  Continuous-time quantum algorithms for combinatorial optimisation problems,
such as quantum annealing, have previously been motivated by the adiabatic
principle. A number of continuous-time approaches exploit dynamics, however,
and therefore are no longer physically motivated by the adiabatic principle. In
this work we take Planck's principle as the underlying physical motivation for
continuous-time quantum algorithms. Planck's principle states that the energy
of an isolated system cannot decrease as the result of a cyclic process. We use
this principle to justify monotonic schedules in quantum annealing which are
not adiabatic. This approach also highlights the limitations of reverse quantum
annealing in an isolated system.
