Researchers Build Custom “Headphones” to Bump Atomic Radio Efficiency

Many industries spanning healthcare to aviation actively use the terahertz (THz) and infrared regions in the electromagnetic spectrum. This region of the spectrum, however, relies on conventional antennas (dipole and loop antennas) that limit the measurement precision of electric field distribution. To complicate the matter, electric field sensors for the infrared regime are almost non-existent.

As a result, researchers have turned their attention toward atom-based sensing, where atoms receive commonly used communication signals. Highly-excited atoms, also called Rydberg atoms, are sensitive to the electric fields in this region. Rydberg atoms can be optically prepared with a diode laser.

Researchers at the National Institute of Standards and Technology (NIST) have been investigating how atoms can receive communication signals. Recently, the NIST team has found a way to boost the reception of an atomic radio by 100 times.

 

The copper headphone-like structure boosts the sensitivity of NIST’s atomic radio receiver. Image used courtesy of NIST

 

What is Atom-based Sensing?

Atom-based sensing is often considered more sensitive, accurate, and stable compared to conventional antenna-based sensing. Rydberg atom-based RF sensing is highly sensitive because of the large transition dipole moments between the Rydberg states.

In an atomic sensor, Rydberg atoms are contained in a vapor cell at room temperature. An electromagnetic method reads out the effect of the RF signals on the Rydberg atoms. The readout method optically detects the light-matter interaction between the electric field and the highly energetic particles.

 

Above a microwave circuit, a Rydberg receiver and spectrum analyzer can detect radio signals including AM, FM, Bluetooth, and Wi-Fi. Image used courtesy of Phys.org and the Army Research Laboratory

 

However, to effectively take advantage of the large transition dipole moments of the Rydberg atoms, the oscillations of the transition dipole must be coherent. The population of the Rydberg atoms has to be low enough inside the vapor cell to increase the coherence time long enough to achieve the target sensitivity. Sensitivity increases as more atoms participate. 

 

NIST Uses Atoms to Receive Modulated Signals

Researchers at NIST used a split-ring resonator (SRR) with a vapor cell to increase the sensitivity of Rydberg atom-based sensors. They report that the SRR improved the electric-field sensitivity by a factor of 100.

In an earlier study, the NIST researchers demonstrated the use of atoms to receive modulated signals. They used two different colored lasers to prepare the high-energy Rydberg atoms. The electric field frequency affects the wavelength of the light absorbed by the Rydberg atoms.

To measure the modulated signals, the team of researchers created an atom-based mixer, in which one RF signal acts as a reference and another RF signal is a modulated signal carrier. The difference in frequency is detected and measured by probing the atoms. The NIST system reportedly received five megabits per second, comparable to third-generation (3G) data rates.

 

Boosting an Atomic Radio

Recently, the researchers reported that they boosted the efficiency of their atom-based receiver. They did this by incorporating Cesium atoms inside a vapor cell surrounded by a square overhead loop connecting square panels. The loop increased the electric fields applied to the vapor cell between the plates.

 

Setup for receiving modulated signals. Image used courtesy of NIST

 

The incoming RF signals to the resonator create a current in the overhead loop, producing a magnetic flux or voltage. The small gap between the metal plates acts as a capacitor and stores energy around the atoms, enhancing the radio signal.

The researchers believe that further development in atom-based receivers may offer benefits beyond conventional radio technologies. The atom-based receivers can be physically smaller in dimensions and less susceptible to interference and noise.

 

Future Research Directions

The loop and gap size determine the frequency of the copper structure. The researchers used a mathematical simulator to determine the loop size to create a frequency of 1.312 GHz with a gap of around 10 mm. 

The model suggests that the incoming signal power could be amplified by 130 times, and a smaller gap could produce even greater amplification. The NIST researchers now plan to investigate other resonator designs, vapor cells, and different frequencies. They also plan to suppress laser noise and other undesired effects to further improve the receiver.