Abstract
Quantum acoustodynamics (QAD) is a rapidly developing field of research, offering possibilities to realize and study macroscopic quantum-mechanical systems in a new range of frequencies, and implement transducers and new types of memories for hybrid quantum devices. Here we propose a novel design for a versatile diamond QAD cavity operating at GHz frequencies, exhibiting effective mode volumes of about $10^{-4}\lambda^3$. Our phononic crystal waveguide cavity implements a non-resonant analogue of the optical lightning-rod effect to localize the energy of an acoustic mode into a deeply-subwavelength volume. We demonstrate that this confinement can readily enhance the orbit-strain interaction with embedded nitrogen-vacancy (NV) centres towards the high-cooperativity regime, and enable efficient resonant cooling of the acoustic vibrations towards the ground state using a single NV. This architecture can be readily translated towards setup with multiple cavities in one- or two-dimensional phononic crystals, and the underlying non-resonant localization mechanism will pave the way to further enhance optoacoustic coupling in phoxonic crystal cavities.
Highlights
New developments in the field of gigahertz (GHz) quantum acoustics are closely mirroring those reported in integrated cavity and waveguide quantum electrodynamics
From the definition of the effective mode volume, we find that Veff,α can be reduced by locally enhancing the energy density h
We propose a simple design of an acoustic cavity capable of localizing GHz mechanical modes into ultrasmall volumes of about 10−4λ3p. Since these cavities are implemented as defects in quasi-one-dimensional phononic crystals, and the localization mechanism is nonresonant, the cavity frequencies can be readily tuned across the few-GHz range by changing geometric parameters
Summary
New developments in the field of gigahertz (GHz) quantum acoustics are closely mirroring those reported in integrated cavity and waveguide quantum electrodynamics. A natural step towards expanding this toolbox is to engineer a more efficient coupling between optically controllable emitters of single phonons—for example, negatively charged nitrogen-vacancy defects in diamond (NV−)—and acoustic resonators [7]. This strain coupling is predominantly determined by a cavity’s ability to spatially confine phonons beyond the diffraction-limited mode volume ∝ λ3, and to suppress the dissipation of phonons into radiative and nonradiative channels. A significant localization characterized with Veff ∼ 10−4λ3 can be achieved in diamond waveguides This leads to a significant enhancement of the coupling between phonon emitters in diamond (e.g., orbital states of NVs), and mechanical modes of the structure, opening pathways to implementing high-cooperativity NV-phonon coupling on the nanoscale. V, we discuss two mechanisms of coupling between the intrinsic strain fields of the acoustic modes and the orbital states of the NV centers, calculate the coupling strengths, resulting cooperativities, and efficiencies of the resonant and off-resonant cooling protocols [21,22,23,24]
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