The world, say many experts, is on the verge of a second quantum revolution. Energy quantization gave us modern electronics via the transistor and the laser, but humans’ burgeoning ability to manipulate individual atoms and electrons could potentially transform industries ranging from communications and energy to medicine and defense. The most talked-about of such technologies is the quantum computer, a device in theory so powerful that it could crack the codes underlying internet security in just a few minutes. But full-scale quantum computers are still potentially decades away. In contrast, devices that exploit quantum phenomena to encrypt codes, rather than break them, are starting to appear on the market.
Yet many scientists believe that quantum will enjoy its first real commercial success in sensing. That’s because sensing can take advantage of the very characteristic that makes building a quantum computer so difficult: the extraordinary sensitivity of quantum states to the environment. Quantum Sensors could be transformative, enabling autonomous vehicles that can “see” around corners, underwater navigation systems, early-warning systems for volcanic activity and earthquakes, and portable scanners that monitor a person’s brain activity during daily life.
Quantum sensing is an area of interest to our military because it is useful on and off the battlefield. For that reason, Army scientists are doing leading-edge research, and exploring the use of quantum sensing for such applications as submarine detection, underwater communications, geolocation, navigation, and communications.
One application of Quantum Sensors that Researchers are in Radio frequency (RF) technologies that are revolutionizing a broad range of industries such as healthcare, entertainment, communications, and radar. The RF spectrum has evolved into a commodity valued at over 1 trillion dollars annually because of its widespread use. The far infrared (FI), or terahertz, region of the spectrum is an active area of research and promises many new application.
Particularly Researchers are interested in Quantum radio receiver. Radio receiver, a wireless or simply a radio is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. The antenna intercepts radio waves (electromagnetic waves) and converts them to tiny alternating currents which are applied to the receiver, and the receiver extracts the desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, and finally recovers the desired information through demodulation.
Thanks to the unique advantage of free-space RF field sensing, the quantum receiver has some great advantages compared with conventional electronics based receivers, including but not limited to possibilities of weak signal and long-distance communication in free space or via a fiber link. While Current receivers are only able to receive over a portion of spectrum band or range of frequencies whereas a quantum receiver could give Soldiers a way to detect communication signals over the entire radio frequency spectrum, from 0 to 100 GHz. Such wide spectral coverage by a single antenna is impossible with a traditional receiver system, and would require multiple systems of individual antennas, amplifiers and other components.
There is no doubt that a functioning quantum radio would provide our military with a significant advantage. However, our scientists are not the only ones doing research on Rydberg atoms. According to Kevin Cox, from a team of research scientists at the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory, over the past few years, there has been growing interest in studying Rydberg atoms for electric field sensing. The other research has primarily been in the United States, Europe, and China. The team is perfecting a quantum sensor that they previously announced as the world’s first quantum radio receiver using Rydberg atoms.
Rydberg atoms as electric field sensors and communications receivers
Atom-based measurements of length, time, gravity, inertial forces and electromagnetic fields are receiving increasing attention. Atoms possess properties that suggest clear advantages as self calibrating platforms for measurements of these quantities. 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. Atom-based measurements have been successfully utilized for magnetometery , time and frequency standards, inertial force sensing as well as searches for local Lorentz invariance and intrinsic electric dipole moments of the neutron and electron , amongst others. Atom-based quantum techniques are also emerging as a completely new and promising tool for
Accurate radio-frequency (RF) electromagnetic field sensing in free-space plays a fundamental role in wireless communication, and Rydberg atoms are remarkable quantum sensors for RF electric(E-) field measurements. Rydberg atoms created from Rubidium, an alkali metal. Alkali metal atoms are used because they have a single valence electron in the outer shell. The valence electron is weakly bound to the atom because it is the only electron in its energy level and is shielded from the nucleus by the inner core electrons. Because a Rydberg electron is relatively weakly bound compared to a valence state, it has a comparably stronger response to an electric field.
To create a Rydberg atom, lasers excite the outer electron to move it up many energy levels. Because of the extreme location of its outer electron, a Rydberg atom is typically very large.
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. Rydberg atoms, with one highly excited, nearly ionized electron, have extreme sensitivity to electric fields, including microwave fields ranging from 100 MHz to over 1 THz.
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. Rydberg atoms can detect and measure RF E-fields through
the use of two related optical phenomena. EIT occurs when a laser field (probe field) that is on-resonance with an atomic transition would normally be absorbed by an atomic vapor, is instead transmitted in the presence of a second laser field (coupling field), which is tuned to another atomic transition. This occurs over a very narrow frequency range. When a third electromagnetic field, tuned to another linked atomic transition, is present, the original transparency region is split into two regions separated in frequency (AT-splitting). The frequency
separation is directly related to the strength of the third RF field (typically an RF field). 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.
