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Researchers May Have Reversed Time on a Quantum Computer

March 25, 2019
Have simulations given us a sneak peek into the “impossible”?

An international team of scientists led by Argonne National Laboratory explored the concept of reversing time in a first-of-its-kind experiment, managing to return a computer briefly to the past. The results present new possibilities for quantum computer program testing and error correction and suggest new paths for exploring the backward flow of time in quantum systems. They also open new possibilities for quantum computer programming. A quantum computer that can effectively jump back and clean up errors as it works also could operate far more efficiently.

To achieve the time reversal, the research team developed an algorithm for IBM’s public quantum computer that simulates a particle’s scattering of a particle. In classical physics, this might appear as a billiard ball struck by a cue, traveling in a line. But in the quantum world, a scattered particle takes on a fractured quality, spreading in several directions. To reverse its quantum evolution is like reversing the ripples created when a pebble is thrown into a pond.

In nature, restoring this particle back to its original state—in essence, putting the humpty Dumpty back together—is impossible.

The main problem is that you would need a “supersystem,” or external force, to manipulate the particle’s quantum waves at every point. But, as researchers note, the timeline required for this supersystem to spontaneously appear and properly manipulate the quantum waves would extend longer than that of the universe itself.

This image shows a time-reversal procedure for a spreading wave packet that represents a quantum particle. The reversed state freely evolves into the original squeezed state, which is recovered with some precision—in this case, 85%. (Image: Argonne National Laboratory)

Undeterred, the team set out to determine how this complexity might be overcome, at least in principle. Their algorithm simulated an electron scattering by a two-level quantum system, “impersonated” by a quantum computer qubit (the basic unit of quantum information) and its related changes through time. The electron goes from a localized (or “seen”) state, to a scattered one. Then the algorithm throws the process in reverse, and the particle returns to its initial state. In other words, it moves back in time, if only by a tiny fraction of a second.

Given that quantum mechanics is governed by probability rather than certainty, the odds for achieving this time-travel feat were pretty good given that algorithm delivered the same result 85% of the time in a two-qubit quantum computer.

The result deepens our understanding of how the second law of thermodynamics (a system will always move from order to entropy, and not the other way around) acts in the quantum world. The researchers demonstrated in previous work that, by teleporting information, a local violation of the second law was possible in a quantum system separated into remote parts that could balance each other out.

“The results also give a nod to the idea that irreversibility stems from measurement, highlighting the role the concept of ‘measurement’ plays in the foundation of quantum physics,” says researcher Gordey Lesovik of the Moscow Institute of Physics and Technology.

This is the same notion Austrian physicist Erwin Schrödinger captured with his famous thought experiment, in which a cat sealed in a box might remain both dead and alive until its status is monitored somehow. Researchers suspended their particle in this superposition, or form of quantum limbo, by limiting their measurements.

“This was the essential part of our algorithm,” says Argonne scientist Valerii Vinokur. “We measured the state of the system in the beginning and the end, but did not interfere in the middle.”

The finding may eventually lead to better methods of error correction on quantum computers, where accumulated glitches generate heat and beget new ones. A quantum computer able to effectively jump back and clean up errors as it works could operate far more efficiently.

“At this moment, it’s very hard to imagine all the implications this will have,” Vinokur said. The study also begs the question: Can researchers now figure out a way to make older folks young again?  “Maybe,” Vinokur jokes, “with the proper funding.”

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