  Quantum bit commitment has long been known to be impossible. Nevertheless,
just as in the classical case, imposing certain constraints on the power of the
parties may enable the construction of asymptotically secure protocols. Here,
we introduce a quantum bit commitment protocol and prove that it is
asymptotically secure if cheating is restricted to Gaussian operations. This
protocol exploits continuous-variable quantum optical carriers, for which such
a Gaussian constraint is experimentally relevant as the high optical
nonlinearity needed to effect deterministic non-Gaussian cheating is
inaccessible.
