physically relevant models
include time-dependent Hamilton operators. When the time-dependence is
slow in comparison to the energy gaps of the spectrum, the adiabatic
theorem implies that a quantum system prepared in the ground state of
an initial Hamiltonian will completely follow the instantaneous ground
state. This can be exploited to prepare interesting ground states from
simple ones by slowly deforming control parameters in the Hamiltonian.
The final ground states can -- for example -- be entangled, be useful
for one-way quantum computation
or even directly encode the solution to a difficult problem.
The scheme has nice robustness features: When the reservoir temperature is significantly smaller than the time-dependent energy gap above the ground state, the decoherent interactions may drag the system towards its ground state and may therefore be even helpful in the goal to solve the problem. However, for critical parameter values the ground state itself may change in a quite abrupt manner, which is typically associated with a nearly vanishing energy gap. In the infinite size limit this corresponds to a quantum phase transition, whereas for finite-size systems one usually has a finite energy gap. This scaling behavior of the energy gap poses a severe problem both for the adiabatic implementation (diverging adiabatic runtime) and its robustness against thermal excitations.
There exist only a few exactly solvable models that exhibit a quantum phase transition in the infinite size limit, such
that in general one has to use numerical simulations.