  Recent experimental and theoretical interest in the superconducting phase of
the heavy fermion material URu$_2$Si$_2$ has led to a number of proposals in
which the superconducting order parameter breaks time-reversal symmetry (TRS).
In this study we measured polar Kerr effect (PKE) as a function of temperature
for several high-quality single crystals of URu$_2$Si$_2$. We find an onset of
PKE below the superconducting transition that is consistent with a TRS-breaking
order parameter. This effect appears to be independent of an additional,
possibly extrinsic, PKE generated above the hidden order transition at
$T_{HO}=17.5$ K, and contains structure below $T_c$ suggestive of additional
physics within the superconducting state.
