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Quantum sensors using Rydberg atoms enable ultrasensitive and ultrawideband electric field sensors, communication receivers from VLF to Terahertz

Quantum sensing uses some nonintuitive properties of nature to measure things like time, magnetic fields, gravity, or acceleration. Quantum sensing has become a distinct and rapidly growing branch of research within the area of quantum science and technology, with the most common platforms being spin qubits, trapped ions and flux qubits. Quantum sensors are measuring device that takes advantage of quantum correlations, such as states in a quantum superposition or entanglement, for better sensitivity and resolution than can be obtained by classical systems.


For instance small particles like cesium atoms have electrons that can be coaxed to oscillate at well-defined frequencies, just like pendulams of mechanical clocks. . Rubidium can also be used for highly precise atomic clocks, and although it is considered less accurate than cesium it is inexpensive and more widely adopted. “Because all cesium atoms or rubidium atoms are identical, they are not only very precise but their frequencies are all the same,” says Kunz. “These nonintuitive properties have applications in timekeeping and positioning, since they are used for GPS. They are also used for magnetometers that can detect submarines or munitions.”


Atom-based measurements have been successfully utilized for magnetometery, time and frequency standards, inertial force sensing, amongst others. The accuracy and repeatability of atom-based measurements significantly surpass conventional methods because the stable properties of atoms and molecules are advantageous for precision measurement. Cesium now for atomic clocks provides the primary standard for the definition of the second in the International System of Units Recently, Rydberg atoms have been introduced to measure the amplitude of radio frequency (RF) electric fields following the same rationale.


It has long been understood that the large Rydberg atom polarizability and strong dipole transitions between energetically nearby states are highly sensitive to electric fields. Because a Rydberg electron is relatively weakly bound compared to a valence state, it has a comparably stronger response to an electric field.


A new imaging system that uses a laser-excited, room-temperature atomic vapour to convert terahertz radiation to visible light has been created by researchers at the University of Durham in the UK. The system can acquire terahertz images rapidly and efficiently using a conventional high-speed camera and the new technique could make it easier to develop practical technologies that use terahertz radiation.


Similarly, the ARL team has been investigating the atoms as a platform for quantum networks. Further, according to the researchers, Rydberg atoms have been showing much progress lately in the broader scientific community, serving as qubits for quantum simulation and computing.


Rydberg  Atom-Based Sensing of Weak Radio Frequency Electric Fields

The Rydberg atom-based RF electric field measurement is promising for performing traceable measurements with a higher sensitivity, accuracy and stability than conventional antenna-based standards. Consequently, Rydberg atom-based RF electrometry has widespread applications in areas such as antenna calibration, signal detection, terahertz sensing and the characterization of electronics and materials in the RF spectrum.


For Rydberg atom-based RF electric field sensing, electromagnetically induced transparency (EIT) is used to readout the effect of a RF electric field on atoms contained in a vapor cell at room temperature. The readout method effectively prepares each participating atom as an interferometer so that the RF electric field induces changes in the light-matter interaction that can be detected optically. The possibility of performing high resolution Rydberg atom spectroscopy in micron sized vapor cells is an important enabler of the method, particularly at higher frequencies.


High-speed terahertz imaging system uses Rydberg atoms

Terahertz radiation lies in the region of the electromagnetic spectrum between infrared light and microwaves. In principle, it has great promise for a wide range of applications including security screening, medical imaging and industrial quality control. Being low-energy, the radiation is non-ionizing and therefore safe for biological and medical applications – though it still has a sufficiently short wavelength for reasonably high-resolution imaging.


Several techniques have already been developed for terahertz imaging. Some systems use a single-pixel detector and build-up images by scanning a terahertz beam across the object – which is a slow process. “There are a small band of focal plane arrays or full-field sensors, which can take a 2D image in one shot,” says Weatherill, “Probably the state of the art is an array of microbolometers [thermal sensors]. Their frame rate is limited to about 30 Hz because the sensitivity is low, so you need to collect photons for a long time to see an image above the background noise.”


