Can Time Flow in Reverse? A Quantum Breakthrough Challenges Our Assumptions

Researchers investigate how quantum time flow can be stretched, blurred, or even reversed.

What if the direction of time isn’t as fixed as it seems? While our everyday experience tells us that time moves relentlessly forward, the microscopic laws governing quantum systems are far less restrictive. In fact, many of the equations of quantum mechanics work just as well if time runs in reverse.

Now, researchers reporting in Physical Review X have developed quantum control protocols that can make certain processes appear more consistent with time flowing backward than forward. By carefully combining measurements, feedback, and tailored control fields, the team showed that they can suppress a quantum system’s arrow of time—or even invert its apparent direction. The work not only offers a new way to explore one of physics’ most fundamental concepts but could also lead to novel methods for extracting energy from quantum systems and preparing quantum states.

Quantum systems, such as collections of qubits, obey the rules of quantum mechanics, where measurements do more than simply observe—they actively alter the system being measured. The researchers exploited this feature to engineer unusual quantum dynamics, including trajectories that resemble time-reversed evolution. As a demonstration, they used the approach to design a measurement-powered engine capable of extracting energy from the act of monitoring a quantum system.

“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.”

Time-reversed trajectories

In classical physics, observing a system usually has little effect on what is being measured. In quantum physics, however, measurement can randomly change the state of a system, which helps create an arrow of time. The research team used measurements and feedback to design stochastic trajectories that resemble time running in reverse, causing quantum systems to behave as though they are moving backward in time.

The team created a control Hamiltonian, a programmed sequence of fields and pulses, that could imitate the effects of measurements. When used in a feedback process, that Hamiltonian allowed the team to cancel, strengthen, or overcorrect measurement disturbances. This produced new trajectories consistent with arrows of time that were stretched, blurred, or even inverted.

In the 19th-century thought experiment called “Maxwell’s demon,” directing hot and cold particles reduces entropy in a system, appearing to challenge the second law of thermodynamics, which holds that entropy should increase or remain constant in natural processes. (Later physics has shown that the second law is not violated when all sources of thermodynamic costs are accounted for.)

The Laboratory team’s quantum “demon” uses information about a quantum system’s state and measurement results to drive similarly unusual processes, reversing the usual arrow of time in a quantum system.

Quantum feedback control for superconducting qubits

The tools developed by the team can change how energy moves into and out of a quantum system. That ability can support a continuous measurement engine that draws energy from the act of monitoring the system. In this setup, quantum measurements become a thermodynamic resource that can supply usable energy, such as energy for another process or storage in a quantum battery.

Future work will include experimental tests of Hamiltonian measurement processes for quantum feedback control. One example is superconducting qubits, a platform that supports fast feedback and highly efficient detection, and where quantum versions of Maxwell’s demon have already been demonstrated. In follow-up work, the new techniques are also being used to design protocols for quantum state preparation.

Reference: “Reshaping the Quantum Arrow of Time” by Luis Pedro García-Pintos, Yi-Kai Liu and Alexey V. Gorshkov, 19 February 2026, Physical Review X.
DOI: 10.1103/l18s-9vmh

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.

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