Quantum leap

Rydberg atom based-sensing could make an important contribution to the enhancement and improvement of radio and radar signal detection.

The city of Halmstad on Sweden’s southwest coast has the distinction of hosting the country’s first ever International Worker’s Day demonstration which took place in 1897. Twelve years earlier, the city’s first trade union was formed by textile workers. In 1854, the city became the birthplace of physicist Johannes Rydberg. It was Dr Rydberg who devised his eponymous formula, which would lay the basis for an emerging form of electromagnetic sensing, which harnesses quantum physics.

The resultant Rydberg formula predicts the wavelength of light which is produced by an electron moving between energy levels as it orbits an atom. Atoms are comprised of a nucleus, which contains neutrons and protons, with the nucleus surrounded by electrons. The number of electrons possessed by an atom can be determined by a chemical element’s atomic number. This number details how many protons are found in the nucleus of each of that element’s atoms, with the atomic number equal to the number of electrons surrounding the nucleus. Electrons will orbit the atom’s nucleus at different distances. Hydrogen has an atomic number of one and hence a single proton and a single electron. The energy of an electron changes as it moves from one orbit to another. As electrons change their energy state, they release or absorb photons, and photons are the particles of which light is comprised. If an electron moves from a low-energy state to a high-energy state, it absorbs a photon. Conversely, when an electron is moving from a high-energy state to a low-energy state, it emits a photon.

An article published in February 2024 by Forbes magazine, written by Paul Smith-Goodson, entitled ‘Quantum Sensing Unleashed: How Rydberg Sensors Will Disrupt Telecom’, provides a good introduction to the Rydberg sensing discipline. Quantum sensing is “a cutting-edge technology that harnesses the strange and unique properties of quantum physics to detect tiny changes in the environment with extreme precision,” said Bruno Desruelle, Vice President of Photonics at Exail. He added that “[by] manipulating particles like photons or atoms, these sensors measure minute variations with unparalleled accuracy.” These properties explain why Rydberg atom sensing are attractive for defence applications like electronic warfare (EW). Joe Spencer, a quantum technology specialist at Global Quantum Intelligence in London, defines a quantum sensor as a “device that can measure or detect the environment in either better ways or new ways than we can currently.”

Smith-Goodson’s article notes the applicability of Rubidium (Rb) to Rydberg sensing. Rubidium is an alkali metallic chemical element with an atomic number of 37. Rubidium has 36 electrons orbiting consecutive distances from its nucleus. However, its 37th electron is a valence electron, which orbits on its own some distance from the nucleus. As the article notes, the 36 electrons orbiting close to the nucleus act as a barrier, weakening the attraction between the nucleus and this single valence electron. By adding energy to a Rubidium atom using a laser, this single valence electron is moved even further away from the nucleus. Much as the electron’s attraction to the rubidium’s nucleus is weakening, so its susceptibility to other factors like electromagnetic forces grows. It is these weakly-bound electrons that can be exploited for radio frequency (RF) sensing.

The properties of atoms vis-à-vis their electrons, and how these electrons behave is denoted by their quantum number. The quantum number essentially describes an electron’s position and energy in an atom. Four specific criteria are used to derive a quantum number: principal, angular momentum, magnetic and spin. The principal quantum number escalates in value from one upwards, denoting how far an electron is orbiting from an atom’s nucleus. One denotes the electron orbit closest to the nucleus and the numbers increase as you move away. The larger the principal quantum number, the further the electron is from the nucleus. The electron nearest the nucleus will also have the lowest energy state. Add energy to that electron, and it will jump to a higher orbit away from the nucleus and experience an increase in principal number. This process also works in reverse and is known as absorption or emission. As noted above, add energy to an electron and photons will be absorbed, weaken the energy and the electron will emit photons. Angular momentum describes the shape of an electron’s orbit around the nucleus. The number of orbits and their orientation is determined by the magnetic quantum number. Finally, the spin quantum number denotes the direction of an electron’s spin. A high quantum number will denote that one or more electrons are comparatively further away from the nucleus than a low quantum number. Rydberg atoms have high quantum numbers with at least one electron a comparatively long distance from the nucleus. Dr Spencer said that this atomic quantum state can be vulnerable to alteration by heat or external electromagnetic force. This fragility provides a means of measuring the very factor that may change the state of an atom, such as a magnetic field.

