  Nanomechanical devices have attracted the interest of a growing
interdisciplinary research community, since they can be used as highly
sensitive transducers for various physical quantities. Exquisite control over
these systems facilitates experiments on the foundations of physics. Here, we
demonstrate that an optically trapped silicon nanorod, set into rotation at MHz
frequencies, can be locked to an external clock, transducing the properties of
the time standard to the rod's motion with the remarkable frequency stability
$f_{\rm r}/\Delta f_{\rm r}$ of $7.7 \times 10^{11}$. While the dynamics of
this periodically driven rotor generally can be chaotic, we derive and verify
that stable limit cycles exist over a surprisingly wide parameter range. This
robustness should enable, in principle, measurements of external torques with
sensitivities better than 0.25zNm, even at room temperature. We show that in a
dilute gas, real-time phase measurements on the locked nanorod transduce
pressure values with a sensitivity of 0.3%.
