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Register a new userPhononic bandgap nano-acoustic cavity with ultralong phonon lifetime
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We present measurements at millikelvin temperatures of the microwave-frequency acoustic properties of a crystalline silicon nanobeam cavity incorporating a phononic bandgap clamping structure for acoustic confinement. Utilizing pulsed laser light to excite a co-localized optical mode of the nanobeam cavity, we measure the dynamics of cavity acoustic modes with single-phonon sensitivity. Energy ringdown measurements for the fundamental $5$~GHz acoustic mode of the cavity shows an exponential increase in phonon lifetime versus number of periods in the phononic bandgap shield, increasing up to $tau approx 1.5$~seconds. This ultralong lifetime, corresponding to an effective phonon propagation length of several kilometers, is found to be consistent with damping from non-resonant two-level system defects on the surface of the silicon device. Potential applications of these ultra-coherent nanoscale mechanical resonators range from tests of various collapse models of quantum mechanics to miniature quantum memory elements in hybrid superconducting quantum circuits.
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Longitudinal relaxation is the process by which an excited spin ensemble decays into its thermal equilibrium with the environment. In solid-state spin systems relaxation into the phonon bath usually dominates over the coupling to the electromagnetic vacuum. In the quantum limit the spin lifetime is determined by phononic vacuum fluctuations. However, this limit was not observed in previous studies due to thermal phonon contributions or phonon-bottleneck processes. Here we use a dispersive detection scheme based on cavity quantum electrodynamics (cQED) to observe this quantum limit of spin relaxation of the negatively charged nitrogen vacancy ($mathrm{NV}^-$) centre in diamond. Diamond possesses high thermal conductivity even at low temperatures, which eliminates phonon-bottleneck processes. We observe exceptionally long longitudinal relaxation times $T_1$ of up to 8h. To understand the fundamental mechanism of spin-phonon coupling in this system we develop a theoretical model and calculate the relaxation time ab initio. The calculations confirm that the low phononic density of states at the $mathrm{NV}^-$ transition frequency enables the spin polarization to survive over macroscopic timescales.
A time-resolved observation of coherent interlayer longitudinal acoustic phonons in 2$H$-MoSe$_2$ is reported. A femtosecond pump-probe technique is used to investigate the evolution of the energy loss of these vibrational modes in a wide selection of MoSe$_2$ flakes with different thicknesses ranging from bilayer up to the bulk limit. By directly analysing the temporal decay of the modes, we can clearly distinguish an abrupt crossover related to the acoustic mean free path of the phonons in a layered system, and the constraints imposed to the acoustic decay channels when reducing the dimensionality. Loses intrinsic to the low dimensionality of single or few layer materials impose critical limitations for their use in optomechanical and optoelectronic devices.
Magnons, namely spin waves, are collective spin excitations in ferromagnets, and their control through coupling with other excitations is a key technology for future hybrid spintronic devices. Although strong coupling has been demonstrated with microwave photonic structures, an alternative approach permitting high density integration and minimized electromagnetic crosstalk is required. Here we report a planar cavity magnomechanical system, where the cavity of surface acoustic waves enhances the spatial and spectral power density to thus implement magnon-phonon coupling at room temperature. Excitation of spin-wave resonance involves significant acoustic power absorption, whereas the collective spin motion reversely exerts a back-action force on the cavity dynamics. The cavity frequency and quality-factor are significantly modified by the back-action effect, and the resultant cooperativity exceeds unity, suggesting coherent interaction between magnons and phonons. The demonstration of a chip-scale magnomechanical system paves the way to the development of novel spin-acoustic technologies for classical and quantum applications.
A phononic crystal can control the acoustic coupling between a resonator and its support structure. We micromachine a phononic bandgap shield for high Q silicon nitride membranes and study the driven displacement spectra of the membranes and their support structures. We find that inside observed bandgaps the density and amplitude of non-membrane modes are greatly suppressed, and membrane modes are shielded from an external mechanical drive by up to 30 dB.
We report on simulations of the degree of polarization entanglement of photon pairs simultaneously emitted from a quantum dot-cavity system that demand revisiting the role of phonons. Since coherence is a fundamental precondition for entanglement and phonons are known to be a major source of decoherence, it seems unavoidable that phonons can only degrade entanglement. In contrast, we demonstrate that phonons can cause a degree of entanglement that even surpasses the corresponding value for the phonon-free case. In particular, we consider the situation of comparatively small biexciton binding energies and either finite exciton or cavity mode splitting. In both cases, combinations of the splitting and the dot-cavity coupling strength are found where the entanglement exhibits a nonmonotonic temperature dependence which enables entanglement above the phonon-free level in a finite parameter range. This unusual behavior can be explained by phonon-induced renormalizations of the dot-cavity coupling $g$ in combination with a nonmonotonic dependence of the entanglement on $g$ that is present already without phonons.
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