08/06/2020

Thermal chaos returns quantum system to its unknown past

Building up on their last year’s breakthrough “time reversal” experiment, two researchers from the Moscow Institute of Physics and Technology and Argonne National Laboratory have published a new theoretical study in Communications Physics. While their previous paper dealt with a predefined quantum state, this time the physicists have devised a way to time-reverse the evolution of an object in an arbitrary, unknown state.

Someday an improved reversal scheme could enable us to confirm the correct functioning of a quantum computer that is so powerful it would otherwise require an even bigger computer than itself to check.


Chaos reigns over time

There’s a certain natural way the state of a quantum chip evolves if left to its own devices: from order to chaos. This is true about other things, too: With time, our bodies grow older, manmade structures deteriorate, and while an ice cube left on the dinner table invariably melts, another ice cube will certainly not pop into existence in a glass right out of the blue — although that might depend on what one has been drinking.

Through everyday experience we acquire a sense of time based on the distinction between the generally more ordered past states and the typically more chaotic future states of closed systems — ones like a glass of water with an ice cube, where melting is a one-way process. Physicists refer to this as temporal asymmetry, or the arrow of time. It stems from the tendency toward disorder, formally expressed by the second law of thermodynamics.

“One of our breakthroughs is the realization — that we put in practice — that a quantum computer is a piece of the real physical world, but allowing for an unprecedented control over its evolution in time,” said one of the authors of the study, Argonne’s Valerii Vinokur.


Spell book of algorithms

What many journalists hailed as a “time machine” last year was the physicists’ experiment that briefly reversed the arrow of time for a quantum computer. That is, the experiment consisted in the computer, initially in an ordered state, evolving toward greater chaos for a brief period of time. After that, the team used its time reversal algorithm to modify the computer’s state in such a way that it started tracing back whatever it had been doing previously, effectively evolving in reverse playback until it assumed the original ordered state.

The catch was that one had to know the state of the computer at the moment when the time reversal algorithm kicked in, because it was not universal. “Even that felt like magic, but the reworked procedure is a whole different kind of genie, if you permit the analogy,” Vinokur commented. “Say you wanted to restore the Parthenon to its original splendor. The old genie would go, ‘Well, I can do that, but you have to give me some information. I want a perfectly detailed plan of the ruins as they are now.’ You see, that genie had no universal spell to turn back time. Instead, he had a great big book of spells, which you would have to leaf through together to find the right one. Very tedious fellow.”

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Image. Genie. Credit: Daria Sokol/MIPT Press Office

Practically speaking, the problem with having to know which state you are reversing is the need to record it. This was not really an issue for the small computer made up of two or three quantum bits, which was used in last year’s study. But scaling up the experiment ramps up the memory requirements really fast: Each additional qubit doubles the amount of memory needed.

To address this, the researchers came up with a universal algorithm, so now they have a beast of a genie to order around that is flexible enough to adapt to any scenario. No matter in which particular way a quantum system has deteriorated, he can do his magic trick and rewind it back to its “orderly” past. Admittedly, he will ask for tons and tons of marble and scorch it with the fires of hell, but it’s never simple with genies. Perhaps this one’s an afreet?