Turnover time! Quantum time machines actually already exist. They are two-way and cannot carry people.

If someone told you that there is a time machine now, or the kind that can transmit in both directions, and can flip the past and the future, would you believe it?

In fact, this "time machine" has been studied in scientists' laboratories for many days, but its passengers are not humans, but particles.

More precisely, photons. Just like when humans transform into werewolves, werewolves transform into humans. In a carefully designed circuit, these photons behave like time flowing in forward and backward quantum combinations.

Sonja Franke-Arnold, a quantum physicist at the University of Glasgow in Scotland, said: "This is the first time in history that we have a similar two-way machine. Time travel machines.

Sadly for science fiction fans, these devices have nothing in common with the 1982 DeLorean. Throughout the experiment conducted by the team, the laboratory clock continued to beat steadily forward.

Only the photons flying through the circuit experienced strange changes in time. Moreover, the researchers also Debating whether this "flip of time's arrow" is real or simulated.

However, this puzzling phenomenon may lead to the emergence of new quantum technologies. ​

Subversion of the concept of time

Ten years ago, physicists It was first realized years ago that the strange rules of quantum mechanics overturned the common sense concept of "time".

Is such that. When you look for a particle, you always detect it in a single, point-like location.

But before being measured, a particle behaves more like a wave, manifesting as a "wave function" that spreads and vibrates in multiple routes. In this undetermined state, the particles exist in a quantum state of possible positions called a "superposition."

In a paper published in 2013, physicist Giulio Chiribella, currently working at the University of Hong Kong, and others proposed a circuit that Events can be put into a chronological overlay, which is a step further than a positional overlay in space.

Four years later, Rubino and her colleagues demonstrated this idea directly experimentally. They sent a photon into the superposition of two paths:

In one path, the photon first experienced event A, then event B, and in the other path, the photon experienced event B first. , and then experiences event A. In a sense, each event seemed to cause another event, a phenomenon that came to be known as indeterminate causation.

Not content with just disrupting the order of events as time marches forward, Chilibela and his colleagues next aimed at the direction of travel of time itself, or the arrow. . They seek a quantum instrument in which time enters a superposition from the past to the future and vice versa - an indeterminate arrow of time.

To do this, the researchers realized they needed a system that could make opposite changes, like a metronome whose arms can swing left or right.

They envision such a system being put into a superposition state, like a musician turning right and left simultaneously It's like turning on a "quantum metronome".

After the idea was proposed, optical wizards immediately began to build models in the laboratory. Last fall, both teams announced successful builds. ​

「Two-dot」game

Researchers have designed a game that only quantum ambidextrous players can play. Playing this game with photons requires firing photons into two crystal gadgets, A and B. Passing the gadget backward causes the polarization to rotate in exactly the opposite way.

Before each round of play, the "referee" secretly sets the gadget to one of two ways. A path forward through A and then backward through B will cause the photon's wave function to shift relative to a time-reversed path (backward through A and then forward through B), but not vice versa.

In this game, players must figure out which choice the referee made. After players arrange their gadgets and other optics however they want, send a photon through the maze.

The photon will eventually appear in one of two detectors. If the player sets up their maze in a clever enough way, the click of a detector holding a photon will reveal the judge's choice.

When the player sets up the circuit so that photons move in only one direction in each gadget, then, even if the causal order of A and B is uncertain, the detector's clicks will only be approximately Matches the secret gadget's settings 90% of the time.

Only when the photon undergoes a superposition, causing it to pass forward and backward through the two gadgets (a phenomenon known as a "quantum time flip"), can Win every round of experiments in theory.

Last year, two teams located in Hefei, China and Vienna, Austria, both built their own "quantum time flip" circuits. After 1 million rounds of testing, the Vienna team increased the game's success rate to 99.45%. The other team won 99.6% of the rounds.

Both results break 90% of the theoretical limits and prove that the photon in the experimental model has experienced the superposition of two opposing transformations, so the arrow representing the direction of time is uncertain. of. ​

Interpretation of "Time Reversal"

Although researchers have performed and named Quantum Time Flip, but they don’t all agree on which word best embodies their work.

In Chilibela’s view, these experiments simulate the reversal of the “arrow of time”. In fact, a true flip requires arranging the space-time structure itself into a superposition of two geometric shapes, in which time points in different directions.

He said: "Obviously, from this perspective, this experiment did not achieve a true time reversal."

Another team It is believed that the greatest significance of these circuits is that they have taken an important step in simulating space and time. The researchers say that the photons' measurable properties change exactly as they would if they passed through a true superposition of two space-time geometries.

And in the quantum world, there is no reality outside of measurable things. "In other words, from the state itself, there is no difference between the simulation and the real thing."​

No inversion time? That's okay

Regardless, physicists hope that the ability to design quantum circuits that flow in two ways at the same time may lead to new devices for quantum computing, communications, and metrology. become possible.

"This allows you to do more than just operate in a sequence," says Cyril Branciard, a quantum information theorist at the Néel Institute in France.

Some researchers speculate that the time-travel flavor of quantum time flipping may make future quantum "undoing" possible. Others anticipate that circuits operating in both directions simultaneously could allow quantum machines to operate more efficiently.

Some researchers said: "This model can be used in games to reduce the so-called query complexity." He refers to the number of steps required to perform certain tasks.

Such practical applications are far from guaranteed. Although time-flip circuits broke the theoretical performance limits in Chiribella and Liu's guessing game, that was a highly engineered task that only served to highlight their advantages over one-way circuits and was still far from practical application.

But weird, seemingly niche quantum phenomena have a knack for proving themselves useful. Renowned physicist Anton Zielinger once argued that quantum entanglement - the connection between separated particles - brings nothing good.

Today, quantum entanglement links nodes in nascent quantum networks to qubits in prototype quantum computers, and Zieringer’s study of this phenomenon has earned him recognition won the 2022 Nobel Prize in Physics. The issue of quantum time reversibility is still at a very early stage.

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