  Superconductivity that spontaneously breaks time-reversal symmetry (TRS) has
been found, so far, only in a handful of 3D crystals with bulk inversion
symmetry. Here we report an observation of spontaneous TRS breaking in a 2D
superconducting system without inversion symmetry: the epitaxial bilayer films
of bismuth and nickel. The evidence comes from the onset of the polar Kerr
effect at the superconducting transition in the absence of an external magnetic
field, detected by the ultrasensitive loop-less fiber-optic Sagnac
interferometer. Because of strong spin-orbit interaction and lack of inversion
symmetry in a Bi/Ni bilayer, superconducting pairing cannot be classified as
singlet or triplet. We propose a theoretical model where magnetic fluctuations
in Ni induce superconducting pairing of the dxy = +- idx^2y^2 orbital symmetry
between the electrons in Bi. In this model the order parameter spontaneously
breaks the TRS and has a non-zero phase winding number around the Fermi
surface, thus making it a rare example of a 2D topological superconductor.
