In 1867, the Scottish physicist, James Clerk Maxwell, published a thought experiment showing how to extract heat from a container of gas.
Maxwell dreamt up a container divided in half by a wall with a trap door that can be opened and closed to allow molecules of gas to pass through.
The experiment begins with all the gas in one half of the container. The gas contains molecules moving at a wide range of speeds. Whenever a high-speed molecule approaches the trap door, Maxwell imagined a ‘demon’ opening it to allow the molecule through.
Eventually, all the fast molecules end up in one half of the container while the slow ones stay in the other half. In effect, the demon has heated one half of the container and cooled the other.
A couple of years ago, we looked at an experimental version of Maxwell’s demon, in which Japanese physicists created a kind of staircase in which they lowered an energy barrier to allow atoms to jump up a step and then raised it to prevent the atom falling back down again.
As a result, the atom slowly climbed the staircase even though no energy was added to the system.
Maxwell’s demon and its experimental counterparts look like a clear violations of the second law of thermodynamics, which states that heat cannot be passed from a cold body to a hot body without doing work and that perpetual motion machines of this type are impossible.
But actually there’s nothing supernatural going on here. Modern physicists have realised that a complete description of thermodynamics must include an assessment of the order and disorder in the system, in other words of the information it contains.
The Japanese physicists have to monitor the position of the atom at all times to know when to raise and lower the barriers. When this monitoring system and the information it generates is taken into account, all is as it should be.
However, what was extraordinary about the Japanese experiment is that it converted information into energy.
Since then, physicists have begun to ask whether there might be some other interesting complexities in the second law, particularly when they take into account the quantum nature of particles.
How might quantum mechanics play a role? One possibility is related to the weird phenomenon of entanglement in which two particles become so deeply linked that they share the same existence, even when they are separated by the width of the universe. When two particles are entangled, a measurement on one gives you information about both particles.
It’s not hard to see how this might be used in a Maxwell demon-type experiment and today that’s exactly what Ken Funo at The University of Tokyo in Japan and a couple of pals do. Here’s how.
Imagine two boxes of particles with trap door between them. You want to use the trap door to guide the faster particles into one box and the slower particles into the other. In a classical experiment you would have to measure the particles in both boxes to do this experiment.
But things are different if the particles in one box are entangled with the particles in the other. In that case, measurements on the particles in one box give you info about both sets of particles.
In essence, you’re getting information for nothing. And since you can convert that information into energy, there is clear advantage when entanglement plays a role.
That’s hugely significant. It means that the laws of thermodynamics depend not only on classical phenomenon and information but on quantum effects too. The breakthrough that Funo and co make is to extend the theory to take this into account. “We show that entangled states can be used to extract thermodynamic work beyond classical correlation,” they say.
That will have important implications for all kinds of phenomenon, from black holes and astrobiology to quantum chemistry and nanomachines.
Now the race will be on to see who can measure it first.
Ref: arxiv.org/abs/1207.6872: Thermodynamic Work Gain from Entanglement