Eliza Cornell, a recent Ph.D. graduate from the Harvard John A. Paulson School of Engineering and Applied Sciences, and Zhujing Xu led experiments demonstrating a new method of extending quantum memory using mechanical vibrations. The researchers successfully protected qubits by “dressing” them with continuous sound waves, or phonons, achieving a roughly threefold increase in silicon-vacancy spin coherence time. “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.” This all-mechanical approach allows for smaller, integrated quantum networks on chips by using phonon wavelengths, which are significantly shorter than those of light, a benefit over more traditional approaches to quantum networking that use light.

“Dressed” Qubit States Extend Silicon-Vacancy Coherence Times

Extending silicon-vacancy spin coherence time by roughly a factor of three, researchers at the Harvard John A. This advancement relies on “dressing” qubits with continuous sound waves, altering their state to become less susceptible to environmental noise and enabling more stable quantum information storage. The team’s approach uniquely avoids reliance on microwave pulses, which prove ineffective when working with qubits housed within phononic cavities designed to trap vibrations for interaction with electron spin. The successful implementation hinges on the unique properties of phonons, sound particles, as both information carriers and protectors within a quantum network. Phonons offer several advantages over more traditional approaches to quantum networking that use light as information carriers at the chip scale, offering a pathway toward more compact and scalable quantum technologies. Phonons readily interact with both solid-state spins and electromagnetic fields, making them versatile components in hybrid quantum systems employing diverse qubit types. Eliza Cornell, a recent Ph.D. graduate from the Lončar lab and current postdoctoral researcher at Boston University, explained that these dressed states exhibit reduced sensitivity to low-frequency noise, a common source of decoherence in quantum systems, and maintain the qubit’s ability to interact strongly with the phonons responsible for information transfer. The team’s work builds on previous efforts to utilize the spin of an electron associated with impurities in diamond as quantum memory, using phononic cavities to enhance interaction between the spin and the phonons. This system offers advantages over other qubit platforms due to the ease with which phonons couple to both solid-state spins and electromagnetic fields. However, extending the coherence time, the duration for which quantum information can be reliably stored, remained a significant hurdle. By employing continuous-wave mechanical noise suppression, the researchers demonstrated a viable path toward overcoming this limitation in real-world devices. The implications of this research extend beyond simply increasing coherence time; it also streamlines the architecture of potential quantum networks. Phonons, in this design, perform dual roles, transmitting quantum information between qubit nodes and simultaneously protecting that information from environmental disturbances. This integrated functionality simplifies network construction and reduces the need for separate control and protection mechanisms. “All-mechanical coherence protection and fast control of a spin qubit,” co-authored by Zhaoyou Wang, Hana K. Warner, Eliana Mann, Michael Haas, Smarak Maity, Graham Joe, Liang Jiang, Peter Rabl, and Benjamin Pingault, details the experimental setup and results. Department of Energy Office of Science National Quantum Information Science Research Centers under award No. DE-FOA-0002253. Work was also conducted at the Harvard Center for Nanoscale Systems, which receives support from National Science Foundation award No. The Harvard Office of Technology Development is actively exploring patent opportunities and commercialization pathways for the innovations stemming from this research, signaling confidence in its potential for real-world application and further development of sound-based quantum technologies.