Rydberg Atoms Detect Clear Signals From a Handheld Radio

For the first time, a team of US researchers has used sensors containing highly excited Rydberg atoms to detect signals from an ordinary handheld radio. Through a careful approach to demodulating the incoming signals, Noah Schlossberger and colleagues at the National Institute of Standards and Technology (NIST) were able to recover audio encoded in multiple public radio channels, with promising implications for everyday uses in consumer electronics. The research has been published in Physical Review Applied.

In a Rydberg atom, a single electron is excited to an extremely high energy level, pushing it far from its host atom's nucleus. From a distance, these atoms resemble a single electron orbiting a positively charged ion.

When any atom is exposed to an external electric field, the positions of its electrons' energy levels shift through a process called the Stark effect. Yet in a Rydberg atom, the shift becomes far more pronounced, causing particularly striking changes in the spectral patterns produced when the atom is probed by a laser.

Untapped potential

This effect ultimately means that Rydberg atoms are ideally suited as electric-field sensors: a possibility the Rydberg Sensor project's group leader, Christopher Holloway, began to explore in 2009. After embarking on the project, Holloway's team soon realized that the possibilities were far more wide-ranging than they first anticipated.

"One of the more intriguing applications is atom-based receivers, where these Rydberg-atom sensors act like an antenna to detect the signal, and perform the demodulation and down conversion automatically," Holloway describes.

"In principle, these Rydberg receivers could eliminate a lot of the front-end devices and electronics when compared to conventional receivers."

So far, however, the possibilities of these atomic sensors have largely been explored within the confines of the lab—leaving the full scope of their potential real-world applications largely unexplored.

Carefully demodulating signals

Building on the earlier results of the Rydberg Sensor project, the NIST team has now applied the principle outside the lab to detect real audio signals encoded in the radio frequencies produced by ordinary handheld radios.

"The key here is that the radio frequencies used in handheld radios are far from the natural resonances of the atom, so while the atoms can sense the radiation, they don't respond to the frequency modulation on which the audio is encoded," Schlossberger explains.

To tackle this challenge, the researchers combined the incoming radio signal with their own artificial signal. By setting the frequency of this added signal extremely close to the signal being measured, they created a pattern of constructive and destructive interference at a frequency equal to the difference between the two signals.

This interference caused the amplitude of the combined radio field inside the vapor cell to vary at the beat frequency, inducing a measurable Stark shift in the Rydberg atoms.

Spectral response of audio transmitted by a handheld two-way radio and received by either another handheld two-way radio or the atomic receiver. The spectra are normalized to the largest transmitted power to account for different electronic gains of the two receivers. 

Recovering speech

From the resulting spectral patterns, the NIST team could clearly detect the audio signals transmitted by a handheld radio—recovering coherent speech. In addition, "because the atoms respond over a very wide instantaneous bandwidth, the system can detect all 22 publicly available Family Radio Service channels at once," Schlossberger says.

"We demonstrated simultaneous reception of neighboring channels with strong isolation between them." This enabled the researchers to monitor numerous radio channels at once, instead of tuning into them individually.

Altogether, the team's results could have promising implications for the use of Rydberg atom sensors beyond the lab—marking an important step toward their real-world application.

"We have demonstrated the flexibility of Rydberg atom receivers: they can be used to detect actual signals from consumer electronics, not just a set of ideal frequencies," Schlossberger says.