Abstract
We resolve phonon number states in the spectrum of a superconducting qubit coupled to a multimode acoustic cavity. Crucial to this resolution is the sharp frequency dependence in the qubit-phonon interaction engineered by coupling the qubit to surface acoustic waves in two locations separated by acoustic wavelengths. In analogy to double-slit diffraction, the resulting interference generates high-contrast frequency structure in the qubit-phonon interaction. We observe this frequency structure both in the coupling rate to multiple cavity modes and in the qubit spontaneous emission rate into unconfined modes. We use this sharp frequency structure to resolve single phonons by tuning the qubit to a frequency of destructive interference where all acoustic interactions are dispersive. By exciting several detuned yet strongly coupled phononic modes and measuring the resulting qubit spectrum, we observe that, for two modes, the device enters the strong dispersive regime where single phonons are spectrally resolved.
1 More- Received 18 February 2019
DOI:https://doi.org/10.1103/PhysRevX.9.021056
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Synopsis
Counting Phonons One by One
Published 20 June 2019
A device enables the detection of single quanta of sound, a step towards using them in quantum technologies.
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Popular Summary
The study of interactions between light and atoms has generated numerous fundamental insights and innovative quantum technologies. Inspired by these accomplishments, researchers have engineered artificial atoms to interact strongly with mechanical excitations. Furthermore, leveraging the unique properties of mechanical systems would enable the realization of physical regimes with technological applications that are inaccessible to light-based platforms. We investigate such a regime by exploiting the slowness of sound to strongly couple a compact mechanical resonator to an artificial atom.
We observe the quantized mechanical excitations of a surface acoustic wave resonator through its strong coupling to a superconducting qubit. Crucial to harnessing the multimode nature of the cavity is coupling the qubit to the phonons at two locations separated by many acoustic wavelengths. In close analogy to double-slit diffraction, this separation leads to self-interference effects that generate sharp fringes in the frequency dependence of the qubit-cavity coupling strengths.
We leverage this frequency dependence to realize a strong coupling to a multimode cavity without sacrificing the ability to control the qubit. By populating several of the acoustic resonances, we resolve multiple peaks in the qubit excitation spectrum that correspond to an integer number of quanta in the acoustic cavity.
Resolving these excitation peaks represents a significant milestone in universal quantum control over acoustic modes, leading to nondestructive quantum measurement of phonons and the creation of multimode phononic entangled states. Moreover, our strategy can take full advantage of the high mode density of acoustic systems to access a giant phase space with a compact device, a prized resource in quantum information processing.