  The search for a material platform for topological quantum computation has
recently focused on unconventional superconductors. Such material systems,
where the superconducting order parameter breaks a symmetry of the crystal
point group, are capable of hosting novel phenomena, including emergent
Majorana quasiparticles. Unique among unconventional superconductors is the
recently discovered UTe2, where spin-triplet superconductivity emerges from a
paramagnetic normal state. Although UTe2 could be considered a relative of a
family of known ferromagnetic superconductors, the unique crystal structure of
this material and experimentally suggested zero Curie temperature pose a great
challenge to determining the symmetries, magnetism, and topology underlying the
superconducting state. These emergent properties will determine the utility of
UTe2 for future spintronics and quantum information applications. Here, we
report observations of a non-zero polar Kerr effect and of two transitions in
the specific heat upon entering the superconducting state, which together show
that the superconductivity in UTe2 is characterized by an order parameter with
two components that breaks time reversal symmetry. These data allow us to place
firm constraints on the symmetries of the order parameter, which strongly
suggest that UTe2 is a Weyl superconductor that hosts chiral Fermi arc surface
states.
