Sound Waves Help Protect Fragile Quantum Information, Harvard Team Finds

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have found a new way to protect quantum information using phonons, tiny packets of mechanical vibration, microscopic sound waves. The work, from Marko Lončar's lab, is published in Nature Physics and could aid chip-based quantum networks and hybrid systems combining different kinds of qubits, or quantum bits.

One approach to quantum networking stores information in an electron's spin tied to an impurity in diamond, then uses phonons to carry it between qubit nodes through a "phononic cavity" that traps vibrations for stronger interaction with the spin. Phonons have shorter wavelengths than light, allowing smaller components, and interact with both solid-state spins and electromagnetic fields.

This creates a problem: preserving quantum memory. Qubits are sensitive to their surroundings and must hold their state long enough to process information, an ability called coherence. Microwave pulses usually protect memory by decoupling it from noise, but they work poorly inside phononic cavities, making strong phonon interaction and long coherence hard to achieve together.

The team, led by Eliza Cornell, a recent Ph.D. graduate from the Lončar lab now a postdoctoral researcher at Boston University, and Zhujing Xu, a former postdoctoral scholar in the group, solved this with "all-mechanical coherence protection": a continuous mechanical field of phonons, instead of microwave pulses, applied to a silicon-vacancy spin in diamond. This turns it into a "dressed" qubit that effectively wears a continuous acoustic field and is less vulnerable to low-frequency noise. "We are solving two problems," Cornell said. "We want the spin to have strong interaction with phonons, and we want the spin to have a long coherence time."

Because the protection comes from a mechanical field compatible with phononic cavities, the same structures could carry quantum information and shield it from noise. Using the new method, the researchers extended the coherence time of the silicon-vacancy spin by roughly a factor of three.