Study Shows Time Trajectories of Qubits Obey Second Law of Thermodynamics

The arrow of time points steadily forward, almost always, even in the abnormal realm of open quantum systems.

Innovative experiments at Washington University in St. Louis perform a comparison between the forward and reverse trajectories of superconducting circuits known as qubits. The results reveal that they obey the second law of thermodynamics. The study has been reported in the Physical Review Letters journal on July 9^th, 2019.

When you look at a quantum system, the act of measuring usually changes the way it behaves. 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.

Kater Murch, Associate Professor of Physics, Arts & Sciences, Washington University in St. Louis

He added, “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.”

Murch asks, “Do quantum movies look funny when you play them backwards?” He and his colleagues, including Patrick Harrington, first author of the study and a graduate student in physics, took the question to the laboratory—where their research is part of the new Center for Quantum Sensors.

We looked at microscopic movies of a quantum system’s movement during measurement, and asked if the movies looked more likely when played forward or backwards; this comparison can be used to determine if entropy increases or not. We found that even at the microscopic scale, the second law seems to hold: entropy generally increases.

Kater Murch, Associate Professor of Physics, Arts & Sciences, Washington University in St. Louis

“This increase happens because we look at it—the process of making the movie seemingly creates the arrow of time,” he further added.

The focus of Murch’s research team is to understand and control open quantum systems. Day-to-day objects follow the laws of classical mechanics; by contrast, single particles of light or matter obey the laws of quantum physics. However, it is not so easy to isolate these particles, and once they interact with the outside world, they tend to lose their quantum properties.

Murch is the 2018 recipient of a Cottrell Scholar Award and a National Science Foundation CAREER Award. The new study featured in the video and publication has been partially funded by his 2015 Alfred P. Sloan Research Fellowship.

Narration and animation by Kater Murch, associate professor of physics in Arts & Sciences. (Video credit: Washington University in St. Louis)