Researchers at the National Institute of Standards and Technology (NIST) have increased the sensitivity of atomic radio receivers by a factor of 100 by surrounding a small glass cylinder of cesium atoms that looks like a custom copper “headphone.”
The structure (a square overhead loop connecting two square panels) increases the incoming radio signal or electric field applied to the gaseous atoms (known as steam cells) in the flask between the panels. With this enhancement, wireless receivers will be able to detect signals that are much weaker than before. The demonstration is explained in a new paper.
The headphone structure is technically a split ring resonator and acts like a metamaterial. This is a material designed with a new structure and produces anomalous properties. “We can call it a metamaterial-inspired structure,” said NIST project leader Chris Holloway.
NIST researchers have previously demonstrated atomic-based radio receivers. Atomic sensors, among other possible advantages, can work better than traditional radio receivers in physically small, noisy environments.
The steam cell is about 14 mm long and 10 mm in diameter, roughly the size of a fingernail or computer chip, but thicker. The resonator overhead loop is about 16mm on a side, and the ear cover is about 12mm on a side.
NIST radio receivers rely on the special state of the atom. Researchers use two different color lasers to prepare the atoms contained in the vapor cell into a high-energy (“Rydberg”) state. It has new properties such as extreme sensitivity to electromagnetic fields. The frequency and intensity of the applied electric field affects the color of the light absorbed by the atom, which has the effect of converting the signal intensity to an accurately measurable optical frequency.
When a radio signal is applied to the new resonator, an electric current is generated in the overhead loop, creating a magnetic flux or voltage. The dimensions of the copper structure are smaller than the wavelength of the radio signal. As a result, this small physical gap between the metal plates has the effect of storing energy around the atom and enhancing the radio signal. This improves performance efficiency or sensitivity.
“The loop captures the incoming magnetic field and creates a voltage between the gaps,” Hollowway said. “The small gap spacing creates a large electromagnetic field across the gap.”
The size of the loop and gap determines the natural or resonant frequency of the copper structure. In NIST experiments, the gap was just over 10 mm and was limited by the outer diameter of the available steam cell. Researchers used a commercially available math simulator to determine the loop size required to create a resonant frequency near 1.312 GHz. Here, the Rydberg atom switches the energy level.
Several outside collaborators helped model the resonator design. According to modeling, the signal can be 130 times stronger, but the measurement is about 100 times stronger, probably due to energy loss and structural defects. The smaller the gap, the greater the amplification. Researchers plan to investigate other resonator designs, smaller vapor cells, and different frequencies.
With further development, atomic-based receivers have the potential to offer many advantages over traditional radio technologies. For example, an atom acts as an antenna, and the atom functions automatically, eliminating the need for traditional electronics to convert and deliver signals to different frequencies. Atomic receivers are micrometer-scale dimensions and can be physically reduced. In addition, atom-based systems may be less susceptible to some types of interference and noise.
This study is partially funded by the Defense Advanced Research Projects Agency and the NIST onaChip program. Modeling assistance was provided by a collaborator at the University of Texas at Austin. City University of New York, New York; University of Technology Sydney, Australia.
Paper: CL Holloway, N. Prajapati, AB Artusio-Glimpse, S. Berweger, MT Simons, Y. Kasahara, A. Enhanced Rydberg atom-based field sensing with Alu, RW Ziolkowski split ring resonators. Letter of applied physics. Published online May 20, 2022. DOI: 10.1063 / 5.0088532