  The twin discoveries of the quantum Hall effect, in the 1980's, and of
topological band insulators, in the 2000's, were landmarks in physics that
enriched our view of the electronic properties of solids. In a nutshell, these
discoveries have taught us that quantum mechanical wavefunctions in crystalline
solids may carry nontrivial topological invariants which have ramifications for
the observable physics. One of the side effects of the recent topological
insulator revolution has been that such physics is much more widespread than
was appreciated ten years ago. For example, while topological insulators were
originally studied in the context of electron wavefunctions, recent work has
led to proposals of topological insulators in bosonic systems: in photonic
crystals, in the vibrational modes of crystals, and in the excitations of
ordered magnets. Here we confirm the recent proposal that, in a weak magnetic
field, the dimerized quantum magnet SrCu$_{2}$(BO$_{3}$)$_2$ is a bosonic
topological insulator with nonzero Chern number in the triplon bands and
topologically protected chiral edge excitations.