Rydberg atoms have been shown to be very useful in performing absolute measurements of the magnitude of a radio frequency (RF) field using electromagnetically-induced transparency (EIT). However, there has been less success in using Rydberg atoms for the measurement of the phase of an RF field. Measuring the phase of a RF field is a necessary component for many important applications, including antenna metrology, communications, and radar. The works have demonstrated the detection and reception of amplitude modulated (AM) and frequency modulated (FM) signals, where the AM and FM detection is based on the measurement of the amplitude of the carrier. To detect and receive BPSK/QPSK/QAM digital communication signals requires the ability to measure the phase of an RF field.
NIST researchers demonstrated a scheme for measuring the phase of an RF field by using Rydberg atoms as a mixer to down-convert an RF field at 20 GHz to an intermediate frequency on the order of kHz. The phase of the intermediate frequency corresponds directly to the phase of the RF field
Block diagram of the Rydberg atom ‘mixer’. The Rydberg atoms separate the difference frequency (IF) from two RF signals
(LO and SIG). This demodulated signal is carried in the probe laser.
The difference frequency, or intermediate frequency (IF), is detected by optically probing the Rydberg atoms. The phase of the IF signal corresponds directly to the relative phase between the LO and RF signals. This technique can be applied over the same frequency range as E-field amplitude measurements, resulting in an atom-based sensor that can measure amplitude, polarization, and now phase for fields from 500 MHz to 1 THz in a single setup.
Chinese Researchers develop Rydberg-atom-based digital communication, reported in Mar 2019
Nowadays, as wireless communication spectrum resources are becoming increasingly scarce, cognitive radio technology is being used to provide a solution for more efficient utilization of the radio spectrum. As the very essence of cognitive radio, continuously tunable RF
reception, which covers an entire frequency band, has become increasingly important for accessing underutilized spare sub-bands of the radio spectrum. In addition, broadband communication in higher frequency bands can provide the high speed, high capacity data transfer, essential to the fifth generation (5G) communication.
Up to now, the measurement of radio-frequency (RF) electric field achieved using the electromagnetically-induced transparency (EIT) of Rydberg atoms has proved to be of high-sensitivity and shows a potential to produce a promising atomic RF receiver at resonance between two chosen Rydberg states. In this paper, we study the extension of the feasibility of digital communication via this quantum-based antenna over a continuously tunable RF-carrier at off-resonance.
In Mar 2019, Researchers from Chinese Academy of Sciences, reported that rather than retrieving the time-varying signals on a specific carrier frequency, we hope to make extensive use of the frequency band near the carrier frequency determined by the energy gap between the neighboring Rydberg states. This requires that Rydberg-atom-based quantum-optical radio communication features be available over the whole RF-carrier near the Rydberg resonance in question. If so, for example, different communication channels can work concurrently in different but close RF-carrier frequencies, which greatly enhances the communication capacity.
1. Experimental setup of the Rydberg-atom-based RF-receiver with differential detection. The coupling and probe lasers counter-propagate through the rubidium vapor cell, and form a ladder-type EIT. An RF E-field couples the two Rydberg states 59D5/2 and 60P3/2
, resulting in an Autler-Townes splitting. AOM: acoustic optic modulator; BS: beam splitter; DM: dichroic mirror; GT: Glan-Taylor prism; HWP: half-wavelength plate; PBS: polarizing beam splitter.
Our experiment shows that the digital communication at a rate of 500 kbps can be performed reliably within a tunable bandwidth of 200 MHz near a 10.22 GHz carrier. Outside of this range, the bit error rate (BER) increases, rising to, for example, 15% at an RF-detuning
of ±150 MHz.
In 2018, Army scientists developed a new quantum antenna using Rydberg atoms
In 2018, it was reported that Army scientists developed a new quantum antenna using atoms excited to unusually high energy levels. Drs. Paul Kunz, Kevin Cox, David Meyer and Fredrik Fatemi from the laboratory’s Sensors and Electron Devices Directorate’s Quantum Technology Branch are leading a research effort that seeks to equip future Soldiers with more accurate sensors that operate with less background noise.
Army researchers used atoms prepared into exotic quantum states known as Rydberg states with super sensitivity to electric fields to achieve communication rates much faster than a comparably-small traditional antenna. A new quantum receiver may allow future Soldiers to perform parallel, fast communications with miniature quantum receivers. This research was published in Applied Physics Letters.
Army scientists were the first in the world to create a quantum receiver that uses highly excited, super-sensitive atoms–known as Rydberg atoms–to detect communications signals, said David Meyer, a scientist at the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory. The researchers calculated the receiver’s channel capacity, or rate of data transmission, based on fundamental principles, and then achieved that performance experimentally in their lab–improving on other groups’ results by orders of magnitude, Meyer said.
