By ARL Public AffairsOctober 25, 2018
ADELPHI, Md. -- Scientists at the U.S. Army Research Laboratory are developing a new quantum sensor using atoms excited to unusually high energy levels.
The research community is also experiencing "high energy and excitement" thinking about the new technological possibilities this discovery could enable.
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; and it all started with what are known as Rydberg atoms.
"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."
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.
Similarly, the ARL team has been investigating the atoms as a platform for quantum networks.
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.
Antennas are used every day in countless military and commercial devices to transmit and receive data using electro-magnetic waves.
By using quantum particles, or atoms, they have been able to achieve much higher data rates than a comparably sized traditional antenna.
"When the outer electron of an atom is highly excited, it becomes very loosely bound, and this greatly increases the atom's sensitivity to electric fields," Kunz said. "It's like making a giant compass needle 3,000 times longer than a normal compass needle, but this is for electric fields instead of magnetic fields; and keep in mind that even giant atoms are still tiny on human scales."
This new antenna looks quite different from a normal antenna.
For the ARL team, their atoms are held in a simple glass cell at room temperature, and they use two colors of laser light to simultaneously excite the Rydberg states and probe their reaction to the external electric fields -- so this system looks quite different from a normal antenna as shown in the graphic.
Not only does it look different, but it behaves in a fundamentally different manner too.
First, these atoms naturally operate over a very wide band of frequencies (from kilohertz (103 Hz) to terahertz (1012 Hz) -- nine orders of magnitude.
Second, the atoms are tiny and absolutely identical (i.e. quantum objects). In fact, the team has demonstrated that the Rydberg receiver can achieve the maximum performance theoretically allowed, such that it was limited only by fundamental collapse of the quantum wave-function.
Thirdly, the atoms convert the communication signal into modulated optical laser light, allowing sensitive and high speed detection.
Lastly, the way in which they detect the RF fields is intrinsically nondestructive (i.e. covert), as opposed to traditional antennas, which strongly absorb the RF energy.
This work falls under ARL's Discovery Essential Research Program, and supports the Network C3I Modernization Priorities.
"This a great example of new potential applications that are uncovered as we explore cutting edge fundamental science," Kunz said. "While we have shown that these sensitive atoms can deliver large bandwidths and signal-to-noise ratio, they are so different from traditional receiver technology that we must step back and creatively consider new applications and new possibilities."
In terms of future steps to be taken to make this technology a reality for Soldiers, the researchers said there are still very many fundamental science questions concerning these Rydberg sensors that need to be answered, and new possibilities to explore.
First, there is still huge untapped potential in terms of better sensitivity from these sensors, and they have some ideas about how to push the boundary of world-record sensitivity.
"We are excited by the response from the scientific community for this research project," Kunz said. "Many groups have reached out to us with questions and positive feedback, and similar research has begun appearing from researchers around the world."
This work was recently accepted for publication in Physical Review Letters.
For further information regarding this research, view:
[1.] David Meyer, Kevin Cox, Fredrik Fatemi, Paul Kunz, "Digital Communication with Rydberg Atoms and Amplitude-Modulated Microwave Fields", App. Phys. Lett. (R) 112, 211108 (2018). [arXiv:1803.03545]
 K. Cox, D. Meyer, F. Fatemi, P. Kunz, "Quantum-Limited Receiver in the Electrically Small Regime", Phys. Rev. Lett. 112, 110502 (2018). [arXiv:1805.09808]
The U.S. Army Research Laboratory is part of the U.S. Army Research, Development and Engineering Command, which has the mission to ensure decisive overmatch for unified land operations to empower the Army, the joint warfighter and our nation. RDECOM is a major subordinate command of the U.S. Army Materiel Command.