  In fermionic systems, superconductivity and superfluidity are enabled through
the condensation of fermion pairs. The nature of this condensate can be tuned
by varying the pairing strength, with weak coupling yielding a BCS-like
condensate and strong coupling resulting in a BEC-like process. However,
demonstration of this cross-over has remained elusive in electronic systems.
Here we study graphene double-layers separated by an atomically thin insulator.
Under applied magnetic field, electrons and holes couple across the barrier to
form bound magneto-excitons whose pairing strength can be continuously tuned by
varying the effective layer separation. Using temperature-dependent Coulomb
drag and counter-flow current measurements, we demonstrate the capability to
tune the magneto-exciton condensate through the entire weak-coupling to
strong-coupling phase diagram. Our results establish magneto-exciton
condensates in graphene as a model platform to study the crossover between two
Bosonic quantum condensate phases in a solid state system.
