The self-winding wristwatch provides a familiar example of a ratchet. An internal mechanism that’s asymmetric with respect to clockwise and counterclockwise converts the energy imparted by random wrist motion into purposeful winding of the clockwork. More generally, a ratchet is a kind of rectifier—macroscopic or microscopic—that forces the otherwise random thermal or mechanical motion of a system that’s out of equilibrium into a specific direction by means of an asymmetric potential. Microscopic ratchets called Brownian motors are thought to be important for motion in biological systems (see the article by Dean Astumian and Peter Hänggi in Physics Today, November 2002, page 33).

Condensed matter physicist Heiner Linke at the University of Oregon concerns himself primarily with quantum ratchets in low-dimensional semiconductor heterostructures, where the goal is to force the random motion of electrons into a particular direction. 1 But his group’s latest paper, 2 which grew out of...

You do not currently have access to this content.