Harvard Uses Sound Waves to Extend Qubit Coherence

Researchers at Harvard have used microscopic sound waves to shield a quantum bit, or qubit, from environmental noise. The method extended the qubit's coherence time by about three times. Paulson School of Engineering and Applied Sciences (SEAS).

Published in Nature Physics, the work was led by former Ph.D. Graduate Eliza Cornell, now at Boston University, and former postdoctoral scholar Zhujing Xu from the lab of Marko Lončar. Their approach centers on using mechanical vibrations, known as phonons, to both protect and potentially transmit quantum information.

Phonons as Quantum Carriers

One path to building quantum networks involves using the spin of an electron within a diamond impurity as a memory qubit. Phonons, which are packets of mechanical vibration, can act as carriers to move quantum information between these qubit nodes. The Lončar lab has previously developed phononic cavities that trap these vibrations to strengthen their interaction with a qubit's electron spin.

Phonons offer potential advantages over light for on-chip networks. At identical frequencies, phonons have much shorter wavelengths than photons of light. This property could allow for the construction of significantly smaller and more densely packed quantum components. Also, phonons interact readily with both solid-state spins and electromagnetic fields, making them suitable for hybrid systems that combine different qubit types.

The Coherence Protection Problem

A major obstacle in using phonons has been preserving the fragile quantum memory of the qubit itself. Qubits are highly sensitive to environmental disturbances, and their usefulness depends on maintaining a coherent quantum state long enough for computation. Conventional protection methods use microwave pulses to decouple the qubit from surrounding noise, but these techniques are not well-suited for qubits inside phononic cavities.

This incompatibility has made it challenging to achieve both strong phonon interaction and long coherence within a single device. The Harvard team sought a method that would work inside the structures intended for future quantum networks.

Creating a 'Dressed' Qubit with Sound

The SEAS researchers demonstrated what they call "all-mechanical coherence protection" for a silicon-vacancy spin qubit in diamond. Instead of microwave pulses, they applied a continuous mechanical driving field made of phonons. This transformed the qubit into a "dressed" state, where it is effectively surrounded by a persistent acoustic field.

In this dressed condition, the qubit becomes less susceptible to low-frequency environmental noise. Because the protective field is mechanical and continuous, it is fully compatible with operation inside a phononic cavity. This gives phonons a dual potential role: they could simultaneously transport quantum information between network nodes and shield that information from decoherence.

"We are solving two problems," said Eliza Cornell. "We want the spin to have strong interaction with phonons, and we want the spin to have a long coherence time."

Experimental Results and Future Path

The new technique increased the coherence time of the silicon-vacancy spin by roughly a factor of three. The result shows that continuous mechanical noise suppression can effectively extend quantum coherence in a real device. This suggests sound waves could become a key tool for building more reliable and compact quantum systems.

The research involved multiple collaborators including Zhaoyou Wang, Hana K. Warner, and Benjamin Pingault. It received U.S. Federal support from the National Science Foundation, the Air Force Office of Scientific Research, and the Q-NEXT quantum research center. The Harvard Office of Technology Development is pursuing patent protection and commercialization opportunities based on this work.