other words, the quantum rangefinder will be invisible and stealthy, including at night time, whereas the classical rangefinder can be visible to the target or others.
Under some circumstances, quantum ghost imaging can play the role of quantum lidar [235], especially when the target does not move or moves very slowly and infinite depth of focus is required for 3D imaging.
5.6 Quantum electronic warfare
Key points:
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Enhancement of current EW by smaller universal quantum antennas, precise timing and advanced RF spectrum analysers.
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The problem with detection of quantum channels.
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When the quantum channel is localised, several types of attacks are considered and developed.
Quantum electronic warfare (EW) can be divided into quantum-enhanced classical EW and quantum EW focusing on countermeasures, counter-countermeasure and support against quantum channels. By a quantum channel is meant any transfer of photons carrying quantum information for quantum internet, quantum radar or another quantum system that uses the free-space or optical fibres channel.
Classical EW systems for electronic support measures can benefit from the quantum antenna. Quantum antenna based on Rydberg atoms can offer a small size independent of the measured signal wavelength (frequency) [122, 123]. This means that even for low-frequency (MHz to kHz [124, 236]) signal interception a few-micrometres of quantum antenna is sufficient. There can be an array of quantum antennas for multi-frequency measurement for different bandwidths or one antenna dynamically changing bandwidth according to the interest. Moreover, Rydberg atoms-based antennas can measure both AM and FM signals, offer self-calibration, and measure both weak and very strong fields and detect the angle-of-arrival [125]. In the future, quantum antennas could look like an array (matrix) of Rydberg atom cells. Different cells can measure different signals, and in the joint measurement of two or more cells, the angle-of-arrival of the signal could be determined. The weakest aspect of such antennas is the cryogenics required for cooling Rydberg atoms that need to be scaled down to an acceptable size. In general, quantum RF sensors are a key enabler for advanced (LPD/LPIFootnote 11) communications, over-horizon directional RF, resistance to RF interference and jamming, RF direction finding, or RF-THz imaging. As an example, an arrayed quantum RF sensor is developed as a potential upgrade for fighter F-35 [237].
Classical EW can also benefit from quantum computing, offering improved RF spectrum analysers for electronic warfare where quantum optimisations and quantum ML/AI techniques can be applied. Higher effectiveness can be reached by the processing and analysing directly of quantum data [55] from RF quantum sensors (Rydberg atoms, NV centres), where the impact of a quantum computer can be more significant. Moreover, other quantum-based solutions and approaches are under development, such as NV centre based RF spectrum analysis or SHB based rainbow analyser [238].
The current EW systems will also benefit from quantum timing. Quantum timing can enhance capabilities such as signals intelligence, counter-DRFM (digital radio frequency memory) and other EW systems that require precise timing; for instance, counter-radar jamming capabilities.
The other area of quantum EW will be signals intelligence (SIGINT) and communications intelligence (COMINT) (detecting, intercepting, identifying, locating) and quantum electronic attack (jamming, deception, use of direct energy weapons). Quantum channels (for quantum communication or quantum imaging) have specific characteristics. First, the simple signal interception is problematic because the quantum data are carried by individual quanta, and their interception can be easily detected. Second, typical quantum imaging technologies use a low signal-to-noise ratio, which means that it is challenging to recognise signal and noise without extra knowledge. Third, coherent photons, usually used as a signal, behave like a laser that is very focused. Finding such a quantum signal without knowing the position of at least one party is very challenging. These characteristics make the classical EW obsolete and blind against quantum channels.
The situation is difficult even for potential quantum electronic warfare systems, since it is open to question whether it will be possible to detect the presence of a quantum (free-space) channel. This will require the development of quantum analogy of laser warning receivers [239]. For quantum EW, it will be critical to get intel on the position of one or both parties using the quantum channel.
