Landauer's principle: erasing one bit of information requires dissipating at least k_B T ln2 of energy as heat. Maxwell's Demon is exorcised: measurement and erasure together satisfy the second law.
Landauer (1961): logical irreversibility implies physical irreversibility. Bennett (1973): computation can be thermodynamically reversible, but erasure cannot. Szilard engine: Demon does k_BT ln2 of work measuring 1 bit; erasure restores entropy. Jarzynski equality: ⟨e⁻βW⟩=e⁻βΔF for nonequilibrium processes.