  The quantum behaviour of electrons in materials lays the foundation for
modern electronic and information technology. Quantum materials with novel
electronic and optical properties have been proposed as the next frontier, but
much remains to be discovered to actualize the promise. Here we report the
first observation of topological quantum properties of chiral crystals in the
RhSi family. We demonsrate that this material hosts novel phase of matter
exhibiting nearly ideal topological surface properties that emerge as a
consequence of the crystals' structural chirality or handedness. We also
demonstrate that the electrons on the surface of this crystal show a highly
unusual helicoid structure that spirals around two high-symmetry momenta
signalling its topological electronic chirality. Such helicoid Fermi arcs on
the surface experimentally characterize the topological charges of $\pm{2}$,
which arise from the bulk chiral fermions. The existence of bulk high-fold
degenerate fermions are guaranteed by the crystal symmetries, however, in order
to determine the topological charge in the chiral crystals it is essential to
identify and study the helical arc states. Remarkably, these topological
conductors we discovered exhibit helical Fermi arcs which are of length $\pi$,
stretching across the entire Brillouin zone and orders of magnitude larger than
those found in all known Weyl semimetals. Our results demonstrate novel
electronic topological state of matter on a structurally chiral crystal
featuring helicoid Fermi arc surface states. The exotic electronic chiral
fermion state realised in these materials can be used to detect a quantised
photogalvanic optical response or the chiral magnetic effect and its optical
version in future devices as described by G. Chang \textit{et.al.,}
`Topological quantum properties of chiral crystals' Nature Mat. 17, 978-985
(2018).
