Lose a memory, use energy

Anyone who has ever tried to forget a bad night knows that erasing memory takes work. Physicists have now shown this is literally true, by measuring the heat released when a single bit of information is deleted. The discovery confirms a 1961 prediction by IBM physicist Rolf Landauer. It links information and heat flow in ways that keep the universe from breaking the second law of thermodynamics. Elaborations on Landauer’s idea may also prove useful in building miniature computers that don’t overheat. “Here you have two quantities which seem to have nothing to do with each other, at least at first sight, and Landauer tells you that they are interconnected,” says physicist Eric Lutz of the Free University Berlin. “That’s a big deal.” Lutz, who led the new work while at the University of Augsburg in Germany, and his colleagues describe the findings in the March 8 Nature. Landauer calculated that erasing a bit of information always releases at least a tiny amount of heat — no less than 3 billionths of a trillionth of a joule (at room temperature). To try to measure such a small number, Lutz went to the laboratory of Sergio Ciliberto, of the French National Center for Scientific Research in Lyon. There the scientists created a one-bit memory with a floating silica bead and a double well, where the bead could rest in one well (representing a 0) or the other (representing a 1). By lowering the barrier between the wells and tilting them, the researchers could coax the bead to jump from one well to the other. Putting the bead back in its original well meant that its memory was erased. Next the scientists carefully measured the speed at which the bead moved from one well to the other, which allowed them to calculate the heat given off during that shift. “This is the first experiment showing that this tiny limit does exist,” says Lutz. Knowing that heat comes out as memory gets erased could help computer manufacturers anticipate problems with overheating in ever-smaller computing devices. Ultimately, strategies for avoiding information erasure may be needed. More fundamentally, Landauer’s principle helps physics avoid breaking the thermodynamics law that holds that entropy, or disorder, in a system always increases over time. A 19th century thought experiment about entropy by Scottish physicist James Clerk Maxwell envisioned a demon sorting cold and hot molecules into two chambers. By separating hot (fast) molecules from cold (slow) ones, such a demon could convert information about molecular speeds into energy, seemingly decreasing entropy. But Landauer’s principle requires “Maxwell’s demon” to use up energy whenever discarding information about the molecules already sorted. Without an infinite memory, the demon would have to erase enough information (using energy each time) to keep the law intact. “When you erase that memory, bring it back to that original state, all the accounts get squared,” says John Bechhoefer, a physicist at Simon Fraser University in Burnaby, British Columbia. Bechhoefer has been using a different particle trap to try to demonstrate Landauer’s principle; he thinks he may be close. “So far we’ve observed the basic effect,” he says. His team is still working out some small discrepancies in the measurements, though. With experimental verification of Landauer’s principle, the quest now turns to exploring it in other realms, such as the quantum world. “I hope this is the first of a long series of experiments,” Lutz says.