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A group of enterprising quantum physicists have managed to reverse time for a single particle challenging our perception that time’s arrow runs only in one direction.

Researchers from across the globe — including institutes in Moscow, the US and Switzerland — have collaborated to reverse time by winding the state of a quantum computer back into the past by a fraction of a second. They also calculated the probability that an electron in empty space will travel back into its recent past.

The study is part of a series of papers that explore the possibility of violating the second law of thermodynamics and the closely linked notion that time’s arrow points only from the past to the future.

Gordey Lesovik, head of the Laboratory of the Physics of Quantum Information Technology at the Moscow Institute of Physics and lead author of the paper, states: “ We began by describing a so-called local perpetual motion machine of the second kind. Then, in December, we published a paper that discusses the violation of the second law via a device called Maxwell’s demon.

“The most recent paper approaches 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.”

Distinguishing the future from the past

Most laws of physics actually make no real distinction between the future and the past. For example, if an equation describes the collision and rebound of two identical billiard balls, it will still describe the event played in reverse. If the event was recorded, you could not tell from the recording which direction of play represented the event as it actually happened.

Both versions look plausible. It would appear that the billiard balls defy the intuitive sense of time.

The four stages of the actual experiment on a quantum computer mirror the stages of the thought experiment involving an electron in space and the imaginary analogy with billiard balls. Each of the three systems initially evolves from order toward chaos, but then a perfectly timed external disturbance reverses this process (@tsarcyanide/MIPT Press Office)

However, imagine that someone has recorded a cue ball breaking the pyramid, the billiard balls scattering in all directions. One need not know the rules of the game to tell the real-life scenario from reverse playback. What makes the latter look so absurd is our intuitive understanding of the second law of thermodynamics — an isolated system either remains static or evolves toward a state of chaos rather than order.

Most other laws of physics do not prevent rolling billiard balls from assembling into a pyramid, or tea from flowing back into the tea bag, or a volcano from “erupting” in reverse.

Obviously, we don’t see any of this happening. That is because to do so would require an isolated system to assume a more ordered state without any outside intervention, something which runs contrary to the second law.

The nature of that law has not been explained in full detail, but researchers have made great headway in understanding the basic principles behind it.

To reverse time–think small

Rather than concern themselves with billiard balls or volcanoes, what the physicists from MIPT tried to check was if time could spontaneously reverse itself, for an individual particle and for the tiniest fraction of a second. A solitary electron in empty interstellar space.

Andrey Lebedev, the study’s co-author from MIPT and ETH Zurich says: “Suppose the electron is localized when we begin observing it. This means that we’re pretty sure about its position in space.

“The laws of quantum mechanics prevent us from knowing it with absolute precision, but we can outline a small region where the electron is localized.”

The evolution of the electron state is governed by the time-independent Schrödinger’s equation, which makes no distinction between the future and the past, the region of space containing the electron will spread out very quickly — becoming more chaotic. The uncertainty of the electron’s position is growing. This is analogous to the increasing disorder in a large-scale system — such as a billiard table — due to the second law of thermodynamics.

Valerii Vinokur, a co-author of the paper, from the Argonne National Laboratory, US, adds: “However, Schrödinger’s equation is reversible. Mathematically, it means that under a certain transformation, called complex conjugation, the equation will describe a ‘smeared’ electron localizing back into a small region of space over the same time period.”

the time-independent Schrodinger’s equation

This phenomenon is not observed in nature, but it could, in theory, happen due to a random fluctuation in the cosmic microwave background found distributed isotropically — evenly — throughout the Universe.

The team then set out to calculate the probability of observing an electron “smeared out” over a fraction of a second spontaneously localizing into its recent past. It turned out that even if one spent the entire lifetime of the universe — 13.7 billion years — observing 10 billion freshly localized electrons every second, the reverse evolution of the particle’s state would only happen once. And even then, the electron would travel no more than a mere one ten-billionth of a second into the past.

Large-scale phenomena involving billiard balls, volcanoes, etc. obviously unfold on much greater timescales and feature an astounding number of electrons and other particles. This explains why we do not observe old people growing younger or an ink blot separating from the paper.

Can we reverse time on-demand?

The researchers then attempted to reverse time in a four-stage experiment. Instead of an electron, they observed the state of a quantum computer made of two and later three basic elements called superconducting qubits.

The researchers found that in 85% of the cases the two-qubit quantum computer indeed returned back into the initial state.

When three qubits were involved, more errors happened, resulting in a roughly 50% success rate. According to the authors, these errors are due to imperfections in the actual quantum computer. As more sophisticated devices are designed, the error rate is expected to drop.

Interestingly, the time reversal algorithm itself could prove useful for making quantum computers more precise. Lebedev explains: “Our algorithm could be updated and used to test programs written for quantum computers and eliminate noise and errors.”

Despite these promising results, it is still early days for the process and the robust nature of the second law of thermodynamics means that many repetitions of this study and variations upon it would be required before it is considered sound.

Another alternative may be that this is some, hitherto unknown, quirk in quantum theory.

Ironically, only time will tell.

Original research https://journals.aps.org/prb/abstract/10.1103/PhysRevB.98.214502





















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