Classical EW would intercept and eavesdrop on the free-space classical channel. However, this is not possible for the quantum channel where it would be detected promptly. One possible attack is a man-in-the-middle type attack [240, 241], since the early quantum network parties can have a problem with authentication or trusted repeaters. Other types of attacks are considered at the quantum physics level; for example, a photon number splitting attack relies on utilising coherent laser pulses for the quantum channel [81] or the Trojan-horse attacks [82], or the collecting of scattered light and its detection [242]. However, these types of attacks are very sophisticated, and their practicability, for example in space, is uncertain.
It is more probable that the quantum EW attack will be just a type of denial of service, where the quantum channel is intercepted, leading to stoppage of use of the channel. Another possibility is the sophisticated jamming of the receivers on one or both sides, leading to enormous noise. When the position of the receiver or transmitter is known, another countermeasure of the classical EW is to make use of directed energy weapons such as laser, leading to damage or destruction of sensors. Such an attack also could help eavesdroppers [155].
In general, new approaches and methods will need to be developed to realise the capabilities of quantum electronic warfare and address the corresponding requirements.
5.7 Quantum radar and lidar
Key points:
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Long-range surveillance quantum radar is unlikely with existing quantum microwave technology.
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Possible applications in the optical regime - quantum lidar.
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Quantum radar could be used for space warfare.
The perception of the quantum radar topic [141, 243, 244] is affected by the hype in the media claiming quantum radar development in China [245, 246] or by optimistic laboratory experiments. Indeed, the theoretical advantages and features of quantum radar are significant (some of them depend on individual quantum protocols):
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Higher resistance to noise—that is, better SNR (signal-to-noise ratio)—higher resistance to jamming and other electronic warfare countermeasures;
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Based on individual photons; that is, the output signal power is so low that it will be invisible to electronic warfare measures;
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Target illumination; that is, a radar allowing identification of the target.
Based on the list of unique quantum radar features, it could be a powerfully disruptive technology that could change the rules of modern warfare. Therefore, attention is being paid to this topic internationally, despite the immaturity of the technology, and the many doubts about whether the quantum radar could work as the standard primary surveillance radar.
Moreover, many people immediately imagine quantum radar as a long-range surveillance radar with a range of hundreds of kilometres, whereas such an application of quantum radar seems unlikely [247, 248]. Such an optimal, long-term surveillance quantum radar would be extremely expensive (many orders of magnitude higher than the classical radar cost for any range) [247], and it would still not fulfil all the advantages and features listed above.
Briefly, the practical problems are the following [247]. Quantum radar too is subject to the radar equation, where the received power is lost with the distance’s fourth power. In parallel, to keep the quantum advantage, it is desirable to have one or fewer photons per mode. In summary, the relatively high power made of low-photon modes in the microwave regime is needed to be generated. This requires a lot of quantum signal generators, cryogenics, large antenna sizes, etc. All this leads to extremely high cost, and impractical design [137, 247]. Scientists need to come up with more practical quantum microwave technology to overcome these difficulties.
Apart from the high price, scepticism also remains about the detection of stealthy targets or jamming resistance. Quantum radar can be advantageous against a barrage jammer, but not necessarily against a DRFM or other smart jammer [247]. In summary, the long-range surveillance quantum radar is unlikely to be achieved even as a long-term prospect. For its realisation, one would need to evolve new technology allowing smaller cryogenics, RF quantum emitter working at a higher temperature or more efficient cryogenics cooling, and a more powerful emitter (high rate of low photon pulses). Note that even if the room-temperature superconducting materials were developed, it would not help in the Josephson parametric amplifier (JPA) method of entangled microwave photon generation [249]. Nevertheless, JPA is not the only method to obtain entangled microwave photons [137]. It is not entirely impossible that a new theory and designs of quantum radar will be discovered in the future. The long-range surveillance quantum radar described above would suffer from large size, weight and power consumption, and it is questionable if such a radar would be stealthy [247].
