As a general rule, if you want sight, you need light. You’re only reading this right now thanks to the light from your screen being beamed onto your retinas, converted into electrical signals, and sent up the optic nerve for your brain to interpret as a bunch of words and images.
But what if you could see things without all that rigamarole? It might sound impossible – perhaps even counter to the very definition of sight – but thanks to the bizarre world of quantum mechanics, it’s actually perfectly possible.
“Since the inception of quantum mechanics, the quest to understand measurements has been a rich source of intellectual fascination,” notes a new paper published this month.
“The interaction-free measurements belong to the class of quantum hypothesis testing, where the existence of an event (for example the presence of a target in a region of space) is assessed,” it explains. “Here… the task is to detect the presence of a microwave pulse… [such] that at the end of the protocol the detector has not irreversibly absorbed the pulse.”
In other words: find a way to “see” a microwave pulse, without using a single photon.
If successful, the Aalto University team behind the new paper wouldn’t be the first to achieve such a feat – in fact, their experiment was based on one originally performed by Anton Zeilinger, one of the winners of the 2022 Nobel Prize in Physics. But there was one crucial difference: Zeilinger had been working with lasers and mirrors, rather than microwaves and superconductors.
For that reason, “we had to adapt the concept to the different experimental tools available for superconducting devices,” study co-author Gheorghe Sorin Paraoanu explained in a statement. Instead of light particles, the team used specially modified transmons – a type of superconducting qubit designed back in 2007 – to detect the presence of the microwave pulses.
“[We] had to change the standard interaction-free protocol in a crucial way: we added another layer of ‘quantumness’ by using a higher energy level of the transmon,” Paraoanu said. “Then, we used the quantum coherence of the resulting three-level system as a resource.”
“Quantum coherence” refers to that particular property that makes quantum mechanics so confusing. It’s the Schrödinger’s Cat paradox: the ability for objects to occupy two different states at the same time – even though under classical physics rules, that should be impossible. The quantum world, however, has no such problems with superpositions – and the team were able not just to work with this effect, but use it to their advantage.
The experiment was a success – and theoretical models confirmed their results. “We also demonstrated that even very low-power microwave pulses can be detected efficiently using our protocol,” added Shruti Dogra, fellow co-author of the paper.
All of which might leave you thinking, well, that’s cool, but it’s a bit niche, isn’t it? Here’s the kicker, though: this result has applications that range far wider than just a cute little demonstration of quantum weirdness.
“In quantum computing, our method could be applied for diagnosing microwave-photon states in certain memory elements,” Paraoanu pointed out. “This can be regarded as a highly efficient way of extracting information without disturbing the functioning of the quantum processor.”
Meanwhile, the team is already looking at further implications of their findings: applications such as counterfactual communication – that is, communication between two parties in which no physical particles are transferred – and counterfactual quantum computing, where the computations can yield results without the computer itself ever being run.
If that sounds bizarre or nonsensical to you, well, you’re not wrong. But in the quantum world, those kinds of mind-boggling concepts are really just a standard Thursday.
The study is published in the journal Nature Communications.
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