  One of the defining properties of electrons is their mutual Coulombic
repulsion. In solids, however, this basic property may change. A famous example
is that of superconductors, where coupling to lattice vibrations make electrons
attract each other and leads to the formation of bound pairs. But what if all
degrees of freedom are electronic? Is it still possible to make electrons
attractive via their repulsion from other electrons? Such a mechanism, termed
'excitonic', was proposed fifty years ago by W. A. Little, aiming to achieve
stronger and more exotic superconductivity, yet despite many experimental
efforts, direct evidence for such 'excitonic' attraction is still lacking.
Here, we demonstrate this unique attraction by constructing, from the bottom
up, the fundamental building block of this mechanism. Our experiments are based
on quantum devices made from pristine carbon nanotubes, combined with cryogenic
precision manipulation. Using this platform we demonstrate that two electrons
can be made to attract using an independent electronic system as the binding
glue. Owing to its large tunability, our system offers crucial insights into
the underlying physics, such as the dependence of the emergent attraction on
the underlying repulsion and the origin of the pairing energy. We also
demonstrate transport signatures of 'excitonic' pairing. This experimental
demonstration of 'excitonic' pairing paves the way for the design of exotic
states of matter.
