Harvard engineers have achieved a direct coupling between a single phonon—a quantum unit of sound vibration—and a single atomic spin for the first time, a breakthrough with implications for quantum communication systems. The work appears in Nature.
The team at Harvard's John A. Paulson School of Engineering and Applied Sciences created conditions where acoustic vibrations at the quantum level interact directly with individual electron spins in a crystal. This coupling enables phonons to carry quantum information in ways previously limited to photons or electrical signals.
The achievement opens new avenues for quantum technologies. Sound-based quantum systems offer distinct advantages over conventional approaches. Phonons occupy a different part of the electromagnetic spectrum than photons, allowing engineers to sidestep certain technical limitations. Acoustic systems can also operate at different temperature ranges and with different material platforms than optical or electrical quantum systems.
The researchers used a specially engineered crystal containing rare-earth ions. By applying precise control techniques, they isolated individual atomic spins and coupled them to quantized acoustic modes within the material. This required unprecedented control over both the phononic and spin states simultaneously.
The significance extends beyond communication. Phonon-spin coupling could enable new quantum sensing applications, where acoustic vibrations detect minute changes in magnetic fields or other physical properties. It also opens pathways for hybrid quantum systems that combine multiple physical platforms.
However, practical quantum technologies remain years away. The coupling strength and coherence times—how long the quantum state persists before degrading—require further optimization. Engineering stable, scalable systems that maintain these quantum properties at useful temperatures and timescales presents substantial engineering challenges ahead.
This work demonstrates that quantum information need not rely exclusively on light or electricity. By harnessing acoustic vibrations at quantum scales, engineers expand the toolkit for building future quantum computers and sensors that exploit properties unavailable through conventional channels.