Short answer: Surface acoustic wave phonons are quantum vibrations that travel along a chip surface, and superconducting qubits can control and read them. A 2017 Nature Communications paper reports a qubit coupled to a surface acoustic wave cavity, a 2018 Nature paper reports full quantum control of a mechanical resonator including a superposition of zero and one phonon, and a 2019 Science paper reports phonon-mediated transfer between two qubits with 67% efficiency and a Bell pair at 84% fidelity. These are laboratory experiments.
Why use phonons
The 2019 abstract says phonons, in particular surface acoustic wave phonons, have been proposed as a means to coherently couple distant solid-state quantum systems. The 2017 abstract notes that quantum devices with mechanical resonators have opened new frontiers in the study of quantum mechanics at a macroscopic level, and that acoustic waves travel about 100,000 times slower than electromagnetic ones, which the authors say may allow manipulation impossible with purely electromagnetic signals.
2017: a qubit coupled to an acoustic cavity
The Nature Communications paper presents measurements of a device in which a superconducting qubit is coupled to a surface acoustic wave cavity, which the authors call a surface acoustic version of cavity quantum electrodynamics. They use measurements of the AC Stark shift between the two systems to determine the coupling strength, which they say agrees with a theoretical model. They suggest the architecture may be used to store quantum information in trapped on-chip acoustic wavepackets. That is a stated possibility.
2018: control of a mechanical state
The Nature paper says that only indirect demonstrations of non-classical states had been achieved in mechanical systems, partly because most mechanical systems respond linearly and have low frequencies that make the quantum ground state hard to reach. The authors strongly couple a surface acoustic-wave 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 with Wigner tomography. They describe such control as essential for applications of surface acoustic waves in the quantum limit, including coupling of disparate quantum systems.
2019: moving a phonon between qubits
The Science paper reports deterministic emission and capture of itinerant surface acoustic wave phonons, which enables entanglement of two superconducting qubits. The acoustic channel is 2 millimeters long, equivalent to a 500-nanosecond delay line. The authors 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, generation of an entangled Bell pair with a fidelity of 84%.
Limits
A transfer efficiency of 67% and a Bell fidelity of 84% are well below what large-scale computing would need, and the abstracts present these as demonstrations. The three experiments use different devices, so the figures are not comparable. The coupling of other quantum systems through phonons is stated as a goal.
FAQ
What is a surface acoustic wave phonon?
A quantum of vibration traveling along a chip surface.
Why are phonons attractive?
Acoustic waves are about 100,000 times slower than electromagnetic waves, per the 2017 abstract, which may allow new kinds of manipulation.
What did the 2018 experiment show?
A superposition of zero and one phonon in a macroscale resonator, mapped with Wigner tomography.
What did the 2019 experiment achieve?
State transfer between two qubits at 67% efficiency and a Bell pair at 84% fidelity.
Is this ready for quantum networks?
No. These are device-level demonstrations.
All field notes