Scientists at Los Alamos National Laboratory have taken a major step toward reversing the quantum arrow of time — and discovered a surprising new way to harvest energy in the process.
Research published July 3 in Physical Review X introduces quantum control protocols that reshape a system's "arrow of time," the concept that time naturally moves in only one direction. Using these techniques, researchers can suppress the usual emergence of the arrow of time or even reverse its apparent direction, making quantum processes look as though they are unfolding backward.
"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 Los Alamos physicist Luis Pedro García-Pintos. "For quantum systems, which operate at that microscopic level, the tools we've constructed can manipulate the perceived arrow of time, leading to surprising, novel ways to control quantum systems."
The team designed a "control Hamiltonian" — a carefully planned sequence of fields and pulses — that reproduces the effects of quantum measurements. When incorporated into a feedback system, the Hamiltonian can cancel, strengthen, or even overcorrect the disturbances caused by measurements, generating trajectories that correspond to stretched, blurred, or inverted arrows of time.
The work also builds on the famous 19th century thought experiment known as "Maxwell's demon." The Los Alamos team's quantum "demon" uses information about a quantum system's state and measurement results to produce similarly unusual behavior.
Most remarkably, the new control methods allow researchers to extract useful energy directly from the monitoring process itself. Quantum measurements become a thermodynamic resource that can be tapped to perform work — such as driving another quantum process or storing energy in a quantum battery.
Looking ahead, the team plans to experimentally demonstrate these techniques using superconducting qubits, which support rapid feedback and highly efficient detection. Future studies will apply the methods to develop improved quantum state preparation protocols, potentially accelerating progress toward practical quantum computing.
