How a Single Sound Quantum Alters Atomic Spin States

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences have, for the first time, demonstrated the interaction between a single quantum of sound and a single atomic spin, opening a new pathway for quantum technologies that utilize phonons, units of vibrational energy, instead of light or electricity. The experiment centered around a 5 mm x 5 mm diamond chip housing a nanometer-scale mechanical resonator engineered around a single color-center spin qubit, enabling strong spin-phonon interactions for quantum information storage. “At the heart of the experiment is a phonon, the smallest possible unit of sound,” explained Marko Lončar, the Tiantsai Lin Professor of Electrical Engineering. This achievement demonstrates an extreme level of sensitivity, as a single phonon was sufficient to alter the quantum state of the atomic spin, and could lead to precision sensing and the connection of disparate quantum systems, according to first author Graham Joe, who stated, “This experiment was both a compelling demonstration of new tools for sensing the environment of a single atom, and a meaningful step towards practical quantum acoustic devices.”

Nanoscale Resonator Enables Single-Phonon, Single-Spin Interaction

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences utilized a diamond chip with a nanometer-scale mechanical resonator to achieve this interaction. This configuration allowed for strong spin-phonon interactions, overcoming a key obstacle in the field and enabling the manipulation of quantum states with increased sensitivity. Lončar explained that while classical systems require significant energy to change state, qubits are far more sensitive, a single phonon can be enough to change their quantum state, either exciting them or, as in this experiment, helping them relax. This is particularly promising because mechanical vibrations can sustain “ringing” for extended periods while occupying a remarkably small volume, making phonons ideal candidates for quantum information carriers and interconnects within future quantum chips. The spin qubit itself functions as an exceptionally sensitive probe of its mechanical environment, potentially enabling the measurement of minute forces, stresses, or temperature fluctuations by “listening” to quantum noise. The research received federal support from the National Science Foundation under grant No. DMR- and the Army Research Office/Department of the Army under award No. W911NF1810432.

This configuration allows the diamond’s atomic defects, acting as quantum memory, to interact with phonons, the fundamental unit of sound, in a way previously unobserved, and represents a significant step toward building quantum systems that leverage mechanical vibrations.