Scientists make quantum time flow backward in stunning physics breakthrough

A quantum system, such as a group of qubits, follows the rules of quantum mechanics rather than classical physics. Using the newly developed control protocols, 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. As a demonstration of the technique, the team also created a measurement engine that can harvest energy from the act of making quantum measurements.

“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 National Laboratory physicist Luis Pedro García-Pintos. “In other words, those laws of physics are symmetrical under time reversal; the equations work just as well if you reverse time. 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.”

Engineering Time Reversed Quantum Behavior

In everyday classical physics, making a measurement has little effect on the object being observed. Quantum systems behave very differently. Measuring them randomly changes their state, naturally creating an arrow of time.

To overcome that effect, the researchers combined measurements with feedback to produce time reversed stochastic trajectories. This allowed quantum systems to follow paths that appear consistent with time flowing in reverse.

The team accomplished this by designing 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. As a result, the system can generate trajectories that correspond to stretched, blurred, or inverted arrows of time.

A Quantum Version of Maxwell’s Demon

The work also builds on the famous 19th century thought experiment known as “Maxwell’s demon.” In that scenario, a hypothetical observer selectively sorts hot and cold particles, apparently reducing entropy and challenging the second law of thermodynamics, which states that entropy naturally increases or remains constant. (Later physics has shown that the second law is not violated when all sources of thermodynamic costs are accounted for.)

The Los Alamos team’s quantum “demon” uses information about a quantum system’s state and measurement results to produce similarly unusual behavior, effectively reversing the system’s natural arrow of time.

Extracting Energy From Quantum Measurements

The new control methods also allow researchers to influence how energy moves into and out of a quantum system. This capability could power a continuous measurement engine that extracts useful energy directly from the monitoring process.

In this framework, 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 researchers plan to experimentally demonstrate Hamiltonian based measurement processes for quantum feedback control using superconducting qubits. These systems support rapid feedback, highly efficient detection, and have already been used to implement quantum versions of Maxwell’s demon. Future studies will also apply the new techniques to develop improved quantum state preparation protocols.

Funding: This work is supported by the U.S. Department of Energy, Office of Science, Advanced Scientific Computing Research program, the Beyond Moore’s Law project of the Advanced Simulation and Computing Program at Los Alamos, and the National Science Foundation.