Physicists Learn to Nudge Time’s Arrow Backward in a Quantum System

Time, in everyday life, runs in one direction: cups shatter but never reassemble, and heat flows from hot to cold. Yet at the smallest scales, the fundamental laws of physics do not actually care which way time points. A team at Los Alamos National Laboratory has now shown, in work reported in mid-2026, how to take advantage of that hidden symmetry, engineering quantum systems so that their “arrow of time” can be slowed, suppressed or even made to run in reverse.

The research, led by physicist Luis Pedro García-Pintos with colleagues Yi-Kai Liu and Alexey V. Gorshkov, was published in the journal Physical Review X. The trick lies in a careful choreography of measurement and feedback. When you measure a quantum system, you inevitably disturb it. The Los Alamos team designed sequences of control fields and pulses, described mathematically as control Hamiltonians, that use the information gained from measurements to cancel out that disturbance, or even overcorrect it, effectively steering the system’s apparent flow of time.

“Yet at the smallest scales, the fundamental laws of physics do not actually care which way time points.”

Physicists have long entertained the idea of a “quantum Maxwell’s demon,” a hypothetical being that uses information to seemingly cheat the tendency of entropy to increase. The new protocols turn that thought experiment into a concrete, programmable recipe. Related ideas point to “measurement engines,” devices that harvest usable energy directly from the act of quantum measurement. “At the microscopic level most fundamental laws of physics see forward and backward movement in time as physically possible,” García-Pintos explained, “and the tools we’ve constructed can manipulate the perceived arrow of time.”

Crucially, this is not about building a time machine or violating the deep principles of thermodynamics; the second law still governs the wider universe. Instead, it is about gaining exquisite control over the tiny quantum systems that power emerging technologies. The payoff could be significant: more powerful and reliable quantum computers, quantum batteries that store and release energy more efficiently, and better ways to prepare delicate quantum states. It is a beautiful example of how understanding nature’s deepest symmetries can quietly hand us new engineering tools.