Qubits ‘Dressed’ For Success

Researchers in the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have demonstrated a promising new way to protect fragile quantum information using nothing but mechanical vibrations — essentially extremely small sound waves.

The breakthrough, which comes from the lab of Marko Lončar, Tiantsai Lin Professor of Electrical Engineering, paves a path toward compact, sound-based quantum networks on chips, as well as hybrid quantum systems that combine many different types of quantum bits, or qubits.

The research is published in Nature Physics. Experiments were led by Eliza Cornell, a recent Ph.D. graduate from the Lončar lab and current postdoctoral researcher at Boston University, and Zhujing Xu, a former postdoctoral scholar in Lončar’s group.

Sound waves as information carriers

One emerging type of quantum network uses the spin of an electron, associated with impurity in diamond, as quantum memory; and sound waves — or more precisely, sound particles, called phonons — as information carriers between qubit nodes. The Lončar lab has been a leader in demonstrating the potential of this kind of system, in part by developing a qubit housing called a phononic cavity that traps the vibrations to make them interact with the electron spin.

Phonons offer several advantages over more traditional approaches to quantum networking that use light as information carriers at the chip scale. First, phonon wavelengths at a given frequency are much shorter than light wavelengths, enabling devices with far smaller footprints and tighter integration. Second, phonons couple easily both to solid‑state spins and to electromagnetic fields, making them attractive components in hybrid quantum systems that employ more than one type of qubit.

But working with phonons has unique challenges — mainly related to memory.

Quantum memories need to be protected from their environment in order to extend their   coherence, or their ability to retain memory for a sufficiently long time. But existing approaches that rely on microwave pulses to de-couple memories from their environment do not work well on qubits housed in phononic cavities.

‘Dressed’ qubits and all-mechanical coherence protection

SEAS researchers solved this bottleneck by demonstrating a unique “all-mechanical coherence protection” of a silicon-vacancy spin in diamond. Rather than traditional microwave pulses, the team applied a continuous mechanical driving field made of phonons to change the qubit into a different state, called a “dressed” qubit.

These states, called “dressed” because they are “wearing” a continuous acoustic field, are less sensitive to the low‑frequency noise in the environment. Because spin coherence is protected here by a continuous mechanical field that is compatible with phononic cavities, the approach is designed to work within the same structures that would eventually be used to connect stationary nodes in a quantum network. Phonons would carry out two jobs in this type of network: transmitting quantum information and also protecting it.

“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. Our paper demonstrates a method of extending the coherence time that is compatible with the silicon-vacancy center being in a cavity.”

Using their new approach, the team extends the silicon-vacancy spin coherence time by roughly a factor of three, establishing that continuous‑wave, mechanical noise suppression can, in real devices, lengthen quantum coherence.

“All-mechanical coherence protection and fast control of a spin qubit” was co-authored by Zhaoyou Wang, Hana K. Warner, Eliana Mann, Michael Haas, Smarak Maity, Graham Joe, Liang Jiang, Peter Rabl, and Benjamin Pingault.

This research received U.S. federal support from: the National Science Foundation under grant number EEC-1941583; the Air Force Office of Scientific Research under award numbers FA9550-23-1-0333 and FA9550-23-1-0338; and Q-NEXT, a U.S. Department of Energy Office of Science National Quantum Information Science Research Centers under award No. DE-FOA-0002253. The work was performed in part at the Harvard Center for Nanoscale Systems, a member of the National Nanotechnology Infrastructure Network, which is supported by National Science Foundation award No. ECS-0335765.

The Harvard Office of Technology Development is actively pursuing patent protection and commercialization opportunities for the innovations arising from this research.