The laws of physics seem to be immutable, right? Well, a team of researchers from MIPT, the Moscow Institute of Physics and Technology, (Russia) together with experts from the United States and Switzerland have managed to move a quantum computer a fraction of a second into the past. They also calculated the probability that an electron in empty interstellar space will spontaneously return to its recent past. The results have been published in the journal Scientific Reports.
“This document 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 the arrow of time that postulates the direction of time in only one sense: from past to future , “commented the study’s lead author, Gordey Lesovik, who heads the Quantum Information Technology Physics Laboratory at MIPT.
What does the second law of thermodynamics say?
The second law of thermodynamics is summarized in that the entropy (the measure of disorder of a system) of any isolated system cannot spontaneously decrease, with the system evolving towards a state of maximum entropy or maximum disorder. This second law is what prevents, for example, the existence of perpetual motion machines.
Future different from the past
“We began by describing a local perpetual motion machine of the second type. Then, in December, we published a document that looks at the violation of the second law through a device called a Maxwell’s demon,” Lesovik said. “The most recent article addresses 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 .”
Most of the laws of physics make no distinction between the future and the past. Regarding the second law of thermodynamics, the nature of it has not been explained in detail, but researchers have come a long way in understanding the basic principles that support it.
Spontaneous time reversal
Russian quantum physicists decided to check whether time could be spontaneously reversed for at least an individual particle and a small fraction of a second. They examined a lone electron in empty interstellar space.
“Suppose the electron is located when we begin to observe it. This means that we are quite 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 located.” says Andrey Lebedev, a co-author of the study.
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 that contains the electron will expand very rapidly. That is, the system tends to become more chaotic. The uncertainty of the electron position is growing. This is analogous to the increasing disorder in a large-scale system, such as a pool table, due to the second law of thermodynamics.
“However, the Schrödinger equation is reversible,” adds Valerii Vinokur, co-author of the work. “Mathematically, it means that, under a certain transformation, called complex conjugation, the equation will describe a ‘Location of electrons in a small region of space during the same period of time.” Although this phenomenon is not observed in nature, in theory it could occur due to a random fluctuation in the cosmic microwave background that permeates the universe.
One time
The team set out to calculate the probability of observing a “spotted” electron in a fraction of a second, spontaneously locating itself in its recent past. It turned out that even if one spent the entire life of the universe (13.7 billion years) observing 10 billion localized electrons every second, the reverse evolution of the particle’s state would only occur once. And even then, the electron would not travel more than ten billion seconds into the past.
Large-scale phenomena involving volcanoes, for example, obviously take place on much longer time scales and feature a staggering number of electrons and other particles. This explains why we don’t see older people getting younger or an ink stain separating from the paper.
Investment time on demand
The researchers subsequently attempted to reverse time in a four-stage experiment. Instead of an electron, they looked at the state of a quantum computer made up of two and later three basic elements called superconducting qubits.
Stage 1: Order. Each qubit is initialized in the ground state, denoted as zero, which corresponds to an electron located in a small region.
Stage 2: Degradation. The order is lost. As the electron is spotted in an increasingly large region of space and the state of the qubits becomes an increasingly complex changing pattern of zeros and ones.
Stage 3: Investment of time. A special program modifies the state of the quantum computer in such a way that it 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.
Stage 4: Regeneration . The second stage evolution program is launched again, this does not lead to more chaos, but rewinds the state of the qubits to the past, to the way a stained electron would locate itself.
The researchers found that in 85% of the cases, the two-qubit quantum computer returned to the initial state . When three qubits were involved, more errors occurred, resulting in a success rate of approximately 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 is expected to decrease.
Interestingly, the time inversion algorithm itself could prove 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,” Lebedev explained.
Referencia: Arrow of Time and its Reversal on IBM Quantum Computer . Scientific Reports 2019.
Crédito imagen: @tsarcyanide/MIPT Press Office
