Paulson School of Engineering and Applied Sciences have demonstrated a method to protect fragile quantum information using microscopic sound waves, according to findings published in Nature Physics. Developed in the lab of Tiantsai Lin Professor of Electrical Engineering Marko Lončar, the advance could facilitate compact quantum networks built directly onto chips and help enable hybrid quantum systems that combine different kinds of quantum bits, or qubits.
Using Sound to Carry Quantum Information on Chips
One promising approach to quantum networking uses the spin of an electron associated with an impurity in diamond to store quantum information. According to researchers, 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 these systems, previously developing structures known as phononic cavities that trap mechanical vibrations so they can interact more strongly with the electron spin inside a qubit.
Pro Tip: Phonons have much shorter wavelengths than light at the same frequency, allowing engineers to build considerably smaller components and pack them more tightly together on chip-scale networks.
Phonons interact readily with both solid-state spins and electromagnetic fields. That versatility makes them attractive for hybrid quantum technologies that bring together different types of qubits in a single system. However, using phonons creates a major difficulty in protecting quantum memory from environmental interference.
Protecting ‘Dressed’ Qubits with Continuous Mechanical Fields
Qubits are extremely sensitive to surroundings and must preserve their quantum state long enough to store and process information—an ability known as coherence. Researchers often protect quantum memories using microwave pulses that decouple the memory from surrounding noise, but those techniques do not work particularly well for qubits placed inside phononic cavities.
To address this, the SEAS team demonstrated “all-mechanical coherence protection” for a silicon-vacancy spin in diamond. Instead of relying on conventional microwave pulses, researchers continuously applied a mechanical driving field made from phonons. This changed the qubit into a “dressed” qubit, meaning it effectively wears a continuous acoustic field that makes it less vulnerable to low-frequency noise.
Did You Know? Eliza Cornell, a recent Ph.D. graduate from the Lončar lab and now a postdoctoral researcher at Boston University, led the experiments alongside Zhujing Xu, a former postdoctoral scholar in Lončar’s group.
“We are solving two problems,” Cornell said regarding the dual role of phonons in transporting information and providing protection. “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.”
Tripling Coherence Time for Reliable Quantum Systems
With the new method, researchers increased the coherence time of the silicon-vacancy spin by roughly a factor of three. This result demonstrates that continuous-wave mechanical noise suppression can extend quantum coherence in real devices.
Warner, Eliana Mann, Michael Haas, Smarak Maity, Graham Joe, Liang Jiang, Peter Rabl, and Benjamin Pingault. Department of Energy Office of Science National Quantum Information Science Research Center. Work was performed in part at the Harvard Center for Nanoscale Systems, and the Harvard Office of Technology Development is pursuing patent protection and commercialization opportunities.
Frequently Asked Questions
What are phonons in quantum networking?
Phonons are tiny packets of mechanical vibration, essentially microscopic sound waves, used as carriers to move information between qubit nodes.
Why use sound waves instead of light?
At the same frequency, phonons have much shorter wavelengths than light, enabling the construction of considerably smaller components packed more tightly together on chips.
What is a “dressed” qubit?
A dressed qubit is a quantum state created by continuously applying a mechanical driving field of phonons, making the qubit less vulnerable to low-frequency environmental noise.
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