NEMS Device Exhibits Quantum Behavior Through Material Properties Alone

Physicists at Caltech and Stanford University have engineered nanoelectromechanical systems (NEMS) capable of exhibiting quantum behavior through the intrinsic properties of their materials, a feat previously requiring an external quantum device like a superconducting qubit. This advancement simplifies the design of compact quantum sensors and qubits, opening new avenues for quantum computing and biophysics. The team achieved this by creating nonlinearity within the NEMS device; evenly spaced energy levels in linear systems hinder the ability to determine a system’s state. “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. The findings, reported in Nature Physics, represent a step toward sensors that use single phonons, the quantum version of sound waves, to detect extremely small changes in materials.

NEMS Devices Enable Intrinsic Single-Phonon Quantum Behavior

The ability to observe quantum behavior in the vibrations of materials, known as phonons, has significantly advanced with the development of nanoelectromechanical systems (NEMS) capable of exhibiting these properties intrinsically. This simplification promises to accelerate progress in compact quantum sensors and computing architectures. This nonlinearity is achieved by leveraging naturally occurring two-level systems, atomic-scale defects within the materials comprising the NEMS device. These defects, previously considered detrimental to quantum systems, are now harnessed to induce the desired nonlinear effects. By carefully tuning the device’s temperature and applying electromagnetic forces, the researchers resonated the NEMS with these defects, creating the conditions for single-phonon sensitivity. “It’s like a radio station, and you can tune it around to listen to the different defects,” Yuksel adds, illustrating the precision of the technique.

Amir Safavi-Naeini (PhD ‘26), now an applied physics professor at Stanford and a co-author, stated he initially doubted the results, saying, “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.” The implications extend beyond fundamental physics, with potential applications in biological measurements and a long-term goal of listening to molecules at the quantum level, according to Yuksel.

Nonlinear Phonon Generation via Material Two-Level Systems

Recent advances in quantum acoustics have largely depended on coupling nanoscale mechanical resonators to external quantum systems, such as superconducting qubits, to observe quantum behavior in phonons, the quantum version of sound waves. However, researchers at Caltech and Stanford University have demonstrated a significant departure from this established methodology, achieving nonlinear phonon generation directly within a nanoelectromechanical system (NEMS) device itself. This intrinsic nonlinearity represents a simplification in the design of quantum sensors and qubits, eliminating the need for complex external apparatus. These two-level systems arise from atoms within the material fluctuating between energetically favorable configurations, similar to comfortably shifting positions. Michael Roukes, the Frank J. Roshek Professor of Physics, Applied Physics, and Bioengineering at Caltech,

Quantum Acoustics for Detecting Molecular Vibrations

Michael Roukes, the Frank J. Roshek Professor of Physics, Applied Physics, and Bioengineering at Caltech, and his team are developing a new approach to quantum sensing, moving beyond the need for external quantum devices to observe phonon behavior. Previously, observing quantum behavior in phonons necessitated coupling the system to another quantum device, such as a superconducting qubit; now, the NEMS device itself facilitates this behavior. This advancement opens possibilities for compact quantum sensors and qubits, potentially revolutionizing fields from quantum computing to biophysics. The team’s ultimate goal extends to molecular-level detection. “Our goal is to basically listen to molecules,” Yuksel says, describing how the NEMS device can sense changes caused by molecules interacting with its phonons. This capability promises to reveal crucial information about molecular structure, function, and interactions with drugs, offering unprecedented insights into biological processes.

Matthew Maksymowych, a Stanford graduate student and co-lead author, highlights the delicate balance required: “For this effort, it is critical that our devices are extremely sensitive to environmental changes, yet stable enough to avoid spurious signals and noise.” Roukes envisions a future where researchers can “listen to internal dynamics of protein structures at the most fundamental level,” marking a new era of quantum measurements.

Lithium Niobate NEMS Design & Defect Utilization

The development of nanoscale sensors capable of detecting individual molecules hinges on increasingly refined designs, and a recent advance from Caltech and Stanford University leverages an unexpected component: material defects. Researchers have demonstrated that nanoelectromechanical systems (NEMS) fabricated from lithium niobate can achieve nonlinear behavior, a crucial requirement for quantum sensing, through the intrinsic properties of naturally occurring two-level systems within the material itself. This simplification stems from a novel approach to utilizing imperfections traditionally viewed as detrimental to quantum systems. The implications extend beyond streamlined sensor design.