  Magnetic adatoms on superconductors give rise to Yu-Shiba-Rusinov (YSR)
states that hold considerable interest for the design of topological
superconductivity. Here, we show that YSR states are also an ideal platform to
engineer structures with intricate wave-function symmetries. We assemble
structures of iron atoms on the quasi-two-dimensional superconductor
$2H$-NbSe$_2$. The Yu-Shiba-Rusinov wave functions of individual atoms extend
over several nanometers enabling hybridization even at large adatom spacing. We
show that the substrate can be exploited to deliberately break symmetries of
the adatom structure in ways unachievable in the gas phase. We highlight this
potential by designing chiral wave functions of triangular adatom structures
confined within a plane. Our results significantly expand the range of
interesting quantum states that can be engineered using arrays of magnetic
adatoms on superconductors.
