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. Meanwhile, at the quantum level, something similar is going on. Atoms inside materials, everything 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.
Now, 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.
This means that through this newly discovered mechanism a solitary NEMS device, can, for example, serve as a greatly simplified and very compact quantum sensor or qubit.
More specifically, the goal of the work is to make the vibrations of the NEMS nonlinear. If you imagine the energy levels in a nonlinear system as steps in a ladder, it is as if the ladder steps are not evenly spaced.
"You don't want linear systems for quantum applications, because then you can't tell what state the system is in—all the step changes that the system can make look the same," says Mert Yuksel (PhD '26), a Caltech postdoctoral scholar and co-lead author of the new study. "So, having nonlinearity is the goal, and now we can achieve this in the NEMS intrinsically."
The new work, part of an emerging field called quantum acoustics, marks a next step toward creating quantum-sensing devices that use single phonons to precisely detect extremely small changes in materials. The findings, reported in Nature Physics, have applications in quantum computing and quantum communications, as well as in biological measurements—a principal focus of the Caltech–Stanford team.
"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 new study, says, "For this effort, it is critical that our devices are extremely sensitive to environmental changes, yet stable enough to avoid spurious signals and noise. Much of our work focuses on tackling this paradox."
By working at the quantum level, the sensors have the potential to unlock deeper information about molecules and their dynamics.
"We are working toward making quantum measurements on individual molecules," says Michael Roukes, the Frank J. Roshek Professor of Physics, Applied Physics, and Bioengineering at Caltech and principal investigator of the new study. "When you bring our devices to the quantum regime by lowering the temperature, then the underlying idea is that we can listen to internal dynamics of protein structures at the most fundamental level."
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. (The word "phonon" comes from the Greek word phōnē, which means sound.) In recent years, researchers have made advances in quantum acoustics devices. For example, 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, these two-level states are considered defects because they can "parasitically steal energy away from quantum systems," Roukes says.
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.
Amir Safavi-Naeini (PhD '13), now an applied physics professor at Stanford and a co-author of the new study, says,"Since the 1970s, people have argued that low-temperature solids are full of two-level system defects, little double-well potentials where an atom or cluster of atoms exist in superpositions of two configurations and tunnel between them. These intrinsic defects are a major headache, and the standard explanation used to explain energy loss and quantum decoherence in amorphous and crystalline materials. I have to admit; I didn't really believe the new results until I saw the data from Mert and Matthew showing that a single defect in a NEMS device is enough to induce single-phonon sensitivity."
Maksymowych adds, "I find it quite remarkable that phonons in our device can sense individual solid-state defects, possibly comprised of a few atoms, in such a repeatable fashion. The fact that we could reproduce single-atom quantum optics experiments with just a slab of material, constituting billions of atoms, was really surprising."
The researchers say a next step is to engineer their own defects into the NEMS devices rather than rely on naturally occurring ones. "We are hopefully opening up a new era of quantum measurements," Roukes says. "One that allows us to tune into the sounds of the quantum world."
The Nature Physics study titled "Intrinsic phononic dressed states in a nanomechanical system" was funded by the Gordon and Betty Moore Foundation, the Wellcome Leap Delta Tissue Program, the National Institutes of Health, the National Science Foundation, the U.S. Defense Advanced Research Projects Agency, Amazon Web Services, the U.S. Department of Energy, and the Natural Sciences and Engineering Research Council of Canada. Other authors include Oliver A. Hitchcock, Felix M. Mayor, and Nathan R. Lee at Stanford as well as Professor Mark I. Dykman of Michigan State University.