We can listen to internal dynamics of protein structures at the most fundamental level.
At the boundary where sound dissolves into quantum mechanics, physicists at Caltech and Stanford have built devices that no longer need external machinery to hear the whisper of individual phonons. By turning a long-standing material flaw — atomic defects — into a precise tuning mechanism, their nanoelectromechanical systems achieve quantum sensitivity from within, the way a tuning fork needs no amplifier to know its own pitch. Published in Nature Physics, the work quietly redraws the frontier of what it means to listen at the smallest scales of matter.
- For decades, observing quantum acoustic behavior required bulky external superconducting qubits — a dependency that limited how small, simple, or practical these systems could ever become.
- The breakthrough hinges on nonlinearity: without it, quantum states are indistinguishable from one another, like trying to read a book where every page looks the same.
- Rather than fighting atomic defects in cooled materials, the team tuned their devices to resonate with them, transforming a persistent engineering nuisance into the very engine of quantum sensitivity.
- A Stanford graduate student and a Caltech postdoc independently confirmed that a single atomic defect is sufficient to grant a device single-phonon sensitivity — a result that surprised even the senior researchers.
- The team is now moving from harvesting natural defects to engineering custom ones, aiming for precise, reproducible quantum control across computing, communications, and molecular diagnostics.
Sound, at its most fundamental, is motion — molecules jostling in waves. At the quantum scale, atoms produce their own vibrational ripples called phonons, and for years studying them required external quantum machinery just to observe a single one. Now, physicists at Caltech and Stanford have built nanoelectromechanical systems, or NEMS, that achieve this sensitivity on their own, using only the properties of the material itself. The work appears in Nature Physics.
The central challenge was nonlinearity. In a linear quantum system, energy levels are evenly spaced and indistinguishable — you cannot tell which state the system occupies. Nonlinear systems break that symmetry, making quantum states readable. The team found a way to produce this nonlinearity intrinsically by exploiting atomic defects: imperfections in cooled solid materials where atoms flip between two stable configurations. Long considered a source of energy loss and noise, these defects were instead tuned like a radio dial — using temperature, electromagnetic fields, and mechanical forces — until they became the mechanism generating nonlinear quantum effects.
The result surprised even the researchers themselves. Stanford professor Amir Safavi-Naeini admitted he doubted the findings until he saw the data confirming that a single defect was enough to induce single-phonon sensitivity. Co-lead author Matthew Maksymowych noted the strangeness of reproducing single-atom quantum optics experiments using a slab of material containing billions of atoms.
The implications extend well beyond physics. Principal investigator Michael Roukes envisions using these devices to listen to the internal dynamics of individual protein molecules — detecting how they fold, how they bind to drugs, how they switch between active and inactive states. The next step is to engineer defects deliberately rather than rely on those that occur naturally, giving researchers precise control over quantum properties. Across quantum computing, communications, and molecular diagnostics, the team believes they are opening an era that allows science to tune into the sounds of the quantum world.
Imagine a guitar string vibrating, or a singer's voice traveling through air. Sound is motion—molecules jostling against each other in waves. At the quantum scale, atoms do something similar. They jiggle constantly, creating ripples of vibrational energy that physicists call phonons: the quantum cousins of sound itself.
For years, researchers who wanted to study these phonons at the quantum level faced a practical problem. They needed help from external devices—superconducting qubits and other quantum machinery—just to observe what the phonons were doing. It was like needing a separate instrument to listen to a single note. Now, physicists at Caltech and Stanford have built devices that don't need that external crutch. Their nanoelectromechanical systems, or NEMS, can exhibit quantum behavior on their own, using only the material properties built into the device itself. The work, published in Nature Physics, represents a significant step forward in a field called quantum acoustics.