Another problem is the ranging in the case of quantum illumination (QI) protocol. QI protocol requires knowledge of the target in advance, and therefore it requires some extension for ranging, whether classical or quantum [6].
For several years, it was believed that the quantum radar cross section (RCS) is higher than the RCS of classical radars [250, 251]. A new precise study of quantum RCS [252] shows that the previously claimed advantage of quantum RCS over the classical RCS results from erroneous approximation. Quantum and classical RCS seem to be equal, at the moment.
Another approach can be the quantum-enhanced noise radar [137, 253, 254]. Noise radar uses noise waveform as a transmission signal, and detection is based on the correlation between the transmitted signal and the received noise waveform radar returns. The advantage is the low probability of interception (LPI), being nearly undetectable by today’s intercept receivers. The quantum noise radar design needs more study to see practical applicability. However, a potential use here is especially for the microwave regime.
Still, the current theory and research have applications in the radar sector, especially that which uses the optical or near-optical photons; that is, quantum lidar. Here, a short-range quantum lidar could be used for target illumination at short distances. Experiments with single-photon imaging were demonstrated from 10 [255] to 45 km [256]. In this range, quantum lidar could operate as an anti-drone surveillance radar or as part of a SHORAD (Short Range Air Defense) complex.
Space can be another example of an advantageous environment for quantum radar/lidar [257] which is low noise for the optical regime, and it even almost eliminates the decoherence problem in the case of entangled photons. For example, Raytheon performs simulations of the quantum radar in the optical regime for space domain [258, 259]. The idea is to place a quantum radar on a satellite and detect small satellites that are difficult to detect because of their small cross-sectional area, reflectivity, and environmental lighting conditions. The deployment of quantum radar/lidar for the space environment can provide almost all the advantages listed above.
A small note is dedicated here to quantum-enhanced radar. Classical radar can be equipped with an atomic or quantum clock. Such quantum-enhanced radars show high precision and reduced noise, and thus demonstrate an advantage in detecting small, slow-moving objects such as drones [260].
5.8 Quantum underwater warfare
Key points:
Quantum technologies can significantly interfere in underwater warfare, with enhanced magnetic detection of a submarine or underwater mines, novel inertial submarine navigation and quantum-enhanced precise sonars. In general, in the maritime environment, sensing based on quantum photo-detectors, radar, lidar, magnetometers, or gravimeters can be applied [257]. For a general overview of the implications of quantum technology for nuclear weapon submarines’ near invulnerability, see [261].
Submarines and other underwater vehicles will benefit from quantum inertial navigation described in Sect. 5.4 about PNT. Large submarines can probably be one of the first adopters of quantum inertial navigation because they can afford to install larger quantum devices, including cryogenics cooling. Moreover, sensitive quantum magnetometers and gravimeters can help map surroundings such as an undersea canyon, icebergs and a wrinkled sea bottom without using sonar that can be easily detected. An example of another type of inertial navigation especially suitable for underwater arctic navigation is based on quantum imaging [262].
The basic tool for anti-submarine warfare could be the quantum magnetometer. Researchers anticipate that the SQUID magnetometers in particular could detect a submarine from 6 kilometres away, with still improving noise suppression [263, 264]. Note that the current classical magnetic anomaly detectors, usually mounted on a helicopter or a plane, have a range of only hundreds of meters. An array of quantum magnetometers, such as along the coast, could cover significant areas, leading to denial area for submarines. Moreover, an array of quantum magnetometers seems to work better with more suppressed noise.
Quantum magnetometers can also be used to detect underwater mines using, for instance, an unmanned underwater vessel [230].
However, the main discussion is about the detection range, sensitivity, etc., as in Sect. 5.5.1. Even other underwater domain technology such as sonar offers longer detection range [229]. It was also pointed out in [261] that quantum technologies will have little impact on SSBN (ballistic missile submarines). It is possible that quantum magnetometers could work with other sensors to aid in detection, identification and classification of targets [229].
