What if time were reversed? Physicists show how time could flow backward on a quantum scale

Researchers have developed a way to flip time to move backward in a quantum system. This level of control could lead to bizarre real-world applications

By Stephanie Pappas edited by Jeanna Bryner

The arrow of time marches forward. Eggs don’t uncrack; milk doesn’t unspill. But now new research has found a way that this arrow could be reversed in a quantum system, flip-flopping events as if time were flowing backward.

The findings are currently theoretical but could be tested experimentally, says Luis Pedro García-Pintos, a physicist at Los Alamos National Laboratory and first author of the new study, published February 19 in the journal Physical Review X.

Ultimately, reversing time on a quantum level could stem the information loss that stymies quantum computers, says Andrea Rocco, a physicist at the University of Surrey in England, who was not involved in the research. “This would immediately be an incredible advantage in terms of the building of these quantum technologies.”

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The idea of reversing time is not new. In the 19th century, physicist James Clerk Maxwell came up with a thought experiment to reverse the second law of thermodynamics, which holds that the total entropy of a system (a measure of disorder) cannot decrease over time. According to this law, heat always flows from a hotter to a cooler object, which in turn increases that cooler object’s entropy. Anyone who has made a mug of hot chocolate to warm their hands on a snowy day can attest to this law. But because of random chance, there will always be some slow-moving molecules in the hot object and some fast-moving molecules in the cold object. That means an outside entity—known as Maxwell’s demon—could theoretically herd those molecules from one object to another preferentially, sorting the faster-moving molecules back to the hot object and the cooler ones to the cold object. Thus, the hot object would get hotter, and the cold object would get colder. To an observer, it would look as though the usual order of things was going in reverse: your cup of hot chocolate would suck the warmth from your hands.

There’s no little demon out there tinkering with hot chocolate mugs. But in minuscule quantum systems, there is an element of outside control. Quantum systems include all the itty-bitty particles, such as atoms and electrons, that behave according to the rules of quantum mechanics. Under these rules, measuring a quantum system changes it: Before an observation, a system can exist in multiple states simultaneously, a concept called superposition. In other words, a particle’s spin, momentum and other properties are not yet defined. But measurement collapses this superposition, yielding one definitive outcome.

Using computer simulations, García-Pintos and his colleagues found that by knowing the original state of a quantum system and the outcome after a measurement is made, they could reverse the arrow of time. For their outside controller, the researchers constructed a sequence of fields and pulses to instantaneously revert the virtual system back to where it started and, in some cases, push it toward the opposite outcome. This control sequence, called a Hamiltonian, acts like Maxwell’s demon, flip-flopping a supposedly irreversible sequence of events forward to backward.

“We’re emulating a universe where things are flowing backward in time,” García-Pintos says.

These Hamiltonian controls could be used to make a continuous measurement engine. The energy put into a quantum system by measurement could be instantly pulled back out by the Hamiltonian and stored in a battery to power other processes, García-Pintos says. Another application might be reversing quantum decoherence, the phenomenon by which a quantum system loses its special quantum behavior and transforms into a classical system because of interactions with the outside environment. Decoherence is a major barrier to quantum computing, Rocco says, so a step toward making it reversible would be significant.

But there are challenges ahead, says Kater Murch, an experimental physicist at the University of California, Berkeley. Creating these Hamiltonians in practice would require perfect measurements without information loss, says Murch, who was not involved in the study. Perfect measurement isn’t possible, though. Currently researchers measure properties of quantum systems by beaming either optical or microwave light at them and then collecting that light to see how its components shift. But the efficiency with which they collect that returning light to see how the system changes is only about 50 percent, he says, which means some details are fuzzy. “Now that we’ve lost some of the measurement signal, we lose track of exactly what the quantum system is doing,” he says. And that means that before researchers can construct the perfect Hamiltonian to reverse time in real quantum systems, they’ll have to get better at measuring them.