The ‘Flipping of Time’ (and Not) by Quantum Physicists

A quantum physicist at the University of Glasgow in Scotland who was not involved in the study said, “For the first time ever, we kind of have a time-traveling machine going in both directions.

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

In either case, the puzzling event might inspire new developments in quantum technology.

“You could imagine circuits in which your knowledge could travel both directions,” said Giulia Rubino, a researcher at the University of Bristol.

Everything at Once, Whenever

Quantum Physicists

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

This is the essence of quantum strangeness: A particle will always be found in a single, point-like location when you search for it. 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 possible positions, in this indeterminate condition.

Giulio Chiribella, a physicist who is currently affiliated with the University of Hong Kong, and coauthors presented a circuit in a 2013 study that would superimpose events’ temporal ordering, going beyond the superposition of their spatial positions. Rubino and her coworkers directly experimentally 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 event 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 indefinitely in both directions, from the past to the future.

Chiribella and Liu discovered they needed 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 success 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 through A and B, two crystal devices. The amount by which a photon’s polarisation rotates while it moves forward through a device relies on the settings for the device. When a signal travels backward through the device, the polarisation rotates in the exact opposite direction.

A referee covertly sets the devices in one of two ways before each gaming round: The player must determine which choice the referee made: the path ahead through A, then backward through B will either move a photon’s wave function relative to the time-reversed path (backward through A, then forward through B), or it won’t. The player then sends a photon through the maze, possibly splitting it into a superposition of two routes using a half-silvered mirror, after arranging the devices and other optical components however they like. 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 if A and B are in an infinite causal chain, the detector’s click will, at best, around 90% of the time, match the secret gadget settings when the circuit is set up so that the photon flows in only one route through each gadget. Only when the photon experiences a superposition that transports it forward and backward through both gadgets—a method nicknamed the “quantum time flip”—can the player conceivably win every round.

A team led by Chiribella in Hefei, China, and one led by the physicist aslav Brukner in Vienna established quantum time-flip circuits last year. The Vienna team correctly predicted outcomes 99.45 percent of the time across a million rounds. Chiribella’s team triumphed in 99.6% of its rounds. In order to demonstrate that their photons underwent a superposition of two opposing transformations and, as a result, an infinite arrow of time, both teams broke the theoretical 90 percent 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. Assembling the fabric of spacetime into a superposition of two geometries with opposing time axes would be necessary to actually flip it. The reversal of the arrow of time is obviously not being implemented in the experiment, he observed.

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 spacetime geometries. Therefore, there is no reality outside of what can be measured in the quantum world. There is no distinction between the simulation and the real thing, according to the state itself, he claimed.

He acknowledges that only photons undergoing polarisation changes can be time-flipped by the circuit; otherwise, if spacetime were truly in a superposition, competing time directions would have an impact on every object.

Circuits with two arrows

Physicists, regardless of their philosophical leanings, anticipate that the capacity to create quantum circuits that run in two 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, remarked that this gives you more options than simply carrying out the processes in a particular order.

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

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 highly fabricated in order to show their superiority to one-way circuits.

Yet, strange, seemingly specialised quantum occurrences frequently turn out to be helpful. The renowned physicist Anton Zeilinger once held the opinion that the connection between separated particles known as quantum entanglement was useless. Nowadays, 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.