  The magnetic ground state of the ideal quantum kagome antiferromagnet (QKA)
has been a long-standing puzzle, mainly because perturbations to the
nearest-neighbor isotropic Heisenberg Hamiltonian can lead to various
fundamentally different ground states. Here we investigate a recently
synthesized QKA representative YCu$_3$(OH)$_6$Cl$_3$, where perturbations
commonly present in real materials, like lattice distortion and intersite ion
mixing, are absent. Nevertheless, this compound enters a long-range
magnetically ordered state below $T_N=15$ K. Our powder neutron diffraction
experiment reveals that its magnetic structure corresponds to a coplanar
$120^\circ$ state with negative vector spin chirality. The ordered magnetic
moments are suppressed to $0.42(2)\mu_B$, which is consistent with the
previously detected spin dynamics persisting to the lowest experimentally
accessible temperatures. This indicates either a coexistence of magnetic order
and disorder or the presence of strong quantum fluctuations in the ground state
of YCu$_3$(OH)$_6$Cl$_3$. The origin of the magnetic order is sought in terms
of Dzyaloshinskii-Moriya magnetic anisotropy and further-neighbor isotropic
exchange interactions.
