As a general rule, if you want to see something, you need light. You are reading these lines right now only because of the light from the screen, which is transmitted to the retina, converted into electrical signals and sent down the optic nerve so that the brain interprets it as a bunch of words and pictures. But what if you could see things without this complicated process? It might seem impossible, but thanks to the bizarre world of quantum mechanics, this is actually perfectly possible.

“Since the dawn of quantum mechanics, the quest to understand measurements has been a rich source of intellectual fascination. Non-interaction measurements belong to the class of quantum hypothesis testing, where the existence of an event (eg, the presence of a target in a region of space) is assessed. Here… the task is to detect the presence of a microwave pulse… (so that) at the end of the protocol the detector has not irreversibly absorbed the pulse,” says a new paper published in the journal Nature Communications.

In other words, find a way to “see” a microwave pulse without using a single photon.

The experiment is based on the project of one of the laureates of the 2022 Nobel Prize in Physics

The team at Finland’s Aalto University behind the new work would not be the first to achieve such a feat. In fact, the researchers’ experiment is based on one originally performed by Anton Zeilinger, one of the 2022 Nobel Prize laureates in physics. But there was one crucial difference: Zeilinger had worked with lasers and mirrors rather than microwaves and superconductors.

For this reason, “we had to adapt the concept to the various experimental tools available for superconducting devices”, explained the co-author of the study, Gheorghe Sorin Paraoanu, in a statement, according to IFLScience.

Instead of light particles, the team used specially modified transmons, a type of superconducting qubit designed in 2007 to detect the presence of microwave pulses.

“(We) had to modify the standard interaction-free protocol in a crucial way: we added another layer of quantumness by using a higher energy level of the transmon. Then, we used the quantum coherence of the resulting three-level system as a resource,” said Paraoanu.

What is quantum coherence?

“Quantum coherence” refers to that special property that makes quantum mechanics so complicated. This is the paradox of Schrödinger’s Cat, which is the ability of objects to occupy two different states at the same time, even though, according to the rules of classical physics, this should be impossible. However, the quantum world has no such problems with superpositions, and the team managed to not only work with this effect, but also use it to their advantage.

The experiment was a success, and theoretical models confirmed the results.

“We also demonstrated that even very low power microwave pulses can be efficiently detected using our protocol,” added Shruti Dogra, co-author of the paper.

A result with varied applications

All of this might make you think that the discovery, while great, seems a little niche. But here’s the interesting part: this result has much wider applications than a cute demonstration of quantum weirdness.

“In quantum computing, our method could be applied to diagnose microwave-photon states in certain memory elements. This can be considered a very efficient way to extract information without disrupting the operation of the quantum processor,” Paraoanu emphasized.

Meanwhile, the team is already looking at other implications of their findings, such as applications such as counterfactual communication, i.e. a communication between two parties where no physical particles are transferred, and counterfactual quantum computing, where calculations can yield results without the intervention of a computer.

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