  Bound states at interfaces between superconductors and other materials are a
powerful tool to characterize the nature of the involved systems, and to
engineer elusive quantum excitations. In-gap excitations of conventional s-wave
superconductors occur, for instance, at magnetic impurities with net magnetic
moment breaking time-reversal symmetry. Here we show that interfaces between a
superconductor and a quantum antiferromagnet can host robust in-gap
excitations, without breaking time-reversal symmetry. We illustrate this
phenomenon in a one-dimensional model system with an interface between a
conventional s-wave superconductor and a one-dimensional Mott insulator
described by a standard Hubbard model. This genuine many-body problem is solved
exactly by employing a combination of kernel polynomial and tensor network
techniques. We unveil the nature of such zero modes by showing that they can be
adiabatically connected to solitonic solutions between a superconductor and a
classical antiferromagnet. Our results put forward a new class of in-gap
excitations between superconductors and a disordered quantum spin phase can be
relevant for a wider range of heterostructures.