The key insight involves making the vibrations nonlinear. In a linear system, energy levels are evenly spaced, like the rungs of a ladder. In a nonlinear system, those rungs are unevenly distributed. This matters because in a linear quantum system, you cannot tell what state the system occupies—all the possible changes look identical. Nonlinearity solves that problem. Mert Yuksel, a Caltech postdoctoral scholar and one of the study's lead authors, explains the practical goal: "You don't want linear systems for quantum applications, because then you can't tell what state the system is in." The team found a way to achieve this nonlinearity intrinsically, without external equipment.
The researchers accomplished this by exploiting something that materials scientists have long considered a nuisance: atomic defects. In solid materials cooled to very low temperatures, atoms can exist in two energetically favorable configurations, flipping back and forth between them like someone shifting positions in a comfortable chair. These two-level systems normally waste energy and degrade quantum performance. But the Caltech-Stanford team realized they could tune their NEMS devices to resonate with these defects, using temperature changes and electromagnetic or mechanical forces. The result: the defects themselves become the mechanism that produces nonlinear quantum effects. Yuksel describes it as tuning a radio dial to listen to different stations—except the stations are the defects themselves.
The implications reach far beyond physics laboratories. The researchers envision using these devices as quantum sensors sensitive enough to detect individual molecules landing on the surface. Matthew Maksymowych, a Stanford graduate student and co-lead author, notes the central challenge: "For this effort, it is critical that our devices are extremely sensitive to environmental changes, yet stable enough to avoid spurious signals and noise." Michael Roukes, the principal investigator and a Caltech professor of physics and bioengineering, articulates the deeper ambition: "We are working toward making quantum measurements on individual molecules. 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."
What makes this breakthrough surprising is its simplicity. Previous work at universities like Chicago and Yale required coupling mechanical devices to superconducting qubits—adding complexity and bulk. The new NEMS devices work alone. Amir Safavi-Naeini, an applied physics professor at Stanford and study co-author, admits his initial skepticism: "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 that reproducing single-atom quantum optics experiments using a slab of material containing billions of atoms felt genuinely remarkable.
The team's next step is to stop relying on naturally occurring defects and instead engineer their own. This would give researchers precise control over the quantum properties of their devices. The applications span quantum computing, quantum communications, and biological measurement—but the most tantalizing possibility is molecular diagnostics. By listening to the quantum vibrations of molecules, researchers could learn their internal structure, how they bind to drugs, how they switch between active and inactive states. Roukes frames it as opening a new era: "One that allows us to tune into the sounds of the quantum world."
Citações Notáveis
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.— Mert Yuksel, Caltech postdoctoral scholar
We are working toward making quantum measurements on individual molecules. 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.— Michael Roukes, Caltech professor and principal investigator
A Conversa do Hearth Outra perspectiva sobre a história
So these devices work without external help now. What was the actual barrier before? Why did you need a superconducting qubit just to listen?
You needed an external device because the phonons alone couldn't tell you what quantum state they were in. The system was too linear. All the possible changes looked the same, so you had no way to read the information. The external qubit was a translator.
And now you've made the vibrations nonlinear. How does that change what you can actually see?
Now the energy levels are unevenly spaced, like an irregular ladder. Each step is different. So when a phonon moves between states, you can actually detect which transition happened. The device itself becomes readable.
The surprising part in the paper is that you're using defects—things that usually ruin quantum systems. Why would you want those?
Because they're already there, and they're tunable. We cool the device and apply forces, and suddenly those defects resonate with the phonons in just the right way to create the nonlinearity we need. We're not fighting the material anymore. We're using what it naturally wants to do.
What can you actually measure with this? What's the real-world application?
A molecule lands on the device. The phonons couple to it. We listen to how the vibrations change. From that, we learn the molecule's structure, how it binds to drugs, whether it's in an active or inactive state. We're essentially eavesdropping on molecular behavior at the quantum level.
And the next step is to engineer your own defects instead of waiting for natural ones?
Exactly. Right now we're tuning to whatever defects happen to be there. If we can design them, we control the whole system. That's when this becomes a real tool.