MARCH 26, 2026 -- Phonons are the quantum units of mechanical vibration. They describe how motion propagates through a solid at the smallest possible scales, in much the same way that electrons describe electric currents. Because phonons can be exceptionally stable and sensitive, they are used in quantum science and technology.

Researchers can already detect and control individual phonons. The problem lies in making phonons interact with each other in a predictable and tunable way, which would be a key requirement for building complex quantum systems like quantum computers.

Interactions are essential in quantum technologies. Whether the goal is sensing tiny forces or processing information, one quantum excitation must be able to influence another. In practice, this requires nonlinearity, which means meaning that adding one excitation changes how the system responds to the next, rather than each excitation behaving independently.

Mechanical systems are attractive in this context because they can store energy for long times. But phonons are usually linear excitations and, left on their own, do not interact. This has made it difficult to move from controlling single phonon modes to building devices where many phonons act together.

Researchers led by Pasquale Scarlino at EPFL, in collaboration with Per Delsing at Chalmers University of Technology, have now found a solution by building a microfabricated chip that hosts both mechanical vibrations and superconducting nonlinear electronics.

By coupling multiple mechanical modes—distinct patterns of vibration within the device—to a nonlinear superconducting circuit built with arrays of Josephson junctions on the same chip, they successfully created a “quantum acoustic” device in which several phonon modes can interact with the resonator at the same time.

The device

The device was fabricated at MyFab Chalmers and combines two main components. The first is a surface acoustic wave cavity, which confines mechanical vibrations on the surface of a solid. These vibrations carry phonons and form a set of discrete mechanical modes.

The second component is a superconducting microwave resonator whose properties can be tuned using a magnetic field. When the mechanical modes and the resonator are brought close in frequency, they hybridize. Each phonon mode then acquires a small contribution from the nonlinear electronic circuit.

Getting phonons nonlinear

The researchers found that even a small contribution from the nonlinear circuit is enough to make phonon modes interact. A key element is the “participation ratio”, which measures how strongly each phonon mode mixes with the superconducting resonator. By extracting this parameter directly from experiments, the team could predict both energy losses and interaction strengths.

Using this approach, the researchers measured interactions between several pairs of phonon modes. Exciting one mode shifted the frequency of another, a clear signature of phonon–phonon interaction. Under a two-photon drive, they also observed bistable behaviour, where a mechanical mode switches between distinct vibration states.

The work establishes a general experimental framework for engineering and characterizing interacting phonons. Such systems could enable new types of quantum sensors, allow the study of collective and nonlinear mechanical phenomena, and support future architectures in which mechanical modes play an active role in quantum information processing. More broadly, the work shows how quantum acoustic devices can scale beyond single-mode operation.