  Electronic ferroelectricity represents a new paradigm where spontaneous
symmetry breaking driven by electronic correlations, in contrast to traditional
lattice-driven ferroelectricity, leads to the formation of electric dipoles.
Despite the potential application advantages arising from its electronic
nature, switchable electronic ferroelectricity remains exceedingly rare. Here,
we report the discovery of an electronic ratchet effect that manifests itself
as switchable electronic ferroelectricity in a layer-contrasting graphene-boron
nitride moir\'e heterostructure. Our engineered layer-asymmetric moir\'e
potential landscapes result in layer-polarized localized and itinerant
electronic subsystems. At particular fillings of the localized subsystem, we
find a ratcheting injection of itinerant carriers in a non-volatile manner,
leading to a highly unusual ferroelectric response. Strikingly, the remnant
polarization can be stabilized at multiple (quasi-continuous) states with
behavior markedly distinct from known ferroelectrics. Our experimental
observations, simulations, and theoretical analysis suggest that dipolar
excitons are the driving force and elementary ferroelectric units in our
system. This signifies a new type of electronic ferroelectricity where the
formation of dipolar excitons with aligned moments generates a macroscopic
polarization and leads to an electronically-driven ferroelectric response,
which we term excitonic ferroelectricity. Such new ferroelectrics, driven by
quantum objects like dipolar excitons, could pave the way to innovative quantum
analog memory and synaptic devices.