Weatherill and colleagues created their terahertz imaging system by filling a cell with caesium vapour and focusing three infrared lasers on it. Each laser is precisely tuned to one of three successive atomic transitions in caesium. When excited by these three lasers in succession, caesium atoms end up in a highly excited “Rydberg state”. Such an atom can then absorb a 0.55 THz photon, which puts it in a different Rydberg state that decays after about a microsecond. This decay involves the emission of a green photon, which can be then detected by a standard optical camera.


The 0.55 THz absorption is a sharp resonance, and terahertz radiation at other frequencies will not be detected. Therefore, unlike other techniques for collecting terahertz photons, the technique can reliably pick out a narrowband signal from broad spectrum thermal noise. The detection process is also about 100 times more sensitive than other techniques.


Two-colour imaging

The researchers acquired terahertz images at up to 3000 frames per second. They are now optimizing their equipment and believe that, in principle, it should be possible to collect data at frame rates up to 1 MHz. They are also keen to extend the research in other ways, such as detecting other terahertz frequencies and even two-colour terahertz imaging.Durham’s Lucy Downes says, “I’m also very keen to try setting this up in reflection mode, so we can look for defects in the surfaces of bulk objects”.


Daniel Mittleman of Brown University in the US, says the most obvious applications of the imaging system are in the laboratory: “Things like explosions, shock wave tests, the fundamental physics of solids and fast, extreme phenomena are where you would need those kinds of frame rates, and any time the material is opaque to optics, terahertz could be an interesting alternative.”


For more commercial applications, he foresees challenges in creating practical devices. “Ultimately, it will be interesting to see how well they can package this for use outside a physics lab. If the applications are fundamental physics, that question becomes irrelevant. If they’re thinking about applications outside the laboratory, that question is relevant and I’ve no idea how to answer it.”



Researchers at NIST  develop Rubidium atoms based VLF receiver

Researchers at the National Institute of Standards and Technology (NIST) have demonstrated that quantum physics might enable communications and mapping in locations where GPS and ordinary cellphones and radios don’t work reliably or even at all, such as indoors, in urban canyons, underwater and underground.


The NIST team is experimenting with low-frequency magnetic radio—very low frequency (VLF) digitally modulated magnetic signals—which can travel farther through building materials, water and soil than conventional electromagnetic communications signals at higher frequencies. The lower a signal’s frequency, the farther it travels, but at the expense of the precision afforded with higher frequencies.


The researchers built a direct-current magnetometer that detects the “spin” of certain atoms using polarized light. Because the atoms are highly sensitive and respond quickly, the resulting quantum sensors would be able to enhance very low-frequency radio with the best of both worlds—precise signals at an ideal bandwidth. A paper detailing the work was published in the Review of Scientific Instruments.


VLF electromagnetic fields are already used underwater in submarine communications. But there’s not enough data-carrying capacity for audio or video, just one-way texts. Submarines also must tow cumbersome antenna cables, slow down and rise to periscope depth (18 meters, or about 60 feet, below the surface) to communicate.


“The big issues with very low-frequency communications, including magnetic radio, is poor receiver sensitivity and extremely limited bandwidth of existing transmitters and receivers. This means the data rate is zilch,” NIST project leader Dave Howe said.


“The best magnetic field sensitivity is obtained using quantum sensors. The increased sensitivity leads in principle to longer communications range. The quantum approach also offers the possibility to get high bandwidth communications like a cellphone has. We need bandwidth to communicate with audio underwater and in other forbidding environments,” he said.


As a step toward that goal, the NIST researchers demonstrated detection of digitally modulated magnetic signals, that is, messages consisting of digital bits 0 and 1, by a magnetic-field sensor that relies on the quantum properties of rubidium atoms. The NIST technique varies magnetic fields to modulate or control the frequency—specifically, the horizontal and vertical positions of the signal’s waveform—produced by the atoms. “Atoms offer very fast response plus very high sensitivity,” Howe said. “Classical communications involves a tradeoff between bandwidth and sensitivity. We can now get both with quantum sensors.”


