The small mass and high coherence of nanomechanical resonators render themthe ultimate force probe, with applications ranging from biosensing andmagnetic resonance force microscopy, to quantum optomechanics. A notoriouschallenge in these experiments is thermomechanical noise related to dissipationthrough internal or external loss channels. Here, we introduce a novel approachto defining nanomechanical modes, which simultaneously provides strong spatialconfinement, full isolation from the substrate, and dilution of the resonatormaterial's intrinsic dissipation by five orders of magnitude. It is based on aphononic bandgap structure that localises the mode, without imposing theboundary conditions of a rigid clamp. The reduced curvature in the highlytensioned silicon nitride resonator enables mechanical $Q>10^{8}$ at $ 1\,\mathrm{MHz}$, yielding the highest mechanical $Qf$-products($>10^{14}\,\mathrm{Hz}$) yet reported at room temperature. The correspondingcoherence times approach those of optically trapped dielectric particles.Extrapolation to $4{.}2$ Kelvin predicts $\sim$quanta/ms heating rates, similarto trapped ions.
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