Army Researchers Develop Tiny Quantum Sensor for 3D Radio Signal Detection

A quantum sensor the size of a paperclip can determine the 3D direction of radio signals using a single compact device, offering a potential tactical advantage on the battlefield.

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Researchers at the U.S. Army Research Laboratory (ARL) have created a quantum sensor that can locate the source of radio signals in three dimensions using a single, tiny device. The sensor, roughly the size of a paperclip, relies on a 2.5-centimeter glass chamber filled with excited atoms known as Rydberg atoms, which are sensitive to electric fields.

Traditional radio direction finding requires multiple large antennas spaced apart to triangulate a signal's origin, each tuned to a specific frequency. That setup is bulky and conspicuous, a liability on battlefields. The new sensor measures the polarization of incoming radio waves—the orientation of their oscillation. Many radio signals propagate in a circular or elliptical corkscrew pattern perpendicular to their travel direction, so determining polarization reveals the source direction. To measure polarization, the researchers inject three reference radio signals oscillating along the X, Y, and Z axes into the device. An external signal alters the electric field along each axis differently depending on its origin, and the Rydberg atoms detect these interactions, allowing the system to map the signal's polarization and direction.

ARL research physicist David Meyer, who leads the research, explained, “You can imagine having a device that provides you a level of spectrum awareness that’s very difficult to get in a single platform.” He added, “The goal is to get more information, and get that information in the hands of soldiers.”

The sensor currently works only with circularly or elliptically polarized signals, but Meyer noted that many signals of interest are already polarized this way, and linear signals often become slightly elliptical after interacting with the atmosphere. For microwave and terahertz signals, researchers can 3D-print a plastic wave plate to convert them to elliptical polarization.

A major hurdle is transitioning the sensor from lab to field. Experiments so far have been conducted on optical tables in controlled environments. The lasers used to excite the Rydberg atoms require high stability. “The stability required is very high,” Meyer said. “And lasers that are that stable don’t generally like being outside in high humidity and high heat.”

To address this, ARL announced in May a partnership with quantum technology company Infleqtion, based in Louisville, Colorado, to develop a field-ready prototype. Seth Caliga, Infleqtion’s director of R&D for quantum RF sensing, said the biggest challenge is making the lasers stable enough for field conditions. The company plans to leverage its optical atomic clock technology, which uses similar components, to create a ruggedized version suitable for military deployment.