A quantum mechanical version of Maxwell's Demon, a 150-year-old thought experiment, has been created. The demon is a simple one, and so far behaving itself.
In 1867 the great physicist James Clerk Maxwell pondered the newly formulated second law of thermodynamics. One framing of the second law is that if two bodies of different temperature are brought together, but isolated from the rest of the universe, their temperatures will converge, and only the input of work can make one hotter than the other – you can't, in other words, get energy for nothing. The law can more generally be summarized as “there's no such thing as a free lunch”, which rules out inventions such as perpetual motion machines.
Maxwell's thought experiment hypothetically contradicted the second law. He proposed the idea of a tiny demon that would sort a gas that contained particles of different energies. Particles of more than a certain energy would be sorted into one bucket, the rest into another. To a physicist, sorting involves no work. Yet the difference between the sorted gases – one high energy and therefore hot, the other cold – could be used to do work of the sort physicists recognize.
So if the second law says that entropy in an isolated system can only increase over time and in the Maxwell's Demon scenario the overall entropy appears to decrease, we have a paradox.
With time, physicists became increasingly confident the second law was right – many seeing it as the one thing in science we could be absolutely sure about. Yet, even though no one could build (or summon) a Maxwell Demon, the idea seemed viable in theory. It was only in the 1980s that the problem was resolved, with the realization that the costs of the information processing involved in assessing the particles and deciding what to do with them, would outweigh the gains from the sorted molecules. Thus, such a demon could not violate the second law, or power an energy supply, but might be interesting in other ways.
In Proceedings of the National Academy of Sciences, physicists describe the creation of a microwave cavity that acts as a Maxwell's Demon, and describes their success in measuring its energy production and loss down to the level of a single photon.