Can deleting a photo make heat? A scientist named Rolf Landauer said yes in 1961. He said that erasing one bit of information always makes a very small amount of heat.

In 2012, scientists in France and Germany tested this idea. They used laser light to hold one tiny bead in water. The bead on the left meant 0, and the bead on the right meant 1. When they erased this memory slowly, the heat came close to Landauer's number.

In the 1860s, the physicist James Clerk Maxwell imagined a tiny creature, later called a demon, guarding a small door between two boxes of gas. By letting only fast molecules pass one way and slow ones the other way, it could make one side hot and the other side cold without doing any work. That would break the second law of thermodynamics.

For a long time, scientists thought the demon paid a price when it looked at the molecules. Charles Bennett argued in 1982 that looking could be free in principle. The real cost comes later, because the demon must clear its memory to keep working. Rolf Landauer had argued in 1961 that erasing information always releases heat. So the heat from the demon's erased notes saves the second law.

For a single bit, Landauer's limit at room temperature is only a few billionths of a trillionth of a joule, so nobody could measure it for decades. In 2012, a team in Lyon, France, working with the theorist Eric Lutz, built a one-bit memory from a tiny bead held in water by laser light. The bead could sit in either of two wells, and the researchers erased the memory by lowering the barrier between them, tilting the system and raising the barrier again.

Averaged over many runs, the heat released fell as the erasure was made slower, and it leveled off at Landauer's value. This study and a 2014 Canadian experiment both showed that single runs can dip below the limit because of random thermal motion, although the average does not. Not everyone treats this as the end of the story. The philosopher of science John Norton has argued that general proofs of the principle rely on hidden assumptions, so seeing the limit in one model system does not show that every possible demon must lose. A Japanese group had already run the idea in reverse in 2010, using information about a particle's position to raise its free energy.