  Superconductivity and quantum Hall effect are distinct states of matter
occurring in apparently incompatible physical conditions. Recent theoretical
developments suggest that the coupling of quantum Hall effect with a
superconductor can provide a fertile ground for realizing exotic topological
excitations such as non-abelian Majorana fermions or Fibonacci particles. As a
step toward that goal, we report observation of Andreev reflection at the
junction of a quantum Hall edge state in a single layer graphene and a
quasi-two dimensional niobium diselenide (NbSe2) superconductor. Our principal
finding is the observation of an anomalous finite-temperature conductance peak
located precisely at the Dirac point, providing a definitive evidence for
inter-Landau level Andreev reflection in a quantum Hall system. Our
observations are well supported by detailed numerical simulations, which offer
additional insight into the role of the edge states in Andreev physics. This
study paves the way for investigating analogous Andreev reflection in a
fractional quantum Hall system coupled to a superconductor to realize exotic
quasiparticles.
