Physicists Learn to Run Time Backward Inside a Quantum System — and Harvest Energy From the Reversal

Physicists at Los Alamos National Laboratory have found a way to reshape one of nature's most stubborn rules — the arrow of time — inside a quantum system, and to squeeze usable energy out of the trick. In work published in the journal Physical Review X, the team described control protocols that can make a quantum system behave as though time were flowing backward, effectively rewinding its evolution on command.

The "arrow of time" is the everyday observation that events run one way: a shattered glass never reassembles, heat spreads out but never spontaneously concentrates. In the quantum world, that directionality is tied to measurement, the act of observing a system, which disturbs it in ways that are normally irreversible. The Los Alamos researchers showed that by carefully designing a control field — a so-called control Hamiltonian — they could emulate the effect of a measurement and then use feedback to cancel, amplify or even overcompensate for the disturbance it caused.

The result is a system whose statistical trajectory can be steered as if the clock were running in reverse. That capability let the team design what they call a "quantum demon," a modern cousin of Maxwell's demon, the 19th-century thought experiment about a tiny being that seems to cheat the second law of thermodynamics by sorting fast and slow molecules. Instead of sorting molecules, the quantum demon extracts energy from the monitoring process itself.

Here is the payoff: the energy pumped into a quantum system by the act of measuring it need not be wasted. Using the control Hamiltonian, the researchers showed it could be pulled back out almost instantly and stored — in principle in a battery — to power other processes. That reframes measurement, usually seen as a nuisance that destroys delicate quantum states, as a potential energy source.

The implications reach into quantum computing, where measurement and state preparation are constant challenges. The same tools that reverse the arrow of time could help scientists prepare quantum states more reliably and manage the fragile qubits used in superconducting quantum processors. The authors suggest the mechanism could one day boost the performance of quantum batteries and certain quantum algorithms.

For now, the work is a blueprint rather than a working machine. The researchers caution that the energy-harvesting "engine" they describe does not yet exist in the lab, and significant experimental hurdles remain before it could be built. Still, the study is a striking demonstration that even something as fundamental as the direction of time can, under the right quantum controls, be bent to human purpose.