Tiny sound waves could help solve a major quantum computing problem

Harvard researchers used microscopic sound waves to protect quantum information, helping a qubit preserve its fragile state about three times longer.

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have demonstrated a new way to protect delicate quantum information using mechanical vibrations, essentially microscopic sound waves.

The advance, developed in the lab of Marko Lončar, Tiantsai Lin Professor of Electrical Engineering, could support the development of compact quantum networks built directly onto chips. It may also help enable hybrid quantum systems that combine several different kinds of quantum bits, or qubits.

The findings are published in Nature Physics. The experiments were led by Eliza Cornell, a recent Ph.D. graduate from the Lončar lab who is now a postdoctoral researcher at Boston University, and Zhujing Xu, a former postdoctoral scholar in Lončar's group.

Using Sound to Carry Quantum Information

One promising approach to quantum networking uses the spin of an electron, associated with impurity in diamond, to store quantum information. Tiny packets of mechanical vibration called phonons can then serve as carriers that move information between qubit nodes.

The Lončar lab has played a major role in exploring the potential of these systems. Among its advances is a structure known as a phononic cavity, which traps mechanical vibrations so they can interact more strongly with the electron spin inside a qubit.

Phonons may have important advantages over light, which is more commonly used to move quantum information across chip-scale networks. At the same frequency, phonons have much shorter wavelengths than light, making it possible to build considerably smaller components and pack them more tightly together.

Phonons also interact readily with both solid-state spins and electromagnetic fields. That versatility makes them especially attractive for hybrid quantum technologies that bring together different types of qubits in a single system.

The Challenge of Preserving Quantum Memory

Using phonons, however, creates a major difficulty: protecting quantum memory.

Qubits are extremely sensitive to disturbances from their surroundings. To remain useful, they must preserve their quantum state long enough to store and process information. This ability is known as coherence.

Researchers often protect quantum memories from environmental interference using microwave pulses that separate, or decouple, the memory from surrounding noise. Those techniques do not work particularly well for qubits placed inside phononic cavities.

That limitation has made it difficult to achieve both strong interaction with phonons and long-lasting quantum memory in the same device.

"Dressed" Qubits Protected by Sound

The SEAS team addressed the problem by demonstrating what they describe as "all-mechanical coherence protection" for a silicon-vacancy spin in diamond.

Instead of relying on conventional microwave pulses, the researchers continuously applied a mechanical driving field made from phonons. This changed the qubit into a different kind of quantum state known as a "dressed" qubit.

The term "dressed" refers to the qubit effectively "wearing" a continuous acoustic field. In this condition, the qubit becomes less vulnerable to low-frequency noise from its surroundings.

Because the protection comes from a continuous mechanical field that is compatible with phononic cavities, the technique could operate inside the same structures that may eventually connect stationary nodes in quantum networks.

That gives phonons a potentially powerful dual role. They could transport quantum information between different parts of a network while simultaneously helping protect that information from environmental 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. Our paper demonstrates a method of extending the coherence time that is compatible with the silicon-vacancy center being in a cavity."

Quantum Coherence Lasts About Three Times Longer

With the new method, the researchers increased the coherence time of the silicon-vacancy spin by roughly a factor of three.

The result demonstrates that continuous-wave mechanical noise suppression can extend quantum coherence in real devices, suggesting that microscopic sound waves could become an important tool for building more reliable and compact quantum systems.

"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.

The 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.