Researchers at Harvard University, the University of Chicago, the Bavarian Academy of Sciences, the Technical University of Munich, and Argonne National Laboratory have demonstrated a new way to protect fragile quantum information using nothing but mechanical vibrations - phonons, the quantum particles of sound.
The work builds on an approach to quantum networking in which the spin of an electron tied to a defect in diamond, known as a silicon-vacancy center, serves as a stationary quantum memory, while phonons carry information between memory nodes. Phonons have real advantages over photons at chip scale: at a given frequency their wavelengths are far shorter, allowing smaller and more tightly integrated devices, and they couple readily to both solid-state spins and electromagnetic fields, making them useful building blocks for hybrid systems that mix different qubit types. The team, based in the lab of Harvard's Marko Lončar, has previously built "phononic cavities" that house a silicon-vacancy qubit and trap vibrations so they interact strongly with its spin.
The catch has been memory protection. Extending a qubit's coherence time - how long it retains quantum information before environmental noise scrambles it - normally relies on microwave pulse sequences that decouple the qubit from its surroundings. Those pulse-based techniques don't work well on qubits already housed inside a phononic cavity built to interact continuously with sound.
The team's solution was to protect coherence mechanically instead of electrically. Rather than pulsing the qubit with microwaves, they applied a continuous mechanical driving field made of phonons, shifting the silicon-vacancy spin into a new "dressed" state - dressed in the sense of being clothed in a continuous acoustic field. That dressed state is less sensitive to the low-frequency noise that normally limits coherence, and because the protection comes from a continuous mechanical field rather than pulsed microwaves, it's compatible with the same phononic cavity structures meant to eventually link network nodes. In this scheme, phonons do double duty: carrying quantum information and protecting it at the same time.
Using the technique, the researchers extended the silicon-vacancy spin's coherence time by roughly a factor of three, and separately demonstrated a Rabi frequency of 800 MHz - a measure of how fast the qubit's state can be controlled - enabling very fast quantum operations on top of the longer memory time.
"We are solving two problems," said Eliza Cornell, the paper's co-lead author and a recent Ph.D. graduate from the Lončar lab, now a postdoctoral researcher at Boston University. "We want the spin to have strong interaction with phonons, and we want the spin to have a long coherence time. Our paper demonstrates a method of extending the coherence time that is compatible with the silicon-vacancy center being in a cavity."