Editing Quantum decoherence (section)

===Dissipation===
{{Main| Quantum dissipation}}
'''Dissipation''' is a decohering process by which the populations of quantum states are changed due to entanglement with a bath. An example of this would be a quantum system that can exchange its energy with a bath through the [[Decoherence-free subspaces#Operator-sum representation formulation|interaction Hamiltonian]]. If the system is not in its [[ground state]] and the bath is at a temperature lower than that of the system's, then the system will give off energy to the bath, and thus higher-energy eigenstates of the system Hamiltonian will decohere to the ground state after cooling and, as such, will all be non-[[Degenerate form|degenerate]]. Since the states are no longer degenerate, they are not distinguishable, and thus this process is irreversible (non-unitary).