For the first time, physicists have measured the minuscule amount of heat released when a single bit of data is erased. Although the value was first predicted more than 50 years ago, it is so small that measuring it has proven impossible—until now. The experiment, which involved trapping a tiny bead in a double well created by a laser and tracking its motion as it rotated between the wells, places a lower limit on the energy dissipated by logic circuits, which could influence the design of future electronic devices. For decades, physicists and computer scientists have established connections between thermodynamics and information theory. In 1961, German-American physicist Rolf Landauer deduced that the irreversible erasure of information involves heat dissipation. The “Landauer principle” applies to computational processes in which the number of bits of information decreases as the computation progresses, which occurs in all conventional computers. A prominent example of irreversible erasure is the “reset-to-one” process, whereby a piece of information containing a bit, which can be either 0 or 1, is reset to 1. Since the information held by the bit is destroyed, this data can no longer be recovered because, once the bit is set to 1, we have no way of knowing its previous value. What has essentially happened is that the entropy—or randomness—of the bit has been reduced. And since the bit and its surroundings are physical entities that must obey the laws of thermodynamics, this entropy must be transferred from the bit to its surroundings in the form of heat. Indeed, according to Landauer’s theory, a minimal amount of heat—about 10-21 J per erased bit—must be dissipated when the information is destroyed. Unfortunately, physicists have struggled to verify this prediction because 10-21 J per erased bit is less than one-thousandth of the electrical energy dissipated when a modern silicon device is reset. Eric Lutz of the University of Augsburg, along with Sergio Ciliberto and colleagues from the École Normale Supérieure de Lyon and Raoul Dillenschneider of the University of Kaiserslautern, are the first to experimentally confirm Landauer’s principle. Instead of using a silicon circuit, the team’s data bit comprises a tiny silica bead just 2 µm in diameter that is immersed in water and trapped using optical tweezers. The laser used to create the tweezers is alternately focused on two different points in rapid succession, creating two different points where the bead can be trapped. The bit is assigned the value “0” when the bead is in the left position and “1” when it is in the right position.
