Physicists invert time using the quantum computer

Researchers from the Moscow Institute of Physics and Technology have teamed up with colleagues from the United States and Switzerland to restore the state of the quantum computer a fraction of a second later. They also calculated the probability that an electron in an empty interstellar space will spontaneously return to its recent past. The study is published in Scientific reports.

"This paper is part of a series of articles on the possibility of violating the second law of thermodynamics.This law is closely related to the notion of time arrow which establishes the meaning of the one-way time of the forward to the future ", The lead author of the study, Gordey Lesovik, heads MIPT's Laboratory of Quantum Information Technology.

"We started by describing a so-called local perpetual motion machine of the second type, and then in December we published an article that dealt with the violation of the second law via a device called Maxwell's Demon," he said. Lesovik. "The last article tackles the same problem from a third angle: we have artificially created a state that evolves in a direction opposite to that of the thermodynamic arrow of time."

What is the difference between the future of the past?

Most laws of physics make no distinction between the future and the past. For example, suppose that an equation describes the collision and rebound of two identical billiard balls. If a close-up of this event is recorded with a camera and read in reverse, it can still be represented by the same equation. Moreover, it is not possible to distinguish from the recording if it has been tampered with. Both versions seem plausible. It seems that billiard balls defy the intuitive sense of time.

However, imagine that you are recording a shock ball breaking the pyramid, billiard balls scattering in all directions. In this case, it is easy to distinguish the real scenario from the reverse reading. Our intuitive understanding of the second law of thermodynamics is that the latter looks so absurd: an isolated system remains static or evolves into a state of chaos rather than order.

Most other laws of physics do not prevent rolling billiard balls from piecing together in a pyramid, tea infused to flow back into the tea bag, or a volcano to "burst" into the # 39; to. But these phenomena are not observed because they would require an isolated system to assume a more orderly state without any outside intervention, which is contrary to the second law. The nature of this law has not been explained in detail, but researchers have come a long way in understanding the basic principles that underpin it.

Spontaneous temporal inversion

The quantum physicists of MIPT decided to check if the time could spontaneously reverse, at least for a given particle and for a fraction of a second. That is, instead of colliding with billiard balls, they examined a solitary electron in an empty interstellar space.

"Suppose the electron is localized when we start to observe it, that means we are pretty sure of its position in space.The laws of quantum mechanics prevent us from knowing it with a absolute precision, but we can define a small region where the electron is located, "said Andrey Lebedev, co-author of the study, MIPT and ETH Zurich.

The physicist explains that the evolution of the electronic state is governed by the Schrödinger equation. Although this makes no distinction between the future and the past, the region of the space containing the electron will spread very rapidly. That is, the system tends to become more chaotic. The uncertainty of the position of the electron increases. This is analogous to the growing disorder in a large-scale system – such as a billiard table – because of the second law of thermodynamics.

"However, the Schrödinger equation is reversible," adds Valerii Vinokur, co-author of the article, from Argonne National Laboratory, USA. "Mathematically, this means that under a certain transformation called complex conjugation, the equation will describe a" smeared. "Electron locating itself in a small region of space over the same period." Although this phenomenon is not not observed in nature, it could theoretically occur due to a random fluctuation of the microwave cosmic microwave background of the universe.

The team undertook to calculate the probability of observing an electron "spread out" over a fraction of a second, locating itself spontaneously in its recent past. It has turned out that even during the entire life of the universe – 13.7 billion years – by observing 10 billion freshly located electrons every second, the l & # 39; Reverse evolution of the particle state would occur only once. And even in this case, the electron would not travel more than a ten billionth of a second in the past.

The large – scale phenomena involving billiard balls and volcanoes obviously take place on much larger time scales and involve an astonishing number of electrons and other particles. This explains why we do not observe rejuvenating older people or ink stains separating from paper.

Reverse time on request

The researchers then tried to reverse time as part of a four-step experiment. Instead of an electron, they observed the state of a quantum computer composed of two and more than three basic elements called superconducting qubits.

• Step 1: Order. Each qubit is initialized to the ground state, denoted zero. This very orderly configuration corresponds to an electron located in a small area or a rack of billiard balls before the break.
• Step 2: Degradation. The order is lost. Just as the electron is spread over a larger and larger area where the frame is broken on the pool table, the state of the qubits becomes an increasingly complex pattern of zeros and voids. This is achieved by briefly launching the evolution program on the quantum computer. In reality, a similar degradation would occur of itself due to interactions with the environment. However, the controlled autonomous evolution program will allow the last stage of the experiment.
• Step 3: Time inversion A special program alters the state of the quantum computer so that it then evolves "backwards" from chaos to order. This operation is apparent to the random fluctuation of the microwave background in the case of the electron, but this time it is deliberately induced. An obviously laudable analogy for the example of billiards would be to give the table a perfectly calculated kick.
• Step 4: Regeneration. The evolution program of the second stage is relaunched. Provided that the kick was successfully given, the program does not cause more chaos but rather brings back the status of the qubits in the past, the way a smeared electron would be located or the billiard balls would trace their trajectories back to the reading, eventually forming a triangle.

The researchers found that in 85% of cases, the two qubit quantum computer had returned to the initial state. When three qubits were involved, other errors occurred, resulting in a success rate of about 50%. According to the authors, these errors are due to imperfections in the real quantum computer. As more sophisticated devices are designed, the error rate should decrease.

It is interesting to note that the time inversion algorithm itself might be useful in making quantum computers more accurate. "Our algorithm could be updated and used to test programs written for quantum computers and eliminate noise and errors," explained Lebedev.

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More information:
Scientific reports (2019). DOI: 10.1038 / s41598-019-40765-6