Scientists at the National Institute of Standards and Technology (NIST) have improved the sensitivity of their atomic radio receiver by enclosing a tiny glass cylinder filled with cesium atoms within what appear to be special copper "headphones."
The structure, which consists of a square overhead loop connecting two square panels, amplifies the incoming radio signal, or electric field, that is delivered to the gaseous atoms in the flask (also known as a vapor cell) between the panels. This improvement allows the radio receiver to identify signals that were previously undetectable. A recent study in the journal Applied Physics Letters describes the experiment.
The headphone structure is a split-ring resonator, which functions as a metamaterial – a material designed with new structures to accomplish strange qualities. “We can call it a metamaterials-inspired structure,”said Chris Holloway, project leader at NIST.
The atom-based radio receiver was previously demonstrated by NIST researchers. Among other advantages, an atomic sensor might be physically smaller and function better in noisy situations than traditional radio receivers.
The vapor cell is about the size of a fingernail or a computer chip, but thicker, at 14 millimeters (0.55 inches) long and 10 millimeters (0.39 inches) in diameter. The overhead loop on the resonator is about 16 mm (0.63 inches) wide, and the ear coverings are about 12 mm (0.47 inches) wide.
The NIST radio receiver is based on a unique atomic state. Researchers employ two distinct color lasers to convert atoms in the vapor cell into high-energy ("Rydberg") states, which have unique traits including extraordinary sensitivity to electromagnetic fields. The colors of light absorbed by the atoms are affected by the frequency and intensity of an applied electric field, which has the effect of transforming the signal strength to an optical frequency that can be measured precisely.
A radio signal supplied to the new resonator causes currents to flow through the overhead loop, resulting in a magnetic flux, or voltage. The copper structure's dimensions are less than the wavelength of the radio transmission. As a result, the small physical space between the metal plates serves to store energy around the atoms while also boosting the radio signal. This improves sensitivity or performance efficiency.
“The loop captures the incoming magnetic field, creating a voltage across the gaps,” Holloway explained. “Since the gap separation is small, a large electromagnetic field is developed across the gap.”
The natural, or resonant, frequency of the copper structure is determined by the loop and gap sizes. The distance in the NIST trials was little over 10 mm, which was limited by the available vapor cell's outer diameter. The researchers utilized a commercial mathematical simulator to figure out how big of a loop they required to make a resonant frequency near 1.312 gigahertz, when Rydberg atoms flip between energy levels.
The resonator design was modeled with the assistance of several other colleagues. Modeling implies that the signal may be 130 times stronger, although the observed result was about a hundredfold, owing to energy losses and structural flaws. Greater amplification would be achieved with a smaller distance. Other resonator designs, smaller vapor cells, and various frequencies are all being investigated by the researchers.
Atom-based receivers may provide several advantages over traditional radio technology with future development. Because the atoms operate as the antenna, typical electronics that transform signals to different frequencies for delivery are not required. Atom receivers can be micrometer-scale in size. Furthermore, atom-based devices may be more resistant to interference and noise.