  Electrons can form a two-dimensional electron gas at metal surfaces, where
lateral confinement leads to quantum-well states. Such states have been
observed for highly itinerant electrons, but it remains an open question
whether quantum-well states can be formed from strongly correlated electrons.
Here we study atomically flat terraces on surfaces of the heavy-fermion
superconductor URu$_2$Si$_2$ using millikelvin scanning tunneling spectroscopy.
We observe two-dimensional heavy fermions (2DHF) with an effective mass 17
times the free electron mass that form quantized states separated by a fraction
of a meV. Superconductivity at the surface is induced by the bulk into the
2DHF. Our results provide a new route to realize quantum well states in
correlated quantum materials.
