Probing Maxwell's Demon with a Nanoscale Thermometer

News Editor

Describing the measurement of temperature across extremely tiny distances such as individual molecules, UA physicists have glimpsed a phenomenon mimicking the actions of Maxwell's Demon, a hypothetical figure in a thought experiment that seemingly defies the laws of physics.

"Maxwell's Demon can't exist because it would violate the laws of thermodynamics," said Charles Stafford, a professor at the department of physics in the UA College of Science. "So you can imagine we were quite surprised to see it appear in our computer-based experiments."

"We showed that if you try to measure the temperature of a system of particles, in this case, electrons, not molecules in a box, with a spatial precision down to the size of individual atoms, then the laws of quantum mechanics result in an effect that is almost identical to what Maxwell's demon would do," Stafford explained.

In their theoretical work, the group simulated a system consisting of a small molecule of carbon and hydrogen atoms with three electrodes attached to it. One electrode transfers heat into the molecule, the second electrode drains heat out of the molecule, and the third measures the temperature at different places within the molecule. The whole setup is called scanning thermal microscopy: A scanning electron microscope uses an ultrafine tip whose apex consists of a single atom to measure temperatures on an atomic scale.

"In our simulations, we found that it is possible to separate the hot from the cold electrons within that single molecule without expending any energy to make this happen, which is exactly what Maxwell's Demon does," Stafford said.

However, it turns out this sorting process does not violate the laws of thermodynamics because of the peculiarities of quantum physics, he explained.

"In the quantum state of the molecule, the hot and cold electrons never mix despite the fact that they exist in the same place at the same time. But that's because they 'remember' where they came from due to quantum wave effects - not because there is a demon at play," he said.

The research project and its unexpected results were several years in the making, Stafford said. The investigation began when undergraduate researcher Shauna Story, who graduated with a Bachelor of Science in physics in 2010, discovered the strange effect while studying simple molecules. This led the group to tests with more complex structures, resulting in a publication co-authored by former graduate student Justin Bergfield and organic chemist Robert Stafford.

For the full story, see

Research supported by the U.S. Department of Energy, Office of Basic Energy Sciences.