Physicists Show How to Reverse the Quantum Arrow of Time — and Harvest Energy Doing It

Physicists have long puzzled over one of nature's deepest asymmetries: at the microscopic level, the fundamental laws of physics work equally well running forward or backward, yet the world we experience only moves in one direction. Now a team at Los Alamos National Laboratory has shown how to bend that rule inside a quantum system, engineering conditions that make it appear to run backward in time.

In a study published in the journal Physical Review X, researchers Luis Pedro García-Pintos, Yi-Kai Liu and Alexey V. Gorshkov describe control protocols that combine carefully designed "control Hamiltonians" — the mathematical rules governing how a quantum system evolves — with measurement and feedback. Because measuring a quantum system randomly disturbs it, that act of observation normally imposes a natural arrow of time. By canceling, amplifying or overcompensating for those disturbances, the team was able to produce time-reversed trajectories that follow paths consistent with time flowing in reverse.

"At the microscopic level most fundamental laws of physics see forward and backward movement in time as physically possible," García-Pintos said, capturing the counterintuitive premise at the heart of the work. The trick was not to break any physical law, but to actively steer a quantum system down one of the reverse-running paths those laws already permit.

The most striking application is a modern reincarnation of Maxwell's demon, the 19th-century thought experiment in which a hypothetical being sorts molecules to seemingly violate the second law of thermodynamics. The Los Alamos team designed a "quantum demon" that uses information gleaned from quantum measurements to extract energy from the monitoring process itself — turning the act of observation into a resource rather than a nuisance.

The researchers say the techniques could have practical payoffs for emerging quantum technologies, including more powerful quantum computers, quantum batteries capable of drawing energy from measurement, and more precise protocols for preparing delicate quantum states. Future experimental demonstrations could be carried out using superconducting qubits, the same building blocks powering many of today's leading quantum machines. The work was supported by the U.S. Department of Energy's Office of Science, its Advanced Simulation and Computing program, and the National Science Foundation, and it adds to a growing body of research probing how far physicists can reshape time's arrow when they take full control of a quantum system.