They managed to stop time with a quantum computer – ElSol.com.ar – Diario de Mendoza, Argentina

As part of a laboratory experiment, a group of scientists managed to reverse the state of a quantum computer a split second back. In the study, published in Scientific Reports, they also calculated the probability that an electron in an empty interstellar space will spontaneously return to its recent past.

"This is part of a series of articles on the possibility of violating the second law of thermodynamics, which is closely related to the notion of time arrow that postulates the direction of time in a unique sense of the past. in the future."explains the lead author of the study, Gordey Lesovik, who heads the quantum information technology physics lab at MIPT (Moscow Institute of Physics and Technology).

"We started by describing a so-called local perpetual motion machine, and then, in December, we published a document badyzing the violation of the second law by means of a device called Maxwell's demon," says Lesovik. . "The last article tackles the same problem from a third angle: "We artificially created a state that moves in a direction opposite to that of the thermodynamic arrow of time."

Most laws of physics make no distinction between the future and the past. The quantum physicists of MIPT decided to check if the time could spontaneously return at least for an individual particle and a small fraction of a second. They examined a solitary electron in an empty interstellar space.

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

The physicist explains that the evolution of the electronic state is governed by the Schrödinger equation. Although it makes no distinction between the future and the past, the region of space containing the electron will grow very rapidly. That is, the system tends to become more chaotic. The uncertainty of the position of the electron increases. This is badogous 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 the location of an electron in a small region of space during the same period. "

Although this phenomenon is not observed in nature, it could theoretically occur due to a random fluctuation of the cosmic microwave background that permeates the universe. The team set out to calculate the probability of observing an electron "spotted" in a fraction of a second, locating itself spontaneously in its recent past.

It turned out that even throughout the life of the universe (13.7 billion years), observing 10 billion electrons located every second, the evolution inverse of the state of the particle 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 researchers tried to reverse the time in a four-step experiment. Instead of an electron, they observed the state of a quantum computer formed of two and three basic elements called superconducting qubits.

Step 1: The command Each qubit is initialized to the ground state, denoted zero. This highly ordered configuration corresponds to an electron located in a small region.

2nd step: Degradation The order is lost. Just as the electron is colored in a region of increasing space, the state of the qubits becomes a changing pattern of increasingly complex zeros and zeros. This is achieved by briefly applying the evolution program in the quantum computer. In fact, 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 investment. A special program alters the state of the quantum computer so that it then evolves "backwards" from chaos to order. This operation is similar to the random fluctuation of the microwave background in the case of the electron, but this time it is deliberately induced.

Step 4: Regeneration The program of evolution of the second stage is again applied. As long as this has been done successfully, the program does not cause more chaos, but instead brings back the status of the qubits in the past, the way a stained electron would be located.

The researchers found that in 85% of cases the two qubit quantum computer had returned to its original state. When three qubits were involved, more errors occurred, giving 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. Interestingly, the time inversion algorithm itself could be useful for improving the accuracy of quantum computers. "Our algorithm could be updated and used to test programs written for quantum computers and eliminate noise and errors," says Lebedev.