Even in the strange world of open quantum systems, the arrow of time points steadily forward—most of the time. New experiments conducted at Washington University in St. Louis compare the forward and reverse trajectories of superconducting circuits called qubits, and find that they follow the second law of thermodynamics. The research is published July 9 in the journal Physical Review Letters.

“When you look at a quantum system, the act of measuring usually changes the way it behaves,” said Kater Murch, associate professor of physics in Arts & Sciences. “Imagine shining light on a small particle. The photons end up pushing it around and there is a dynamic associated with the measurement process alone.

”We wanted to find out if these dynamics have anything to do with the arrow of time—the fact that entropy tends to increase as time goes on.”