A ‘quantum time flip’? Scientist explains how light can travel back and forth in time

Coincidentally, two separate teams of scientists worked simultaneously on a similar problem and released their findings around the same time this year — on Oct. 31 and Nov. 2. Unlike the photons they were studying, the two teams were working in the same direction.

We reached out to Teodor Strömberg, lead author of the second paper, titled 'Experimental superposition of time directions' to see what he could tell us about how his team’s experiment worked and how their findings could be applied to various fields, including the potentially revolutionary world of quantum computing.

What does light traveling back and forth in time simultaneously actually mean?

For those uninitiated in the world of quantum mechanics, the idea of a particle of light simultaneously existing in two different time states likely sounds bizarre, to say the least. It’s important to note, however, that what we’re not talking about a photon literally traveling back in time. “I want to clarify what we did in the experiment, because one needs to be a bit more precise than simply saying “traveling backward in time,” Strömberg told IE. 

To make the experiment possible, Strömberg and his team made use of quantum superposition and charge, parity, and time-reversal (CPT) symmetry, which are principles of quantum mechanics. “The light wasn't traveling into the past,” he emphasized. “Instead, a quantum state that was encoded in a degree of freedom of a single photon (in this case the polarization) evolved in a superposition of time evolutions, where one was the time reverse of the other.”

A good analogy, Strömberg explained, would be a “tennis ball flying through the air in a superposition of the forwards and backwards direction,” or “an object that is rotating in a superposition of clockwise and counterclockwise rotations.” 

“Both of these analogies are examples of processes where one is the time reverse of the other,” Strömberg continued, “but one can't definitively say which one is the forward time direction, and which is the backward time direction (imagine that I show you a movie of a tennis ball flying over the net, or an object spinning, you wouldn't be able to tell me if the movie was being played forwards or backwards). To get a bit more technical, this is because the processes we implemented conserve entropy.”

‘Quantum time flip’ scientists gamify quantum superposition

Superposition is a quantum principle that describes the fact that a quantum system, such as an atom, can exist in two states at the same time until it is observed. 

It is famously described by the thought experiment Schrödinger's cat, in which a hypothetical cat is considered simultaneously alive and dead due to the fact that its state is determined by a random subatomic event that both takes place and doesn't take place until observed. 

Quite aptly, then, Strömberg’s team utilized their own thought experiment as part of their research into the superposition of time direction. 

The researchers "considered a sequence of two distinct time evolutions," Strömberg explained. "In the example of a spinning object, this would correspond to first spinning around one axis for some time, and then spinning around another."

They then formulated a game to determine whether the combined effect of the two rotations changes depending on the spinning direction.

"Under the restriction that the player can only let the object be rotated once, it turns out that you can't always successfully answer this question," Strömberg said. "However, if the player is able to let the object be rotated in a superposition, where the two constituent rotations have opposite relative time direction, then this property can be ascertained with 100% success probability."

“What we did experimentally,” he continued, “was to show that we could get really close to 100%, and showed mathematically that any physical process in which the state wasn't evolving in a superposition of relative time directions could at best have achieved a success probability of around 90%. We could therefore rule out such an explanation to the experiment.”

New findings are a "motivating example for exploring models of quantum computation"

The team’s findings could allow for faster and more efficient processing in quantum computing, as they demonstrated that they can solve computational problems more efficiently by utilizing a method that falls outside the standard model for quantum computing frameworks.

“When working on quantum computation one typically does it within a framework that abstracts away the physical hardware, and distills the relevant concepts,” Strömberg explained. 

The most widely-used framework to describe quantum computation is the quantum circuit model, which is made up of sequences of quantum gates operating on qubits — the quantum computing equivalent of a bit in classical computing. 

“The quantum circuit model has some rules for how quantum information can be manipulated, and these in turn lead to some restrictions on what is and isn't possible,” Strömberg said. “The significance of our work in this context is that we define a task that can be understood as a computational problem, and we show that we can solve it more efficiently using the process we demonstrate in the lab.”

The lead researcher emphasized that this process is “allowed by quantum mechanics, but it is not allowed within the quantum circuit model. This is analogous to indefinite causal processes, which can also speed up certain tasks, and fall outside the quantum circuit model. Our work, therefore, serves as a motivating example for exploring models of quantum computation more general than the standard framework.”

Next, Strömberg explained that he and his team want to see “whether the particular advantage we observe in our computational "game" can translate into an advantage in a task that has more concrete applications” or “systems with more qubits”.