Physicists Learn to Run Time Backward in a Quantum System — and Harvest Energy From the Trick

Researchers have found a way to reshape the "arrow of time" inside a quantum system — stretching it, blurring it, and even making the system behave as if time were running in reverse — and then exploited the effect to extract energy from the act of measurement itself.

The work, carried out at Los Alamos National Laboratory and published in the journal Physical Review X, tackles one of the deepest puzzles in physics: why time seems to flow in only one direction. In everyday experience, a shattered cup never reassembles and heat never flows from cold to hot. At the quantum scale, however, the underlying equations are largely symmetric in time, and the team set out to see how much control an experimenter could exert over that asymmetry.

Their answer relies on quantum control protocols that combine measurement with feedback. By designing a specially tailored "control Hamiltonian" — a mathematical description of the forces acting on a system — the researchers could emulate the disturbances that measurements normally cause, then use feedback to cancel, amplify or overcompensate for them. The upshot is a system whose statistical path through time can be engineered to look time-reversed, as if a movie of its evolution were being played backward.

The most eye-catching application is what the team calls a "quantum demon," a nod to Maxwell's demon, the 19th-century thought experiment in which a tiny intelligent being appears to violate the second law of thermodynamics by sorting fast and slow molecules. In the Los Alamos version, the demon extracts usable energy from the monitoring process — turning the information gained by measuring a quantum system into a resource rather than a mere disturbance.

Beyond the philosophical intrigue, the technique has concrete uses in mind. Engineering time-reversed trajectories could sharpen the preparation of delicate quantum states, a routine but error-prone step in building quantum computers, and could feed into new schemes for quantum-enhanced sensing. The ability to counteract or deliberately amplify the disturbances caused by measurement gives experimenters a new lever over systems that are notoriously easy to disrupt.

The next phase, the researchers say, is to move from theory and proof of principle to hardware — demonstrating the measurement-and-feedback protocols in superconducting qubits, the same building blocks used by leading quantum computers. If it works there, the ability to bend the quantum arrow of time could become a practical tool for controlling the fragile machines that many hope will define the future of computing.