Physicists reverse time using quantum computer

In collaboration with colleagues from the United States and Switzerland, scientists from the Moscow Institute of Physics and Technology were able to send a quantum computer back in time by a little amount of time. The likelihood that an electron in empty interstellar space may spontaneously journey back into its recent past was also computed. Published in Scientific Reports is the study.

This article is one of several that discuss the potential for defying the second rule of thermodynamics. The arrow of time, which proposes that time moves in a single direction from the past to the future, is strongly tied to this rule, according to the study's primary author Gordey Lesovik, who also serves as the director of MIPT's Laboratory of the Physics of Quantum Information Technology.

"We started out by outlining a second type of local perpetual motion machine. Then, in December, we released a study that covered the Maxwell's demon as a means of violating the second rule, according to Lesovik. "The most recent paper approaches the same problem from a third angle: we have artificially created a state that evolves in the opposite direction to that of the thermodynamic arrow of time."

What distinguishes the future from the past

The majority of physical laws do not distinguish between the present and the past. Let's use the collision and rebound of two identical pool balls as an example and write an equation to explain it. The same equation may still be used to describe the event if a close-up of it is photographed and then played backward. Furthermore, if the recording has been altered, it is impossible to tell. Both explanations seem reasonable. The billiard balls seem to defy our sense of time, it seems.

However, picture capturing a cue ball smashing the pyramid and the pool balls flying everywhere. In such circumstance, the difference between the real-life scenario and reverse playback is evident. Our intuitive grasp of the second rule of thermodynamics, which states that an isolated system either remains static or progresses toward a state of chaos rather than order, is what makes the latter seem so ludicrous.

The majority of other physical rules do not stop infused tea from returning to the tea bag, rolling pool balls from forming a pyramid, or a volcano from "erupting" backwards. However, these events are not seen since they would call for an isolated system to spontaneously enter a more ordered state, which is against the second law. Although the specifics of that law have not been fully elucidated, academics have made significant progress in comprehending the fundamental ideas that underlie it.

Unplanned time-reversal

MIPT quantum scientists made the decision to investigate if time might spontaneously reverse itself, at least for one particle and for a very brief period of time. In other words, rather than studying crashing pool balls, they looked at a single electron in a barren region of interstellar space.

"Let's assume that the electron is confined when we start to see it. This indicates that we are quite certain of its location in space. According to research co-author Andrey Lebedev from MIPT and ETH Zurich, "We can sketch a tiny region where the electron is confined, but we cannot know it with exact precision due to the limitations of quantum mechanics.

The scientist discusses how Schrödinger's equation controls the development of the electron state. The zone of space holding the electron will stretch out very fast, making no distinction between past and present. In other words, the system tends to get messier. The electron's location is becoming more erratic. This is comparable to how the second law of thermodynamics causes growing disorder in a large-scale system, like a pool table.

"However, Schrödinger's equation is reversible," continues Valerii Vinokur, a co-author of the study from the United States' Argonne National Laboratory. Mathematically, it means that the equation will depict a "smeared" electron localizing back into a constrained area of space during the same time period under a certain transformation known as complex conjugation. Although this event hasn't been seen in the wild, it is theoretically possible because of a chance variation in the cosmic microwave background, which permeates the whole universe.

The researchers set out to determine the likelihood of observing an electron spontaneously localizing into its recent past after being "smeared out" over a brief period of time. The reverse development of the particle's state would only occur once, even if observers observed 10 billion newly localized electrons per second for the entire universe's 13.7 billion year history. Even then, the electron would only go back in time a scant one ten billionth of a second.

Large-scale events like volcanoes and pool balls clearly occur over considerably longer durations and involve a huge quantity of electrons and other particles. This explains why we never see an ink blot separate from paper or an old person get younger.

On-demand time travel

In a subsequent four-stage experiment, the researchers tried to turn back time. They saw the state of a quantum computer composed of two, and subsequently three, fundamental components termed superconducting qubits instead of one electron.

Order is stage one. The ground state, represented by the number zero, is used to initialize each qubit. This highly organized form resembles a rack of pool balls just before the break or an electron that is confined in a narrow area.

Degradation is stage two. Order is misplaced. The state of the qubits transforms into an ever-more intricate shifting pattern of zeros and ones, much to how the electron is smeared out over a larger and larger area of space or how the rack breaks on the pool table. This is accomplished by momentarily starting the quantum computer's evolution program. Actually, environmental interactions would cause a comparable deterioration to happen on their own. However, the last phase of the experiment will be made possible by the regulated program of autonomous development.

Time travel is stage three. The quantum computer's state is altered by a unique software so that it will eventually evolve "backwards," from chaos to order. In the case of the electron, this action is analogous to the random microwave background fluctuation, except this time, it is intentionally created. The billiards scenario may be compared to someone delivering the table a perfectly timed kick, which is plainly an absurd analogy.

Regeneration is stage four. The second step of the evolution program is restarted. The program rewinds the state of the qubits back into the past, similar to how a smeared electron would be localized or the billiard balls would retrace their trajectories in reverse playback, eventually forming a triangle, provided that the "kick" has been successfully delivered.

The two-qubit quantum computer, according to the researchers, returned to its starting state in 85% of instances. Three qubits increased the number of mistakes, resulting in a success rate of about 50%. The authors claim that these mistakes are a result of flaws in the real quantum computer. The error rate is anticipated to decrease as devices get more complex.

It's interesting to note that the time reversal method itself could help improve the accuracy of quantum computers. According to Lebedev, "Our algorithm could be improved and used to test programs created for quantum computers and eliminate noise and errors."