Researchers at Los Alamos National Laboratory have developed quantum control protocols that can make quantum systems behave as though time is moving backward rather than forward. The work, published in Physical Review X, manipulates the "arrow of time" at the microscopic level, where the laws of physics are symmetrical under time reversal. The team used a combination of measurements and feedback to generate time-reversed stochastic trajectories, effectively canceling or reversing the natural direction of time's flow.

“Unlike phenomena we observe around us, at the microscopic level most fundamental laws of physics see forward and backward movement in time as physically possible,” said physicist Luis Pedro García-Pintos. The researchers also built a measurement engine that harvests energy from quantum measurements, turning the act of observation into a thermodynamic resource. This builds on the 19th-century "Maxwell's demon" thought experiment, where a hypothetical observer sorts particles to reduce entropy - though, as always, the second law of thermodynamics remains intact when all costs are accounted for.

The team plans to experimentally demonstrate their Hamiltonian-based feedback control using superconducting qubits, which could lead to improved quantum state preparation and even quantum batteries. The work was funded by the U.S. Department of Energy and the National Science Foundation.