What Happened When Quantum Physicists "Flipped Time"—and What Didn't

 By Charlie Wood

January 27, 2023

Scientists have forced light particles to go through opposing changes at the same time, much like a person changing into a werewolf and the werewolf changing into a human. The photons behave as though time were moving in a quantum mix of forward and backward in precisely designed circuits.

For the first time ever, according to Sonja Franke-Arnold, a quantum physicist at the University of Glasgow in Scotland who was not involved in the study, "we kind of have a time-traveling machine moving in both directions."

Unfortunately for science fiction lovers, the gadgets are completely unrelated to a DeLorean from 1982. Laboratory clocks kept moving ahead consistently during the studies, which were carried out by two different teams in China and Austria. The only particles that had temporal hiccups were the photons fluttering across the circuitry. Researchers disagree about whether the time-arrow flipping observed in photons is real or artificial.

In any case, the puzzling event may inspire new developments in quantum technology.

According to Giulia Rubino, a researcher at the University of Bristol, "you might imagine circuits in which your information could travel both ways."

Everything at Once, Anytime

It took physicists a decade to understand that conventional ideas of time are contradicted by the bizarre laws of quantum physics.

In order to find a particle, you must always seek in a single, pointlike spot, which is the core of quantum weirdness. A particle behaves more like a wave before being measured, with a "wave function" that spreads out and ripples along many paths. A particle is in a superposition, or quantum combination of potential positions, in this indeterminate condition.

Giulio Chiribella, a physicist who is currently affiliated with the University of Hong Kong, and co-authors presented a circuit in a 2013 study that would superimpose temporal ordering on occurrences, going beyond the superposition of spatial locations. Rubino and her coworkers immediately empirically proved the concept four years later. A photon was dispatched down a superposition of two paths: one along which it first encountered event A, followed by event B, and another along which it encountered B, then A. Each occurrence appeared to have some causal relationship with the others, leading to the term "indefinite causality" for this situation.

Chiribella and a colleague, Zixuan Liu, decided to target the arrow, or marching direction, of time itself since they were not satisfied with altering only the sequence of events as time moved on. They were looking for a quantum device that would allow time to flow endlessly in both directions, from the past to the future.

Chiribella and Liu discovered they required a mechanism that could experience opposing changes, similar to a metronome with a swinging arm to the left or right, to accomplish this. They envisioned superpositioning such a system, analogous to a musician simultaneously moving a quantum metronome to the right and left. They provided details on a plan to implement such a system by 2020.

Optics wizards started building duelling arrows of time in the lab right away. Two teams announced triumph in the fall.

A Game With Two Times

A game that could only be won by a quantum two-timer was created by Chiribella and Liu. In order to play the light game, photons must be fired via A and B, two crystal devices. The amount by which a photon's polarisation spins while it moves forward through a device relies on the parameters for the device. When a signal travels backward via the device, the polarisation spins in the exact opposite direction.

A referee covertly sets the devices in one of two ways before each gaming round: The wave function of a photon travelling through A and B in reverse order will either change in relation to the time-reversed path (backward via A and then forward through B), or it won't. The athlete has to determine the decision the referee made. The player then sends a photon through the maze, perhaps dividing it into a superposition of two routes using a half-silvered mirror, after arranging the devices and other optical components however they choose. One of two detectors receives the photon in the end. The detector that holds the photon will click, revealing the referee's decision whether the player has constructed their maze in a sophisticated enough manner.

Even though A and B are in an infinite causal chain, the detector's click will, at best, match the secret gadget settings 90% of the time when the player configures the circuit so that the photon flows in only one direction through each gadget. The player can theoretically win every round only when the photon undergoes a superposition that sends it forward and backward through both devices, a strategy known as the "quantum time flip."

A team in Hefei, China, under Chiribella's guidance, and one in Vienna under the guidance of scientist Saslav Brukner, established quantum time-flip circuits last year. The Vienna team accurately predicted the outcome 99.45% of the time over a million rounds. The team led by Chiribella won 99.6% of its games. By demonstrating that their photons underwent a superposition of two opposing transformations and, as a result, an infinite arrow of time, both teams exceeded the theoretical 90% limit.

Understanding the Time Switch

Although the researchers have carried out and called the quantum time flip, they disagree on the exact phrases that best describe what they have done.

Chiribella believes that the studies have mimicked the arrow of time flipping. To actually flip it, the space-time fabric would have to be set up in a superposition of two geometries with opposing time axes. The experiment, he observed, "clearly is not executing the reversal of the arrow of time."

Brukner, on the other hand, believes that the circuits only go a little beyond simulation. He draws attention to the fact that the photons' measurably changing characteristics closely match what would happen if they travelled through a real superposition of two space-time geometries. Furthermore, there is no reality outside of what can be measured in the quantum realm. There is no distinction between the simulation and the actual thing, according to the state itself, he claimed.

He acknowledges that only photons experiencing polarisation changes may be time-flipped by the circuit; otherwise, everything would be affected by the conflicting time directions if space-time were indeed in a superposition.

Circuits with two arrows

Whatever their philosophical preferences, physicists are hopeful that the capacity to create quantum circuits that can flow in both directions simultaneously will lead to the development of novel quantum computing, communication, and metrology devices.

Cyril Branciard, a quantum information theorist at the Néel Institute in France, said: "This allows you to do more things than merely apply the operations in one sequence or another."

The Laws of Cause and Effect are Rewritten by Quantum Mischief

Is Time Really Moving? New Insights from a Century-Old Mathematical Method.

Thermodynamic Clocks: A New Perspective

Some academics think that a future quantum "undo" function could be made possible by the time-travel aspect of the quantum time flip. Others believe that circuits that operate in two directions simultaneously might improve the performance of quantum machines. Rubino stated, "You might use this for games where you want to lower the so-called query complexity," which is the quantity of steps necessary to complete a job.

Such real-world uses cannot be taken for granted. While the time-flip circuits in Chiribella and Liu's guessing game exceeded a theoretical performance limit, that challenge was heavily fabricated in order to show their superiority versus one-way circuits.

However, strange, apparently specialised quantum occurrences frequently turn out to be helpful. The renowned physicist Anton Zeilinger once thought that the link between separated particles known as quantum entanglement was useless. Today, entanglement connects qubits in experimental quantum computers and nodes in developing quantum networks. Zeilinger shared the 2022 Nobel Prize in Physics for his research on the phenomenon. It's still extremely early for the flippable character of quantum time, according to Franke-Arnold.

Under the terms of a Creative Commons licence, this article has been taken from Quantmagazine. Go here to read the original article.