“The most precise measurement devices in the world are based on atoms and are revolutionizing critical Department of Defense capabilities such as timekeeping and magnetic field sensing,” Kunz said. “We have recently been investigating the characteristics of highly excited atoms, known as Rydberg atoms, for applications as electric field sensors and communications receivers.” Along the way, they realized that the Rydberg atoms’ exquisite sensitivity to electric fields could open new possibilities within more traditional application spaces like classical radio-frequency communications.
US Army scientists develop Ultra-broadband quantum receiver the first to detect entire radio frequency spectrum reported in Jan 2021
“These new sensors can be very small and virtually undetectable, giving Soldiers a disruptive advantage,” Meyer said. “Rydberg-atom based sensors have only recently been considered for general electric field sensing applications, including as a communications receiver. While Rydberg atoms are known to be broadly sensitive, a quantitative description of the sensitivity over the entire operational range has never been done. A big plus is that the device can also act as its own antenna.
To assess potential applications, Army scientists conducted an analysis of the Rydberg sensor’s sensitivity to oscillating electric fields over an enormous range of frequencies–from 0 to 10^12 Hertz. The results show that the Rydberg sensor can reliably detect signals over the entire spectrum and compare favorably with other established electric field sensor technologies, such as electro-optic crystals and dipole antenna-coupled passive electronics.
“Quantum mechanics allows us to know the sensor calibration and ultimate performance to a very high degree, and it’s identical for every sensor,” Meyer said. “This result is an important step in determining how this system could be used in the field.”This work supports the Army’s modernization priorities in next-generation computer networks and assured position, navigation and timing, as it could potentially influence novel communications concepts or approaches to detection of RF signals for geolocation.
In the future, Army scientists will investigate methods to continue to improve the sensitivity to detect even weaker signals and expand detection protocols for more complicated waveforms. The Journal of Physics B published the research, “Assessment of Rydberg atoms for wideband electric field sensing,” in its special issue on interacting Rydberg atoms. Army scientists David H. Meyer, Kevin C. Cox and Paul D. Kunz led this research, as well as Zachary A. Castillo from the University of Maryland. This work was supported by the Defense Advanced Research Projects Agency.
In Jan 2021, it was reported that Army researchers built the quantum sensor, which can analyze the full radio-frequency spectrum—from zero frequency up to 20 GHz—and detect AM and FM radio, Bluetooth, Wi-Fi and other communication signals, unleashing new potentials for soldier communications, spectrum awareness and electronic warfare.
The Rydberg sensor uses laser beams to create highly-excited Rydberg atoms directly above a microwave circuit, to boost and hone in on the portion of the spectrum being measured. The Rydberg atoms are sensitive to the circuit’s voltage, enabling the device to be used as a sensitive probe for the wide range of signals in the RF spectrum.
“All previous demonstrations of Rydberg atomic sensors have only been able to sense small and specific regions of the RF spectrum, but our sensor now operates continuously over a wide frequency range for the first time,” said Dr. Kevin Cox, a researcher at the U.S. Army Combat Capabilities Development Command, now known as DEVCOM, Army Research Laboratory. “This is a really important step toward proving that quantum sensors can provide a new, and dominant, set of capabilities for our Soldiers, who are operating in an increasingly complex electro-magnetic battlespace.”
The Rydberg spectrum analyzer has the potential to surpass fundamental limitations of traditional electronics in sensitivity, bandwidth and frequency range. Because of this, the lab’s Rydberg spectrum analyzer and other quantum sensors have the potential to unlock a new frontier of Army sensors for spectrum awareness, electronic warfare, sensing and communications—part of the Army’s modernization strategy.
“Devices that are based on quantum constituents are one of the Army’s top priorities to enable technical surprise in the competitive future battlespace,” said Army researcher Dr. David Meyer. “Quantum sensors in general, including the one demonstrated here, offer unparalleled sensitivity and accuracy to detect a wide range of mission-critical signals.”
The researchers plan additional development to improve the signal sensitivity of the Rydberg spectrum analyzer, aiming to outperform existing state-of-the-art technology. “Significant physics and engineering effort is still necessary before the Rydberg analyzer can integrate into a field-testable device,” Cox said. “One of the first steps will be understanding how to retain and improve the device’s performance as the sensor size is decreased. The Army has emerged as a leading developer of Rydberg sensors, and we expect more cutting-edge research to result as this futuristic technology concept quickly becomes a reality.”
Although the researchers have a long-term goal of perfecting quantum radio receivers for military purposes, Kuntz noted that the sensors also have the potential to be used as a calibration standard. Although Rydberg sensors are quantum in nature, it will be many years before sensors incorporate one of the most useful quantum properties called entanglement, a state when particles act similarly, and they are dependent on each other. With entanglement, the received signal will be greatly enhanced relative to the noise and lead to even more advanced receiving capabilities.
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