  Using a neutron double-slit setup, we construct a quantum bit commitment
scheme in which time development of quantum states plays an essential role. Our
scheme evades the widely accepted no-go theorem by the fact that it is neither
possible to stop the time evolution of spreading wave packets, nor is it
possible to evolve them backward in time. Moreover, for unstable particles such
as neutrons, one cannot delay detecting their positions indefinitely, or they
would disintegrate spontaneously and escape detection. We find that using
non-stationary states instead of stationary ones, unconditionally secure
quantum bit commitment is possible.