5.9 Quantum space warfare
Key points:
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Important for long-distance quantum communication.
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Low Earth orbit will be important for the future deployment of quantum sensing and imaging technologies.
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Space warfare will lead to new quantum radar/lidar and quantum electronic warfare technologies for deployment in space.
The space domain is gaining in importance and will be an important battlefield used by advanced countries. Space used to be a place mainly for satellites for navigation, mapping, communication and surveillance, often for military purposes. Nowadays, space is becoming more weaponised [265]; for example, satellites with laser weapons or ‘kamikaze’ satellites are placed in Earth orbit, and anti-satellite warfare is growing in parallel. Another surging problem is the amount of space garbage, with the number of satellites estimated at 2,200 and several more planned to be released [266].
Space also will be key for placing quantum sensing and communication technology in satellites [267–271], as well as for space countermeasures.
For many quantum technology applications described in previous sections, it would be desirable to place quantum sensing technology such as quantum gravimeter, gravity gradiometer or magnetometer on satellites in Earth orbit, especially the low one (LEO). Such applications are in development; for example, a low-power quantum gravity sensing device that can be deployed in space on board a small satellite for accurately mapping resources or to aid in assessing the impact of natural disasters [272]. However, such an application does not require too high spatial resolution. See Sect. 5.5.1 for a detailed discussion. The same applies to satellite-based quantum imaging. For example, China claimed the development of a spy satellite that uses ghost imaging technology [273]. However, what spatial resolution it has is uncertain. Nevertheless, quantum ghost imaging would have the advantage of being usable in cloudy, foggy weather or at night as well.
On the other hand, utilisation of satellites for quantum communication has already been demonstrated [62, 274]. Satellite-based quantum communication will be essential for the near-term integrated quantum network at long distances [275]. The present quantum communication satellites suffer from the same problems as trusted repeaters for optic fibre channel. In fact, present quantum satellites are trusted repeaters. The issue with trusted repeaters is that they keep the doors open to possible cyber attacks on the satellite control system. A better security situation is with the presently demonstrated MDI-QKD protocol [276], where the central point works as a repeater or switch, but in a safe regime, and later with quantum repeaters. For a space quantum communication overview, see [270, 271].
A new required military capability will be technology to detect other satellites, space-borne objects, space garbage and track them. classical radars are used for this purpose; for instance, the Space Fence project as part of the US Space Surveillance Network [277]. However, most of these space surveillance radars have problems with objects with a size of about 10 cm and smaller [266] (in the case of Space Fence, the minimal size is about 5 cm), and another problem is the capacity, as to how many objects they can track. This is the case with most of the space garbage that is only a few centimetres in size. Instead of classical radar, quantum radar or lidar is considered [6, 257, 259] as an alternative. Specifically for the space environment, the quantum radar in optical regime is considered [259], since the optical photons do not suffer from losses such as in the atmosphere. Space quantum radar can offer most of the advantages of quantum radar as described in Sect. 5.7, including stealth. According to simulations [259], quantum radar in space can offer at least one order of magnitude higher detection sensitivity and object tracking sensitivity in space in comparison with GEODSS (Ground-based Electro-Optical Deep Space Surveillance). Space quantum radar would be very useful for tracking small, dark and fast objects, such as satellites, space garbage or meteoroids.
The increasing presence of quantum sensing and communication devices in space will lead to increased interest in quantum electronic warfare as described in Sect. 5.6.
5.10 Chemical and biological simulations and detection
Key points:
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∼200 qubits are sufficient to carry out chemical quantum simulation research.
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The capability of achieving more complex simulations increases with the number of logical qubits.
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Chemical detection in the air or in samples.
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Suitable for detecting explosives and chemical warfare agents.
