Quantum information is fragile, and the usual tool for shielding it — sequences of microwave pulses that decouple a qubit from surrounding noise — works poorly for one of the most promising hardware platforms: spins held inside phononic cavities. Writing in Nature Physics, researchers in the lab of Marko Lončar at Harvard's John A. Paulson School of Engineering and Applied Sciences describe a way around that limit by making the protection mechanical.
The team worked with a silicon-vacancy spin in diamond, a leading candidate for storing quantum information, and drove it continuously with a mechanical field made of phonons — tiny packets of vibration. The result is a "dressed" qubit: one effectively wearing an ongoing acoustic field, which makes it less vulnerable to low-frequency environmental noise. The approach, which the authors call "all-mechanical coherence protection," increased the spin's coherence time by roughly a factor of three.
The significance lies in compatibility. Phonons have a practical edge over light for chip-scale quantum networks: at the same frequency their wavelength is much shorter, so components can be built smaller and packed more densely, and they interact readily with both solid-state spins and electromagnetic fields. But the same cavities that make phonons useful had resisted conventional decoupling. Because the new protection comes from a continuous mechanical field, it can operate inside those cavities — giving phonons a potential dual role, transporting quantum information between nodes while simultaneously helping to preserve it.
"The spin needs to interact strongly with phonons, and it also needs a long coherence time," said Eliza Cornell, who led the experiments as a Ph.D. student in the Lončar lab and is now a postdoctoral researcher at Boston University. The result suggests microscopic sound waves could become a practical tool for more reliable, compact quantum devices and for hybrid systems that combine different kinds of qubits. The work was supported by the NSF, the Air Force Office of Scientific Research and the DOE's Q-NEXT center, and Harvard's Office of Technology Development is pursuing patent protection.
