  In a zero-dimensional superconductor, quantum size effects(QSE) not only set
the limit to superconductivity, but are also at the heart of new phenomena such
as shell effects, which have been predicted to result in large enhancements of
the superconducting energy gap. Here, we experimentally demonstrate these QSE
through measurements on single, isolated Pb and Sn nanoparticles. In both
systems superconductivity is ultimately quenched at sizes governed by the
dominance of the quantum fluctuations of the order parameter. However, before
the destruction of superconductivity, in Sn nanoparticles we observe giant
oscillations in the superconducting energy gap with particle size leading to
enhancements as large as 60%. These oscillations are the first experimental
proof of coherent shell effects in nanoscale superconductors. Contrarily, we
observe no such oscillations in the gap for Pb nanoparticles, which is ascribed
to the suppression of shell effects for shorter coherence lengths. Our study
paves the way to exploit QSE in boosting superconductivity in low-dimensional
systems.
