Bulk acoustic resonators coupled to superconducting qubits

Short answer: A high-overtone bulk acoustic resonator, or HBAR, is a slab of crystal whose sound modes can be coupled to a superconducting qubit through the piezoelectric effect. A 2018 Physical Review B paper reported coupling bulk acoustic phonons to a transmon at the quantum level, a 2023 Applied Physics Letters paper reported state transfer between two qubits through their HBAR modes, and a 2025 Nature Communications paper reported generating a large phonon population from a single qubit. These are laboratory demonstrations in single devices.

2018: phonons meet a transmon

The Physical Review B abstract says mechanical resonators are a promising way to interface qubits in hybrid quantum systems, because mechanical systems can have very long energy lifetimes and can be interfaced to other systems. HBARs combine high frequency with high quality factors, can reach their quantum ground state at dilution refrigerator temperatures, and can be strongly coupled to superconducting qubits through their piezoelectric effect.

The authors demonstrate a system in which bulk acoustic phonons of a high-overtone resonator are coupled to a transmon qubit in a planar circuit architecture. They show that the phonons interact with the qubit at the quantum level in a simple design, which they present as progress toward quantum control of mechanical motion.

2023: two qubits, two resonators

The Applied Physics Letters article (Crump, Välimaa and Sillanpää) presents a device with two coupled transmon qubits, each coupled to an independent HBAR. Both resonators support many acoustic modes, which can couple near-resonantly to the qubit. The authors first show qubit-qubit interaction in the multimode system and then demonstrate quantum state transfer, where an excitation is swapped from an HBAR mode of one qubit to an HBAR mode of the other. The text read here is the opening abstract and not the full paper, so no fidelity is quoted.

2025: a large phonon state

The Nature Communications abstract says quantum acoustics has enabled cooling of mechanical objects to the quantum ground state, and the generation of Fock states and Schrödinger cat states. It adds that generating large-amplitude phonon states had been elusive. The authors use a single superconducting qubit coupled to an HBAR to generate a large phonon population in an acoustic mode. They observe extended ringdowns of the qubit, which confirm the generation of a large-amplitude phonon state, and an upper threshold behavior, a consequence of phonon quenching predicted by their model. They call this a key tool for generating arbitrary phonon states in circuit quantum acoustodynamics, an expected use and not a demonstration.

What the abstracts do not say

None of the three abstracts reports using acoustic modes as a quantum memory with a stated lifetime, and none compares acoustic storage with electromagnetic storage. The 2018 abstract makes the general point that mechanical systems can have very long energy lifetimes, but it does not give a number.

Limits

Each paper uses a different device. The 2023 result is a state swap between modes, and the 2025 result is a classical-like large phonon population measured through the qubit ringdown. Neither is an error-corrected memory.

FAQ

What is an HBAR?

A high-overtone bulk acoustic resonator, a crystal supporting many acoustic modes at high frequency and quality factor.

How does it couple to a qubit?

Through the piezoelectric effect, per the 2018 abstract.

What was transferred in 2023?

An excitation between HBAR modes of two different qubits.

What is phonon quenching?

An upper threshold behavior the 2025 authors observe and predict with their model.

Is this a working memory?

Not in these abstracts.