Quantum Simulation Offers Brief Peek into Time Reversal Possibilities

People mark days with calendars and clocks, but possibly no timepiece is more direct than a mirror. Over the years, changes are noticed which richly demonstrate science’s “arrow of time”—the probable evolution from order to disorder. It is not possible to reverse this arrow just like one cannot erase all their wrinkles or restore a broken teacup to its original form.

Or would there be a possibility?

An international group of scientists led by the U.S. Department of Energy’s (DOE) Argonne National Laboratory investigated this question in a first-of-its-kind experiment, managing to return a computer fleetingly to the past. The outcomes, reported in the March 13^th issue of the journal Scientific Reports, propose new paths for discovering the backward flow of time in quantum systems. They also pave the way for new possibilities in quantum computer program testing and fault correction.

To realize the time reversal, the team created an algorithm for IBM’s public quantum computer that mimics the scattering of a particle. In classical physics, this might seem as a billiard ball struck by a cue, moving in a line. But in the quantum realm, one scattered particle absorbs a fractured quality, dispersing in numerous directions. To reverse its quantum progress is similar to reversing the rings formed when a stone is flung into a pond.

In nature, reinstating this particle back to its original state—essentially, repairing the broken teacup—is not possible.

The key issue is a “supersystem,” or external force would be needed to control the particle’s quantum waves at every point. But, the researchers observe, the timeline needed for this supersystem to instinctively appear and correctly manipulate the quantum waves would range longer than that of the universe itself.

Undismayed, the team embarked to establish how this complexity might be handled, at least in principle. Their algorithm replicated an electron scattering by a two-level quantum system,“impersonated” by a quantum computer qubit—the standard unit of quantum information—and its associated evolution in time. The electron goes from a localized, or “seen,” state, to a scattered one. Then the algorithm puts the process in reverse, and the particle goes back to its original state—in other words, it travels back in time, if only by a minute fraction of a second.

Given that quantum mechanics is ruled by probability instead of certainty, the chances for realizing this time-travel feat were quite good: The algorithm gave the same result 85% of the time in a two-qubit quantum computer.

“We did what was considered impossible before,” said Argonne senior researcher Valerii Vinokur, who led the research.

The outcome extends the insight of how the second law of thermodynamics—that a system will at all times move from order to entropy and not in reverse—acts in the quantum domain. The scientists showed in an earlier work that, by teleporting information, a local abuse of the second law was conceivable in a quantum system divided into remote parts that could balance each other out.

The results also give a nod to the idea that irreversibility results from measurement, highlighting the role that the concept of “measurement” plays in the very foundation of quantum physics.

Gordey Lesovik, Study Co-Author, Moscow Institute of Physics and Technology.

This is the same idea Austrian physicist Erwin Schrödinger used with his well-known thought experiment, wherein a cat sealed in a box might stay both dead and alive until its status is observed somehow. The scientists suspended their particle in this superposition, or state of quantum limbo, by controlling their measurements.

“This was the essential part of our algorithm,” Vinokur said. “We measured the state of the system in the very beginning and at the very end, but did not interfere in the middle.”

The finding may ultimately enable better approaches of error correction on quantum computers, where amassed glitches produce heat and yield new ones. A quantum computer able to successfully bounce back and rectify errors as it works could function a lot more efficiently.

At this moment, it’s very hard to imagine all the implications this can have. I am optimistic, and I believe that it will be many.

Valerii Vinokur, Study Leader and Senior Researcher, Argonne National Laboratory.

The research also implores the question; can the scientists now discover a way to make older people young again? “Maybe,” Vinokur jokes, “with the proper funding.”

The research was carried out by an international team including scientists from the Moscow Institute of Physics and Technology (Gordey Lesovik, Andrey Lebedev, Mikhail Suslov), ETH Zurich (Andrey Lebedev), and Argonne National Laboratory, U.S. (Valerii Vinokur, Ivan Sadovskyy).

Funding for this study was given by the DOE Office of Science and Strategic Partnership Projects (Swiss National Foundation and the Foundation for the Advancement of Theoretical Physics “BASIS”).