Landauer's principle

Landauer's principle states that erasing one bit of information in a physical system must dissipate at least k_BT ln 2 of energy as heat into the environment — about 3 × 10⁻²¹ joules at room temperature. Erasure is logically irreversible: it maps two distinct states (0 and 1) onto a single standard state, merging two regions of phase space into one, and that loss of one bit of phase-space volume must be compensated by an entropy increase of at least k_B ln 2 somewhere outside the system.

The principle draws a sharp line between reversible and irreversible computation. Operations that preserve information can in principle be performed with arbitrarily little energy; only the destruction of information carries an unavoidable thermodynamic cost. Its slogan — 'information is physical' — captures the recognition that a bit is not an abstraction but a physical degree of freedom subject to thermodynamics.

Landauer's principle resolves Maxwell's demon: the demon's memory is finite and must be erased to continue operating, and that erasure pays back exactly the work the demon extracted. It also sets the ultimate energy floor for computation, a limit measured directly in single-particle experiments since 2012.

Formulated by Rolf Landauer at IBM in 1961, and used by Charles Bennett in 1982 to give the definitive resolution of the Maxwell's-demon paradox by locating the entropy cost in memory erasure rather than in measurement.