  In condensed-matter physics, electronic Mott insulators have triggered
considerable research due to their intricate relation with high-temperature
superconductors. However, unlike atomic systems for which Mott phases were
recently shown for both bosonic and fermionic species, the fingerprint of a
Mott phase implemented with bosons is yet to be found in the solid state. Here
we unveil such signature by emulating the Bose-Hubbard hamiltonian with
semiconductor excitons confined in a two-dimensional lattice. The exciton fluid
is characterised by strong repulsive dipolar interactions. These mostly
constrain the system to behave classically, but nevertheless allow for a very
narrow parameter space where two Mott phases are formed, with either one or two
excitons per lattice site. Our ability to program the lattice geometry explored
by excitons then paves the way towards quantum simulations of long-sought
many-body phases such as supersolids.
