How Quantum Physicists ‘Flipped Time’ (and How They Didn’t)

Physicists have coaxed particles of light into undergoing opposite transformations simultaneously, like a human turning into a werewolf as the werewolf turns into a human. In carefully engineered circuits, the photons act as if time were flowing in a quantum combination of forward and backward.

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

Regrettably for science fiction fans, the devices have nothing in common with a 1982 DeLorean. Throughout the experiments, which were conducted by two independent teams in China and Austria, laboratory clocks continued to tick steadily forward. Only the photons flitting through the circuitry experienced temporal shenanigans. And even for the photons, researchers debate whether the flipping of time’s arrow is real or simulated.

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

“You could conceive of circuits in which your information could flow both ways,” said Giulia Rubino, a researcher at the University of Bristol.

Anything Anytime All at Once

Physicists first realized a decade ago that the strange rules of quantum mechanics topple commonsense notions of time.

The essence of quantum strangeness is this: When you look for a particle, you’ll always detect it in a single, pointlike location. But before being measured, a particle acts more like a wave; it has a “wave function” that spreads out and ripples over multiple routes. In this undetermined state, a particle exists in a quantum blend of possible locations known as a superposition.

In a paper published in 2013, Giulio Chiribella, a physicist now at the University of Hong Kong, and co-authors proposed a circuit that would put events into a superposition of temporal orders, going a step beyond the superposition of locations in space. Four years later, Rubino and her colleagues directly experimentally demonstrated the idea. They sent a photon down a superposition of two paths: one in which it experienced event A and then event B, and another where it experienced B then A. In some sense, each event seemed to cause the other, a phenomenon that came to be called indefinite causality.

Not content to mess merely with the order of events while time marched onward, Chiribella and a colleague, Zixuan Liu, next took aim at the marching direction, or arrow, of time itself. They sought a quantum apparatus in which time entered a superposition of flowing 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 undergo opposite changes, like a metronome whose arm can swing left or right. They imagined putting such a system in a superposition, akin to a musician simultaneously flicking a quantum metronome rightward and leftward. They described a scheme for setting up such a system in 2020.

Optics wizards immediately started constructing dueling arrows of time in the lab. Last fall, two teams declared success.

A Two-Timing Game

Chiribella and Liu had devised a game at which only a quantum two-timer could excel. Playing the game with light involves firing photons through two crystal gadgets, A and B. Passing forward through a gadget rotates a photon’s polarization by an amount that depends on the gadget’s settings. Passing backward through the gadget rotates the polarization in precisely the opposite way.

Before each round of the game, a referee secretly sets the gadgets in one of two ways: The path forward through A, then backward through B, will either shift a photon’s wave function relative to the time-reversed path (backward through A, then forward through B), or it won’t. The player must figure out which choice the referee made. After the player arranges the gadgets and other optical elements however they want, they send a photon through the maze, perhaps splitting it into a superposition of two paths using a half-silvered mirror. The photon ends up at one of two detectors. If the player has set up their maze in a sufficiently clever way, the click of the detector that has the photon will reveal the referee’s choice.

When the player sets up the circuit so that the photon moves in only one direction through each gadget, then even if A and B are in an indefinite causal order, the detector’s click will match the secret gadget settings at most about 90% of the time. Only when the photon experiences a superposition that takes it forward and backward through both gadgets — a tactic dubbed the “quantum time flip” — can the player theoretically win every round.

Last year, a team in Hefei, China advised by Chiribella and one in Vienna advised by the physicist Časlav Brukner set up quantum time-flip circuits. Over 1 million rounds, the Vienna team guessed correctly 99.45% of the time. Chiribella’s group won 99.6% of its rounds. Both teams shattered the theoretical 90% limit, proving that their photons experienced a superposition of two opposing transformations and hence an indefinite arrow of time.

Interpreting the Time Flip

While the researchers have executed and named the quantum time flip, they’re not in perfect agreement regarding which words best capture what they’ve done.

In Chiribella’s eyes, the experiments have simulated a flipping of time’s arrow. Actually flipping it would require arranging the fabric of space-time itself into a superposition of two geometries where time points in different directions. “Obviously, the experiment is not implementing the inversion of the arrow of time,” he said.

Brukner, meanwhile, feels that the circuits take a modest step beyond simulation. He points out that the measurable properties of the photons 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 beyond what can be measured. “From the state itself, there is no difference between the simulation and the real thing,” he said.

Granted, he admits, the circuit can time-flip only photons undergoing polarization changes; if space-time were truly in a superposition, dueling time directions would affect everything.

Two-Arrow Circuits

Whatever their philosophical inclinations, physicists hope that the ability to design quantum circuits that flow two ways at once might enable new devices for quantum computing, communication and metrology.

“This allows you to do more things than just implementing the operations in one order or another,” said Cyril Branciard, a quantum information theorist at the Néel Institute in France.

Some researchers speculate that the time-travel flavor of the quantum time flip might enable a future quantum “undo” function. Others anticipate that circuits operating in two directions at once could allow quantum machines to run more efficiently. “You could use this for games where you want to reduce the so-called query complexity,” Rubino said, referring to the number of steps it takes to carry out some task.

Such practical applications are far from assured. While the time-flip circuits broke a theoretical performance limit in Chiribella and Liu’s guessing game, that was a highly contrived task dreamt up only to highlight their advantage over one-way circuits.

But bizarre, seemingly niche quantum phenomena have a knack for proving useful. The eminent physicist Anton Zeilinger used to believe that quantum entanglement — a link between separated particles — wasn’t good for anything. Today, entanglement threads together nodes in nascent quantum networks and qubits in prototype quantum computers, and Zeilinger’s work on the phenomenon won him a share of the 2022 Nobel Prize in Physics. For the flippable nature of quantum time, Franke-Arnold said, “it’s very early days.”