  Many present and future applications of superconductivity would benefit from
electrostatic control of carrier density and tunneling rates, the hallmark of
semiconductor devices. One particularly exciting application is the realization
of topological superconductivity as a basis for quantum information processing.
Proposals in this direction based on proximity effect in semiconductor
nanowires are appealing because the key ingredients are currently in hand.
However, previous instances of proximitized semiconductors show significant
tunneling conductance below the superconducting gap, suggesting a continuum of
subgap states---a situation that nullifies topological protection. Here, we
report a hard superconducting gap induced by proximity effect in a
semiconductor, using epitaxial Al-InAs superconductor-semiconductor nanowires.
The hard gap, along with favorable material properties and gate-tunability,
makes this new hybrid system attractive for a number of applications, as well
as fundamental studies of mesoscopic superconductivity.
