  New physics can emerge in magnetic materials where quantum fluctuations are
enhanced due to reduced dimensionality and strong frustration. One long sought
example is the resonating-valence-bond (RVB) state, where atomic magnetic
moments are strongly correlated but do not order or freeze even in the limit of
T -> 0. The RVB ground state does not break conventional symmetries, such as
lattice translation or spin-rotation. The realization of such a quantum spin
liquid in two-dimensions would represent a new state of matter. It is believed
that spin liquid physics plays a role in the phenomenon of high-Tc
superconductivity, and the topological properties of the spin liquid state may
have applications in the field of quantum information. We present neutron
scattering measurements of the spin excitations on single crystal samples of
the spin-1/2 kagom\'{e} lattice antiferromagnet ZnCu3(OD)6Cl2 (also called
herbertsmithite). Our observation of a spinon continuum in a two-dimensional
magnet is remarkable first. The results serve as a key fingerprint of the
quantum spin liquid state in herbertsmithite.
