When a singer hits a high note and a guitar string vibrates in sympathy, something remarkable happens at a scale far too small to see: atoms inside materials jiggle and dance, creating tiny vibrational waves called phonons—the quantum version of sound itself. Now physicists at Caltech and Stanford University have engineered devices that harness these quantum vibrations without needing external quantum machines, opening a new frontier in what scientists call quantum acoustics.
The breakthrough centers on nanoelectromechanical systems, or NEMS, fabricated from piezoelectric lithium niobate and actuated with aluminum electrodes. The key innovation is elegantly simple: the researchers made the vibrations inside these devices nonlinear—meaning the energy levels don't space evenly, like unevenly spaced rungs on a ladder. "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," explains Mert Yuksel, a Caltech postdoctoral scholar and co-lead author of the study published in Nature Physics. "So, having nonlinearity is the goal, and now we can achieve this in the NEMS intrinsically."
What makes this achievement significant is that the intrinsic material properties of the device itself—including natural defects within the resonator—create the quantum behavior. Previously, observing such behavior required coupling the NEMS to an external quantum device, such as a superconducting qubit, adding complexity, size, and cost. By eliminating that requirement, a single NEMS device can now serve as a greatly simplified quantum sensor or even a qubit—potentially transforming how researchers detect infinitesimal changes in materials.
The team's ultimate ambition reveals why this matters beyond the lab. "Our goal is basically to listen to molecules," Yuksel says. The phonons generated within their device act as sensors, detecting when molecules land on the device and revealing their secrets: internal structure, how they function, how they bind to drugs, how they switch between active and passive states. This could fundamentally change biological measurement and drug discovery.
Matthew Maksymowych, a Stanford graduate student and co-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." The team has navigated this paradox by working at the quantum level, where single phonons can precisely detect the subtlest shifts in their surroundings.
Michael Roukes, the Frank J. Roshek Professor of Physics, Applied Physics, and Bioengineering at Caltech and principal investigator, frames the larger vision: "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." This approach mirrors the success of quantum optics, which revolutionized physics by focusing on single photons—discrete packets of light. Quantum acoustics now applies the same principle to phonons: discrete packets of vibrational energy.
While teams at the University of Chicago and Yale University have developed small mechanical devices operating at the single-phonon level, they required coupling to external devices to function. The Caltech-Stanford team tuned their NEMS to operate at the single-phonon level intrinsically, marking a decisive step forward. As quantum acoustics emerges as a field with applications spanning quantum computing, quantum communications, and molecular biology, this simplified approach could accelerate innovation across all three domains—and eventually let scientists truly listen to the quantum whispers inside living molecules.