How Quantum Physicists ‘Reversed Time’ (And Didn’t)


Physicists deny it. Particles of light simultaneously undergo opposite transformations, like a werewolf turning into a human. In carefully designed circuits, photons function as if they flow in a quantum forward and backward combination.

“For the first time, we have a time traveling machine that goes in both directions,” said Sonja Franke-Arnold, a quantum physicist at the University of Glasgow in Scotland who was not involved in the research.

Unfortunately for science fiction fans, the equipment has nothing in common with the 1982 DeLorean. Laboratory clocks have continued to advance steadily throughout the experiments conducted by two independent groups in China and Austria. Only photons flying through the circuit experience transient shenanigans. Even for photons, researchers debate whether the reversal of time’s arrow is real or simulated.

Either way, the perplexing phenomenon could lead to new quantum technology.

“You can think of circuits where your data can go both ways,” says Julia Rubino, a researcher at the University of Bristol.

All at once at any time

Physicists first realized ten years ago that the strange laws of quantum mechanics were time-defying notions of intelligibility.

The essence of quantum weirdness is this: when you look for a particle, you always find it in a point-like location. But before it can be measured, a particle acts like a wave; It has a “wave function” that extends and rotates on several lines. In this indefinite state, a particle exists in a quantum mixture known as a superposition.

In the year In a paper published in 2013, Giulio Chiribella, now a physicist at the University of Hong Kong, and collaborators proposed a circuit that places events on temporal orders, taking precedence over locations in space. Four years later, Rubino and her colleagues demonstrated the idea directly in experiments. Send a photo highlighting two paths: one with event A followed by event B, and another with B followed by A. It is called an indefinite cause.

Not content with just messing around with the sequence of events as time progressed, Chiribella and his colleague Zixuan Liu, next took the direction, or arrow, of the march. They sought a quantum device in which the superspace flows from the past to the future and vice versa—an indefinite arrow of time.

To do this, Chiribella and Liu realized they needed a system that could make counter-changes, like a metronome swinging an arm left or right. They intend to place such a system in a high position, similar to a musician swinging a quantum metronome left and right. They have expressed a plan to establish such a system in 2020.

Optics wizards immediately began building arrows of time in the lab. Last fall, two teams announced success.

Double game

Chiribella and Liu created a game where only one quantum two-timer stood out. Playing the game with light involves shooting photons through two crystal gadgets, A and B. Passing through a gadget rotates the photon’s polarization by an amount that depends on the gadget’s settings. Passing backwards through the device, the polarization rotates in exactly the opposite direction.

Before each round of the game, a judge secretly places the gadgets in one of two ways: forward through A, then through B. Either the photon wave function changes with the time-reversed path (backward through A, then forward through B), or it doesn’t. The player must know which choice the referee made. After the player arranges the gadgets and other optical elements in any way they want, they send a photo through the maze, perhaps using a half-silver mirror to split it into two high-altitude areas. Photon ends up on one of two pointers. If the player has prepared their regrets cleverly enough, the inspector with the photo will reveal the judge’s choice when tapped.



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