The defence-related chemical and biological simulations are primarily interesting for the military and national laboratories, the chemical defence industry or CBRN (Chemical, Biological, Radiological and Nuclear) defence forces. Research on new drugs and chemical substances based on quantum simulations will require an advanced quantum computer, classical computing facility and quantum-chemical experts. The quantum simulations for chemical and biological chemical warfare agents, in principle, have the same requirements as civil research, such as the already ongoing protein folding, nitrogen fixation and peptides research.
The number of required qubits depends on the number of spatial basis functions (various basis sets exist, e.g., STO-3G, 6-31G or cc-pVTZ); for example, using the 6-31G basis, the Benzene and Caffeine molecules can be simulated by approx. 140 and 340 qubits, respectively [278]. Then, the Sarin molecule simulation, for instance, requires about 250 qubits. Based on quantum computer roadmaps [27, 279] and logical qubit requirements, one can come to 100 logical qubits in 10 years, but probably earlier with more effective error corrections and error-resisting qubits. This is sufficient for medium-sized molecule simulations.
The threat could be the design and precise simulation of structures and the chemical properties of new small- to medium-sized molecules that could play the role of chemical warfare agents similar to, for example, Cyanogen, Phosgene, Cyanogen chloride, Sarin or Yperit. On the other hand, in general the same knowledge can also be used for CBRN countermeasures and new detection technique development.
The research on protein folding, DNA and RNA exploration, such as motifs identification, Genome-wide association studies and De novo structure prediction [280] could impact the research on biological agents as well [281]. However, more detailed studies are needed to assess the real threat from quantum simulations.
Photoacoustic detection with quantum cascade laser will be effective as a chemical detector. For example, quantum chemical detectors can detect TNT and triacetone triperoxide elements used in improvised explosive devices (IED) that are a common weapon used in asymmetric conflicts. The same system for detecting Acetone can be used to discover baggage and passengers with explosives boarding aircraft. In general, quantum chemical detection can be used against chemical warfare agents or toxic industrial chemicals [282, 283].
In the mid- to long term, such detectors can be placed on autonomous drones or ground vehicles that are inspecting an area [284].
5.11 New material design
Key points:
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General research impacts; for example, room-temperature superconducting allowing the highly precise SQUID magnetometers to operate without cooling can have a remarkable impact on military quantum technology applications.
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Defence industry research on camouflage, stealth, ultra-hard armour or high-temperature tolerance material.
Modern science is developing new materials, metamaterials, sometimes called quantum material, by exploiting the quantum mechanical properties (e.g. graphene, topological insulator). Material as a quantum system can be simulated by a quantum computer; for example, the electronic structure of the material. The considered applications can be, for instance, the room-temperature superconductor, better batteries and improvement of specific material features.
To explain in greater detail, the room-temperature superconductivity material, for example, exploits superconductivity at high temperatures [285]. That would allow building Josephson junctions, usually used as the building blocks of SQUIDs or superconducting qubits. So far, cooling near absolute zero is required. It is expected that a quantum computer with about 70 logical qubits [286] could be sufficient for the basic research on high-temperature superconductors.
For the defence industry, opportunities for research on new materials such as better camouflage, stealth (electromagnetic absorption), ultra-hard armour or high-temperature tolerance material design are considered without any details being revealed.Footnote 12
5.12 Brain imaging and human-machine interfacing
Key points:
MEG (magneto-encephalography) scanner is a medical imaging system that visualises what the brain is doing by measuring the magnetic fields generated by current flowing through neuronal assemblies. Quantum magnetometers—based, for instance, on optically pumped magnetometers [287]—can enable high-resolution magnetoencephalography for real-time brain activity imaging. This technology is safe and non-invasive, and is already laboratory tested. The technology itself is small, and wearable [287].
In the near term, quantum MEG could be a part of a soldier’s helmet for continuous and remote medical monitoring and diagnosis in case of injury. The long-term expectations include enhanced human–machine interfacing, i.e. practical non-invasive cognitive communication with machines and autonomous systems [11].