A Rydberg atom’s furthest electron from the nucleus will be very sensitive to electromagnetic energy. RF energy, which radios, radars and satellite navigation systems depend on, is a form of electromagnetic energy. The radio segment of the electromagnetic spectrum covers a waveband of 3 KHz to 3 THz. A January 2024 article published in the EE Times written by Robert Huntley detailed an approach to Rydberg atom sensing using spectroscopy, which measures the interaction of atoms and molecules with light, which can include visible light as well as ultraviolet, and lasers (coherent light). Light will be absorbed, emitted or scattered by particles as the former interacts with the latter.

Rydberg atoms and RF sensing

How can Rydberg atom sensing help detect RF energy? This process requires spectroscopy, electromagnetically-induced transparency (EIT) and a cell containing a chemical such as Rubidium or Caesium. EIT involves making a medium, such as a specific material, transparent when subjected to a probe field such as a laser beam. The probe field will interact with this medium that is usually opaque, and this process causes the absorption of the light by that material. Initially, when the probe field enters one side of the medium, there will be no transmission of light out of the other side. At this point, the probe field is being entirely absorbed by the medium. However, by applying a second stronger probe field, light from the first probe field can be transmitted through the other side of the medium, making it transparent. This process is possible through the behaviour of the medium’s atoms. When the initial probe field is applied, it excites the atoms from their ground state. The ground state is the condition when an atom, and its accompanying electrons, are all at their lowest energy state. Applying the probe field excites the atoms, causing their electrons to absorb photons and preventing any transmission of the probe field through the material. By applying the second, stronger field this process is altered, enabling the transmission of the probe field through medium.

The standard method of sensing RF energy is to use an antenna, much like that which would equip a radio, television or your cell phone. Antennas work well as a means of transmitting and receiving radio signals but have drawbacks: Firstly antennas can have their performance and accuracy enhanced or degraded according to the quality of their construction. A general rule of thumb in RF engineering is that a dipole antenna, such as metal rod or wire, is one half or one quarter of the wavelength that it is designed to receive and transmit; wavelength is the measurement of the distance between a peak and trough in a radio wave. Antennas must be sited and calibrated to ensure they can work is desired; for example, the higher an antenna is placed, the wider its field-of-view will be, and the better its performance in receiving and transmitting signals. This is why television antennas are placed on top of houses and not in their basements. Moreover, the performance of antennas can change as they age. Environmental factors such as heat may also affect their performance as metal expands and contracts.

Atoms, on the other hand, have specific properties which make them attractive for RF sensing as they have their own accurate properties, and these differ from element to element. For example, the resonant frequency of hydrogen is 1.24 GHz. This means a hydrogen atom will emit a frequency of 1.24 GHz when it transitions from its ground state to a higher energy level and vice versa. As these frequencies are fixed, devices using these mediums for RF sensing are self-calibrating. Importantly, they do not suffer the physical changes experienced by metals. Given that atoms do not need to be size-dependent in a similar fashion to an antenna, they can potentially cover a much larger waveband of frequency detection. Rydberg atoms can be sensitive to external stimuli such as RF energy, as they are very sensitive to microwave electromagnetic energy. Microwaves typically stretch from 300 MHz to 300 GHz. From a military perspective this encompasses the ultra-high frequency (UHF; 300 MHz to 3 GHz), L-band (1-2 GHz), S-band (2-4 GHz), C-band (4-8 GHz), X-band (8-12 GHz), Ku-band (14-18 GHz), K-band (18-27 GHz) and Ka-band (27-40 GHz) wavebands routinely used for convention radio and satellite communications, satellite navigation and radar.

To understand how all these factors come together to detect microwaves, it is necessary to consider the following architecture. An atomic vapour derived from a material such as Rubidium or Caesium is used, precisely because these materials have a single valance atom which can be employed to detect microwaves. Rubidium atoms are excited by lasers using the EIT process described above. The probe laser and strong field, or coupling, lasers will have frequencies of 351.9 THz and 589 THz. The goal is to use these Rydberg atoms to transfer their activity into an optical readout to demonstrate the presence of microwave radiation. The architecture will also need an optical converter to change the incoming RF energy to be analysed to an optical signal. This optical signal can then be used to drive the lasers which will initiate the EIT process. Dr Spencer noted that the components required for Rydberg sensing include a physics package which comprises the atoms that will do the work; “laser systems to control them, photonic circuitry to direct light, detectors to detect signals and feedback systems to maintain atomic states and control the states, and finally a robust housing to shield such a system from unwanted environmental noise.”