Traditionally, such atomic magnetometers are used to measure naturally occurring magnetic fields, but in this NIST project, they are being used to receive coded communications signals. In the future, the NIST team plans to develop improved transmitters. The researchers have published their results (link is external) in the Review of Scientific Instruments. The quantum method is more sensitive than conventional magnetic sensor technology and could be used to communicate, Howe said. The researchers also demonstrated a signal processing technique to reduce environmental magnetic noise, such as from the electrical power grid, which otherwise limits the communications range. This means receivers can detect weaker signals or the signal range can be increased, Howe said.


For these studies, NIST developed a direct-current (DC) magnetometer in which polarized light is used as a detector to measure the “spin” of rubidium atoms induced by magnetic fields. The atoms are in a tiny glass container. Changes in the atoms’ spin rate correspond to an oscillation in the DC magnetic fields, creating alternating current (AC) electronic signals, or voltages at the light detector, which are more useful for communications.


Such “optically pumped” magnetometers, in addition to high sensitivity, offer advantages such as room-temperature operation, small size, low power and cost, and reduced interference. A sensor of this type would not drift or require calibration. In the NIST tests, the sensor detected signals significantly weaker than typical ambient magnetic-field noise. The sensor detected digitally modulated magnetic field signals with strengths of 1 picotesla (one millionth of the Earth’s magnetic field strength) and at very low frequencies, below 1 kilohertz (kHz). (This is below the frequencies of VLF radio, which spans 3-30 kHz and is used for some government and military services.) The modulation techniques suppressed the ambient noise and its harmonics, or multiples, effectively increasing the channel capacity.


The researchers also performed calculations to estimate communication and location-ranging limits. The spatial range corresponding to a good signal-to-noise ratio was tens of meters in the indoor noise environment of the NIST tests, but could be extended to hundreds of meters if the noise were reduced to the sensitivity levels of the sensor. “That’s better than what’s possible now indoors,” Howe said.


Pinpointing location is more challenging. The measured uncertainty in location capability was 16 meters, much higher than the target of 3 meters, but this metric can be improved through future noise suppression techniques, increased sensor bandwidth, and improved digital algorithms that can accurately extract distance measurements, Howe explained. To improve performance further, the NIST team is now building and testing a custom quantum magnetometer. Like an atomic clock, the device will detect signals by switching between atoms’ internal energy levels as well as other properties, Howe said. The researchers hope to extend the range of low-frequency magnetic field signals by boosting the sensor sensitivity, suppressing noise more effectively, and increasing and efficiently using the sensor’s bandwidth.


The NIST strategy requires inventing an entirely new field, which combines quantum physics and low-frequency magnetic radio, Howe said. The team plans to increase sensitivity by developing low-noise oscillators to improve the timing between transmitter and receiver and studying how to use quantum physics to surpass existing bandwidth limits


US Army Research Laboratory work using Rydberg atoms – for electric field sensors and communication receivers.

In October 2018, the US Army Research Laboratory revealed work it had carried out using Rydberg atoms – an atom excited to high energy levels – for electric field sensors and communication receivers.


“In this instance with the Rydberg electric field sensor, we were initially interested in Rydberg atoms’ promise as a quantum repeater for transmitting quantum information over long distances, but through that work we realised their potential as a sensor for traditional classical information. So we began experiments to investigate, and are excited by the results and new possibilities that this could open up,” says Kunz.


“It boils down to the dramatic differences between our Rydberg sensor and today’s existing antenna/receiver technologies.”


“Traditional antennas will have use long into the future, but atom-based antennas may find use in some situations to augment them or even as an alternative.”


A Rydberg receiver has several advantages including being able to operate on any frequency from DC to THZ (0 to 1,000,000,000,000 Hz), naturally integrating with optical technologies, and detecting a field without absorbing the energy. According to Kunz, traditional antennas will have use long into the future, “but such unique atom-based antennas may find use in some situations to augment them or even as an alternative”.


“Since the atom-based antenna research is cutting edge, these technologies will continue to develop, improve, and be refined over time, and they will be engineered in more robust and cost effective manners,” Kunz explains. “Initially, they would only be adopted by customers who really need this level of performance and can afford to develop them; so this means governments and defense agencies.


“But we can look at the analogy with the development of GPS, as the systems and technologies become more refined and more affordable, they will trickle down through society and can ultimately change the way we all operate on a daily basis.”




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