  We present nanofabrication and mechanical measurements of single-crystal
diamond cantilevers with thickness down to 85 nm, thickness uniformity better
than 20 nm, and lateral dimensions up to 240 um. Quality factors exceeding one
million are found at room temperature, surpassing those of state-of-the-art
single-crystal silicon cantilevers of similar dimensions by roughly an order of
magnitude. Force sensitivities of a few hundred zeptonewtons result for the
best cantilevers at millikelvin temperatures. Single-crystal diamond could thus
directly improve existing force and mass sensors by a simple substitution of
resonator material, and lead to quantum nanomechanical devices with
exceptionally low energy dissipation.
