The small mass and high coherence of nanomechanical resonators render them the ultimate mechanical probe, with applications ranging from protein mass spectrometry and magnetic resonance force microscopy, to quantum optomechanics. A notorious challenge in these experiments is thermomechanical noise related to dissipation through internal or external loss channels. Here, we introduce a novel approach to defining nanomechanical modes, which simultaneously provides strong spatial confinement, full isolation from the substrate, and dilution of the resonator material’s intrinsic dissipation by five orders of magnitude. It is based on a phononic bandgap structure that localises the mode, without imposing the boundary conditions of a rigid clamp. The reduced curvature in the highly tensioned silicon nitride resonator enables mechanical Q > 108 at 1 MHz, yielding the highest mechanical Qf-products (> 1014 Hz) yet reported at room temperature. The corresponding coherence times approach those of optically trapped dielectric particles. Extrapolation to 4.2 Kelvin predicts ~quanta/ms heating rates, similar to trapped ions.
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