When a singer belts out a tune while a guitar player strums along, sound waves travel through the air, driving collective oscillations of the molecules within. Something similar goes on at the quantum level. Atoms inside materials, from our bodies to metals and more, naturally jiggle around, creating tiny vibrational waves that ripple across the material. These vibrations are known as phonons: the quantum version of sound waves.

Physicists at Caltech and Stanford University have developed devices called nanoelectromechanical systems (NEMS) that allow phonons to exhibit their quantum behavior purely through the intrinsic properties of the material that makes up the device. Previously, it was not possible to observe such behavior without the help of an external quantum device, such as a superconducting qubit. Through this newly discovered mechanism—a solitary NEMS device—can serve as a greatly simplified and very compact quantum sensor or qubit. The goal of the work is to make the vibrations of the NEMS nonlinear.

According to Caltech postdoctoral scholar and co-lead author of the study, Mert Yuksel (PhD ’26), “When all the step changes that a system can make look the same, you can’t tell what state it’s in. So, having nonlinearity is the goal, and now we can achieve this in the NEMS intrinsically.” 

This new work is part of an emerging field called quantum acoustics and marks a next step toward creating quantum-sensing devices that use single phonons to precisely detect extremely small changes in materials. Applications include quantum computing and quantum communications, as well as biological measurements. “Our goal is to basically listen to molecules,” Yuksel says. "The phonons live in our device, and what we sense is whatever couples to those phonons, such as a molecule landing on the device. We want to learn about molecules' unique properties: internal structure, how they function, how they bind to drugs, how they switch between active and passive states, and so on.”

Matthew Maksymowych, a Stanford graduate student and co-lead author of the study, said, “For this effort, it is critical that our devices are extremely sensitive to environmental changes, yet stable enough to avoid spurious signals and noise.” By working at the quantum level, the sensors have the potential to unlock deeper information about molecules and their dynamics.

While the field of quantum optics focuses on single photons, which are discrete packets of light, quantum acoustics investigates discrete packets of vibrational energy, known as phonons. Teams at the University of Chicago and at Yale University have been developing small mechanical devices to work at the single-phonon level, but they must be coupled to another device, such as a superconducting qubit, to function. In the new study, the researchers tuned a NEMS device to operate at the single-phonon level intrinsically, without the need for an additional external device.

This new NEMS design scheme takes advantage of a phenomenon in materials known as two-level systems. In these systems, atoms flip between two spatial configurations within a material that are both energetically favorable—for a human, this would be like going back and forth between two comfortable positions in a lounge chair, normally considered defects. However, the new work takes advantage of these defects, which naturally occur in the materials from which NEMS devices are patterned. By lowering the temperature of a device and applying electromagnetic or mechanical forces, the researchers were able to tune the device to be in resonance with the defects in such a way that nonlinear effects are produced. “It's like a radio station, and you can tune it around to listen to the different defects,” Yuksel says.

The researchers say a next step is to engineer their own defects into the NEMS devices rather than rely on naturally occurring ones. The hope is that this will open up a new era of quantum measurements that allow researchers to tune into the sounds of the quantum world. 

Images courtesy of Caltech. 

For information: 

Caltech 

Stanford University

Nature Physics