Connecting the quantum and classical physics

In this week’s issue of Science, a Dartmouth researcher comments about a new experiment that brings us closer to connecting the macroscopic and the microscopic worlds.

Miles Blencowe, a quantum theorist with the Physics and Astronomy Department at Dartmouth, wrote the article “Nanomechanical Quantum Limits” for the “Perspectives” section of the April 2 issue of Science. In it, he explains the problem of reconciling the inherent contradiction between the quantum or atomic world and the macroscopic word of trees, buildings and cars that we live in.

“The world we live in follows the principles of classical physics,” says Blencowe. “We see objects in one place. In the microscopic world, the quantum world, things can be in two places at once. The Heisenberg Uncertainty Principle asserts that the more you try to localize an object the more you disturb it and it zooms away and then you don’t know where it is anymore. Somehow the atomic world becomes ours as we go to larger and larger systems. Scientists want to know how that crossover from quantum to classical occurs.”

As a theorist, Blencowe and his colleagues propose experiments and hypothesize about the results. His commentary in Science discusses the findings of M.D. LaHaye and his collaborators, researchers with the Laboratory for Physical Sciences in Maryland. LaHaye’s group based their experiment on Blencowe’s theories.

“About three years ago, my colleagues and I proposed that we could see this quantum motion in the macroscopic realm with an extremely sensitive motion detector, called a single electron transistor. We came up with this idea to look at quantum effects in mechanical systems that are really tiny, but still much larger than a single atom.”

The Maryland researchers cooled a tiny mechanical beam to close to absolute zero, and they measured its movement using a single electron transistor. As the beam is cooled, it slows down and the classical physics that normally dictate its movements are frozen out, leaving the quantum zero-point fluctuations, which is as close to still as you can get. Getting the beam to reveal its zero-point fluctuations, where quantum classical physics cross over to quantum physics, is the goal of the experiment, and LaHaye’s group comes close.

“It’s very exciting. They have achieved great sensitivity; they’ve come to within a factor of 10 of this zero point measurement,” says Blencowe.

Blencowe is hopeful that the next generation of experiments will “reach the quantum limit for motion of mechanical systems well outside the microscopic domain,” according to his article.