time they have measured the tiny amount of heat released when an individual bit of data is erased. In 2014, physical experiments tested Landauer's principle and confirmed its predictions. In 2016, researchers used a laser probe to measure the amount of energy dissipation that resulted when a nanomagnetic bit flipped from off to on. Flipping the bit required 26 millielectronvolts (4.2 zeptojoules). A 2018 article published in Nature Physics features a Landauer erasure performed at cryogenic temperatures on an array of high-spin (S = 10) quantum molecular magnets. The array is made to act as a spin register where each nanomagnet encodes a single bit of information. The experiment has laid the foundations for the extension of the validity of the Landauer principle to the quantum realm. Owing to the fast dynamics and low "inertia" of the single spins used in the experiment, the researchers also showed how an erasure operation can be carried out at the lowest possible thermodynamic cost—that imposed by the Landauer principle—and at a high speed. Rationale Landauer's principle can be understood to be a simple logical consequence of the second law of thermodynamics, which states that the entropy of an isolated system cannot decrease—together with the definition of thermodynamic temperature. For, if the number of possible logical states of computation were to decrease as the computation proceeded forward (logical irreversibility), this would constitute a forbidden decrease of entropy, unless the number of possible physical states corresponding to each logical state were to simultaneously increase by at least a compensating amount so that the total number of possible physical states was no smaller than it was originally (i.e. total entropy has not decreased). Yet, an increase in the number of physical states corresponding to each logical state means that, for an observer who is keeping track of the physical states of the system but not the logical states, the number of possible physical states has increased; in other words, entropy has increased from the point of view of this observer. The maximum entropy of a bounded physical system is finite. (If the holographic principle is correct, then physical systems with finite surface area have a finite maximum entropy; but regardless of the truth of the holographic principle, quantum field theory dictates that the entropy of systems with finite radius, energy, and surface area is finite due to the Bekenstein bound.) To avoid reaching this maximum over