  Penrose's solution was effectively rediscovered independently by Bennett (1982), after Bennett had demonstrated that logically reversible computation could avoid the necessity of storing large quantities of additional information (Bennett 1973). Bennett presented an explicit physical model for reversible measurement. In logical terms, he represented a measurement by the state of a measuring system becoming correlated to the state of the measured system. The measuring system starts in a fixed state (which can be taken to be logical state zero) and moves through a correlated interaction, into a copy of the logical state, zero or one, of the system being measured. For (measuring system, measured system), a combined input logical state of (zero, zero) is left unaffected with the output logical state (zero, zero), while a combined input logical state of (zero, one) becomes the output logical state (one, one). As the combined output state can be used to uniquely identify the combined input state, the operation is logically reversible.