Next steps

Although Rydberg sensing shows promise in microwave detection, it may be some years until this technology is deployed on the battlefield. As this article has illustrated, much of the work on using Rydberg atoms to aid microwave detection has been confined to the laboratory. It would be reasonable to assume that the technology largely meets a Technology Readiness Level (TRL) of four. US Department of Defense (DOD) and European Union (EU) TRL levels are similar: For both, TRL-4 denotes that the technology has been validated in a laboratory environment. This is just under halfway through the nine US DOD and EU TRL steps that must be taken before an actual system is demonstrated and proven operationally. Relevant operational environments could include representative military exercises or actual battle. The challenge will be developing Rydberg atom sensing to a point where it can equip an Electronic Support Measure (ESM).

ESMs are routinely used by sea, land, air and space forces to detect, locate, identify and characterise RF transmissions from radios and radars. ESMs play a key part in the Electronic Support (ES) mission. EW contains three subdisciplines of Electronic Attack (EA), Electronic Protection (EP) and ES. The US DOD Dictionary of Military and Associated Terms defines EA as a division of EW which employs electromagnetic, directed energy or anti-radiation weapons “to attack personnel, facilities, or equipment with the intent of degrading, neutralising or destroying enemy combat capability”. EP concentrates on protecting blue forces against EA. ES aids “immediate threat recognition, targeting, planning and (the) conduct of future operations” from an EW perspective.

One of the key contemporary challenges is that ESMs must find and process a Signal-of-Interest (SOI) within what are often very noisy electromagnetic environments. We are continually surrounded by electromagnetic noise which affects the radio spectrum. Some of this noise emanates from space and some occurs naturally on Earth. RF noise is also caused by human activity, cell phone use and broadcasting being two good examples. An ESM’s work is further complicated by the fact that military communications and radars transmit RF signals using low probability of detection/interception (LPI/D) techniques. Understandably, users of such equipment are keen for their systems to be as difficult as possible to discover via their radio signals. Once signals are discovered, their source can be pinpointed: Find a signal and you find the soldier, platform, weapon, sensor or base using this radio or radar. Once the source of transmission is determined, coordinates can be derived for the target to be attacked kinetically. Furthermore, a radio signal can be exploited for communications intelligence (COMINT). A radar signal can be similarly mined for electronic intelligence (ELINT). Both ELINT and COMINT provide useful information on the characteristics of these radar and radio signals. By understanding these characteristics, EW cadres can ascertain how these signals could be jammed.

LPI/D adornments for signals are intended to help these signals ‘hide’ in prevailing electromagnetic noise. One standard LPI/D technique is to make these signals as discreet, or electromagnetically quiet, as possible meaning that the signal will be very weak. In fact, a signal may be so weak that it seemingly disappears within the prevailing noise. An ESM depends on a signal amplifier to strengthen a weak SOI that the apparatus has detected so that this signal can be properly analysed. However, the amplifier’s components also produce noise. It may be possible to amplify a very weak signal, but this will see a corresponding amplification of the system noise. As Rydberg sensing is not dependent on an amplifier; an ESM thus equipped may be able to extract the characteristics of a very pure signal with minimal accompanying noise. Such techniques may help cut through LPI/D signal adornments.

NATO interest

Rydberg sensing properties are motivating research efforts in the military domain in the US and France. Rydberg Technologies is one company which is forging ahead with commercialising Rydberg sensing innovation. The company’s Rydberg Field Measurement System uses spectroscopy to collect and characterise RF signals. RF signals can also be collected across a wide waveband using the equipment which would ordinarily require multiple receive antennas. Using multiple antennas adds complexity to an ESM as it demands additional hardware and software. Traditionally, ESMs detect signals across wavebands of between 18 MHz and 2 GHz, to 18-40 GHz, depending on customer preferences. Rydberg Technologies has stated that its technology can detect electromagnetic waves from almost 0 Hz to 100 GHz. Such epic wavebands could be very useful in the future as militaries begin to exploit signals above 40 GHz for communications and sensing.

In the public sector, the US Defence Advanced Research Projects Agency (DARPA), has Rydberg sensing as a key part of its SAVANT (Science of Atomic Vapours for New Technologies) effort. Across the Atlantic, the Agence Nationale de la Recherche (ANR; ENG: French National Research Agency) has embarked on the ‘Cardamone’ programme. As the ANR’s literature makes clear, Cardamone is examining the potential and limitations of Rydberg sensing technology with an interest in RF sensing. Partners in the Cardamone initiative include Thales, the Lumière, Matière et Interfaces (ENG: Light, Material and Interfaces) laboratory at the Paris-Saclay University and the Charles Fabry laboratory at the same institution. Dr Spencer emphasised that “quantum sensors are being tested in the real world right now”, although noting that “there is still work to be done to make them fully operational.” Johannes Rydberg would no doubt have been pleased to learn about the consequences of his pioneering work over 150 years later.

Dr Thomas Withington