Maxwell's demon is one of the most famous thought experiments in physics. In its traditional formulation, a demon sits next to a small hatch that separates two chambers. It observes the velocity of any gas molecules heading toward the hatch from one room and only opens the hatch when the velocity exceeds a certain value. Over time, the demon will raise the temperature of one room while cooling the second—something we know is thermodynamically impossible.

Over time, the demon's domain has been expanded, as researchers realized the same issue applied to a variety of other problems. One reformulation came from physicist Leo Szilard, who noted you can have a demon-based engine. Now, 90 years later, researchers have built a Szilard engine that operates using a single electron. In the process, the researchers confirm that setting the digital bit of information describing the engine's state has an energetic cost.

In its original formulation, the Szilard engine was a chamber with pistons at either end and a single gas molecule in the middle. Slide a divider down in the middle, and the gas molecule will wind up on one side or the other. This will push one of the pistons out, providing the potential for doing some work for "free" without the input of energy. (This being a thought experiment, the pistons are assumed to move without friction.) You can then remove the divider, let the chamber re-equilibrate, and do it all over again.

The Szilard engine also helps demonstrate how thermodynamic energy and information are equivalent. When the engine is in operation, each output of work is equivalent to a single bit of information: it tells you which of the two chambers the gas molecule is in. Based on this, people have been able to calculate the energetic cost of setting or erasing a bit. Over the past few years, researchers started implementing simplified Maxwell's demons, allowing them to test whether these calculations hold in the real world (see sidebar).

Eventually an international team of researchers decided to tackle the Szilard engine. In their implementation, the gas molecule has been replaced with the charge of a single electron. That charge is held in one of two locations that are separated by a barrier that allows quantum tunneling. When the system is left on its own, the extra charge will tunnel randomly back and forth between the two locations.

Enter the demon. Near one of the two locations, the researchers placed a sensor that could register whether the location carried a charge. If it did, the equipment would apply a voltage to the setup, which would be sufficient to trap the electron on that side of the barrier. Although this isn't performing any useful work, it is the equivalent of setting the value of a bit, which goes from indeterminate (nobody knows where the electron might be) to determinate (the electron is in a specific location).

That's not especially exciting, except for the fact that the entire process takes a tiny bit of energy, which the authors supply using a source of heat. As the demon repeatedly traps and releases the electron, the energetic cost keeps adding up. There's a corresponding drop in the temperature of the heat bath that's powering the whole thing.

This lets the authors measure the energetic cost of setting the bit. Although the system is near absolute zero, there's still a bit of random thermal fluctuations in it. And with enough measurements, it's possible to determine that the value of energy lost is roughly equal to that predicted by theoretical work. Thermodynamics is still the law.

PNAS, 2014. DOI: 10.1073/pnas.1406966111 (About DOIs).