  We provide a physically motivated definition for the binding energy (or
bond-dissociation) of a bipartite quantum system. We consider coherently
applying an external field to cancel out the interaction between the
subsystems, to break their bond and separate them as systems from which no work
can be extracted coherently by any cyclic evolution. The minimum difference
between the average energies of the initial and final states obtained this way
is defined as the binding energy of the bipartite system. We show the final
optimal state is a passive state. We also discuss how required evolution can be
realized through a sequence of control pulses. We illustrate utility of our
definition through two examples. In particular, we also show how quantum
tunneling can assist or enhance bond-breaking process. This extends our
definition to probabilistic events.
