Surface acoustic waves coupled to superconducting qubits

Short answer: Surface acoustic wave (SAW) resonators let a superconducting qubit exchange energy with sound at the single-phonon level. A 2017 Nature Communications paper reported a qubit coupled to a SAW cavity, a 2018 Nature paper reported full quantum control of a SAW resonator's mechanical state, and a 2019 Science paper reported emission and capture of itinerant phonons with 67% state-transfer efficiency between two qubits. These are laboratory experiments in small devices.

2017: a surface acoustic cavity

The Nature Communications abstract says experiments with mechanical resonators in quantum devices have opened new frontiers in macroscopic quantum mechanics. SAWs can be piezoelectrically coupled to superconducting qubits and confined in high-quality Fabry-Perot cavities in the quantum regime. The authors measure a device in which a superconducting qubit is coupled to a SAW cavity, realizing a surface acoustic version of cavity quantum electrodynamics. They measure the AC Stark shift between the two systems to determine the coupling strength, which agrees with a theoretical model.

They say this architecture could be used to store quantum information in trapped on-chip acoustic wavepackets and manipulate it in ways that are impossible with purely electromagnetic signals, because mechanical waves are about 10 to the fifth times slower. This is a proposed use.

2018: quantum states of a mechanical resonator

The Nature abstract says that only indirect demonstrations of non-classical states had been achieved in mechanical systems, in part because their response is highly linear and their frequencies are low. The authors demonstrate full quantum control of the mechanical state of a macroscale mechanical resonator. They strongly couple a SAW resonator to a superconducting qubit and use the qubit to control and measure the resonator's quantum states. They generate a non-classical superposition of the zero- and one-phonon Fock states and map this and other states using Wigner tomography. They say such programmable control is essential to applications of SAWs in the quantum limit, including coupling disparate quantum systems.

2019: sending a phonon between qubits

The Science abstract notes that phonons, especially SAW phonons, have been proposed as a way to couple distant solid-state quantum systems coherently. The authors report deterministic emission and capture of itinerant SAW phonons, which enables quantum entanglement of two superconducting qubits. Using a 2-millimeter acoustic channel, equivalent to a 500-nanosecond delay line, they demonstrate emission and recapture of a phonon by one qubit, quantum state transfer between two qubits with 67% efficiency, and, by partial transfer of a phonon, an entangled Bell pair with a fidelity of 84%.

What the papers have in common

Each paper reaches a stage past the previous one: a coupled cavity, then controlled phonon states, then a phonon traveling between qubits. The progression is the post author's reading of the three abstracts, and the abstracts do not describe it as a single program.

Limits

An 84% Bell fidelity and 67% transfer efficiency are well below the values reported for some other platforms. All three experiments are single-device studies at cryogenic temperatures, and none reports acoustic quantum memory outperforming electromagnetic alternatives.

FAQ

What is a surface acoustic wave?

A sound wave traveling along a chip surface, coupled here to a qubit through a piezoelectric material.

What state was made in 2018?

A superposition of zero and one phonon in the mechanical resonator.

How good was the 2019 entanglement?

A Bell pair with 84% fidelity, per the authors.

Why use sound?

The 2017 authors say mechanical waves are about 100,000 times slower, which allows new ways to store and handle information.

Is this a quantum computer?

No. These are two-qubit and single-qubit demonstrations.