Tiny copper 'headphones' boost atomic radio reception

Researchers at the US National Institute of Standards and Technology (NIST) have boosted the sensitivity of their atomic radio receiver 100-fold by enclosing the small glass cylinder of cesium atoms inside what looks like custom copper ‘headphones’. An atomic sensor has the potential to be physically smaller and to work better in noisy environments than conventional radio receivers, among other possible advantages.

The team’s structure — a square overhead loop connecting two square panels — increases the incoming radio signal, or electric field, applied to the gaseous atoms in the flask (known as a vapour cell) between the panels. This enhancement enables the radio receiver to detect much weaker signals than before. Their work is described in the journal Applied Physics Letters.

The headphone structure is technically a split-ring resonator, which acts like a metamaterial — a material engineered with novel structures to produce unusual properties. The vapour cell is about 14 mm long with a diameter of 10 mm, while the resonator’s overhead loop is about 16 mm on a side, and the ear covers are about 12 mm on a side.

The NIST radio receiver relies on a special state of the atoms. Researchers use two different colour lasers to prepare atoms contained in the vapour cell into high-energy (‘Rydberg’) states, which have novel properties such as extreme sensitivity to electromagnetic fields. The frequency and strength of an applied electric field affects the colours of light absorbed by the atoms, and this has the effect of converting the signal strength to an optical frequency that can be measured accurately.

A radio signal applied to the resonator creates currents in the overhead loop, which produces a magnetic flux, or voltage. The dimensions of the copper structure are smaller than the radio signal’s wavelength. As a result, this small physical gap between the metal plates has the effect of storing energy around the atoms and enhancing the radio signal. This boosts performance efficiency, or sensitivity.

“The loop captures the incoming magnetic field, creating a voltage across the gaps,” NIST project leader Chris Holloway said. “Since the gap separation is small, a large electromagnetic field is developed across the gap.”

The loop and gap sizes determine the natural, or resonant, frequency of the copper structure. In the NIST experiments the gap was just over 10 mm, limited by the outside diameter of the available vapour cell. The researchers used a commercial mathematical simulator to determine the loop size needed to create a resonant frequency near 1.312 GHz, where Rydberg atoms switch between energy levels.

Several outside collaborators helped model the resonator design. Modelling suggests the signal could be made 130 times stronger, whereas the measured result was roughly 100-fold, likely due to energy losses and imperfections in the structure. A smaller gap would produce greater amplification. The researchers plan to investigate other resonator designs, smaller vapour cells and different frequencies.

With further development, atom-based receivers may offer many benefits over conventional radio technologies. For example, the atoms act as the antenna, and there is no need for traditional electronics that convert signals to different frequencies for delivery because the atoms do the job automatically. The atom receivers can be physically smaller, with micrometre-scale dimensions. In addition, atom-based systems may be less susceptible to some types of interference and noise.

Image caption: Copper ‘headphones’ boost the sensitivity of NIST’s atomic radio receiver, which is composed of a gas of cesium atoms prepared in a special state inside the glass container. When an antenna located above the set-up sends down a radio signal, the headphones boost the strength of the received signal 100-fold. Image credit: NIST.