WHAT can a demon do for a quantum computer? It can slyly trap and cool atoms so that their properties can be measured.

Now a team has brought such a creature to life – using lasers. The demon in question is based on the one in a thought experiment devised by the Scottish physicist James Clerk Maxwell in 1871. He imagined a creature capable of seeing individual atoms in a container of gas that had a central barrier with a tiny trapdoor. Maxwell’s demon could sort the atoms according to their energy by opening or shutting the trapdoor to incoming …