Quantum Sound: Generating and manipulating phonons at the quantum level

Research Project, 2016 – 2025

This application aims at developing new quantum circuits where sound, in the form of Surface Acoustic Waves (SAW), is studied at the quantum level. We use the unique properties of superconducting circuits, which can be engineered into superconducting qubits, or artificial atoms and let them interact with SAW. The interaction between SAW and the artificial atoms will allow us to create non-classical states of sound, manipulate phonons in flight, and study new regimes for artificial atoms.

During the last decade, it has become possible to study quantum optics phenomena in the microwave domain. This new research area called circuit-QED, offers strong atom-photon interactions, very low dissipation due to superconductivity, and efficient guiding of photons in transmission lines. Many artificial atoms, transmission lines, and other devices can be integrated on one chip. In a parallel development, it has also become possible to study mechanical degrees of freedom in suspended beams and membranes at the quantum level. These experiments have all addressed localized modes.

Here, we suggest to combine the circuit QED technologies with propagating surface acoustic waves. We focus on the quantum information encoded directly in the mechanical degree of freedom of the propagating SAW. The use of SAW extends the prospects of mechanical quantum processing to propagating phonons, which can potentially be used to in quantum information applications. Building on our experience of doing quantum optics on-chip, we now enter the area of quantum acoustics on-chip.

Three years ago we showed that SAW waves can be studied at the single phonon level (M.V. Gustafsson et al., Nature Physics, 2012). More recently we have also demonstrated how SAW phonons can interact with superconducting qubits. In a series of experiments we could both demonstrate non-linear reflection of SAW on an artificial atom, and we could "listen" to a decaying atom (M.V. Gustafsson et al., Science, 2014). We have also theoretically investigated the very special situation that the artificial atom can be substantially larger than the wavelength of the sound (A. Frisk-Kockum et al., Phys. Rev. A, 2014). These findings open a new door to investigate sound at the quantum level and to reach completely new regimes for the artificial atoms.

While SAW based systems share many of the features of circuit QED, they also have some additional unique properties:
i) The low propagation speed for sound allows for manipulation of the SAW phonons ”in-flight” on-chip.
ii) Ultrastrong coupling is possible by going to a strongly piezoelectric substrate.
iii) The artificial atoms can be much larger than the wavelength. This is a new regime that has not been achieved in any other system.
iv) The SAW-wavelength can be matched to the wavelength of optical light, and may allow coherent coupling between phonons and optical photons.
iv) The frequency dependent coupling of the SAW waves can be engineered with artificial atoms so that some transitions are enhanced whereas others are suppressed, resulting in population inversion.

The research plan has two parts. The first part is physics oriented - we aim to demonstrate new physical phenomena using the interaction between SAW and artificial atoms. The second part is supporting technological research where we develop new tools necessary and/or useful for performing the physics experiments.

In this project we are aiming for the following specific goals:
1. Generating nonclassical states of sound, e.g. a single phonon generator.
2. Catching a single phonon in flight.
3. Investigating atom phonon interaction in the long atom limit.
4. Demonstrating ultra-strong coupling between a qubit and SAW.