  In strained mechanical resonators, the concurrence of tensile stress and
geometric nonlinearity dramatically reduces dissipation. This phenomenon,
dissipation dilution, is employed in mirror suspensions of gravitational wave
interferometers and at the nanoscale, where soft-clamping and strain
engineering have allowed extremely high quality factors. However, these
techniques have so far only been applied in amorphous materials, specifically
silicon nitride. Crystalline materials exhibit significantly lower intrinsic
damping at cryogenic temperatures, due to the absence of two level systems in
the bulk, as exploited in Weber bars and silicon optomechanical cavities.
Applying dissipation dilution engineering to strained crystalline materials
could therefore enable extremely low loss nanomechanical resonators, due to the
combination of reduced internal friction, high intrinsic strain, and high yield
strength. Pioneering work has not yet fully exploited this potential. Here, we
demonstrate that single crystal strained silicon, a material developed for high
mobility transistors, can be used to realize mechanical resonators with
ultralow dissipation. We observe that high aspect ratio ($>10^5$) strained
silicon nanostrings support MHz mechanical modes with quality factors exceeding
$10^{10}$ at 7 K, a tenfold improvement over values reported in silicon
nitride. At 7 K, the thermal noise-limited force sensitivity is approximately
$45\ \mathrm{{zN}/{\sqrt{Hz}}}$ - approaching that of carbon nanotubes - and
the heating rate is only 60 quanta-per-second. Our nanomechanical resonators
exhibit lower dissipation than the most pristine macroscopic oscillators and
their low mass makes them particularly promising for quantum sensing and
transduction.
