  The symmetry properties of the order parameter characterize different phases
of unconventional superconductors. In the case of the heavy-fermion
superconductor UPt$_3$, a key question is whether its multiple superconducting
phases preserve or break time-reversal symmetry (TRS). We tested for asymmetry
in the phase shift between left and right circularly polarized light reflected
from a single crystal of UPt$_3$ at normal incidence, finding that this
so-called polar Kerr effect appears only below the lower of the two zero-field
superconducting transition temperatures. Our results provide evidence for
broken TRS in the low-temperature superconducting phase of UPt$_3$, implying a
complex two-component order parameter for superconductivity in this system.
