ESPOO, Finland — The word “quantum” has become something of a scientific and cultural buzzword in recent years. Whether we’re talking about quantum physics, the Higgs boson particle, or perhaps the fictitious “quantum realm” seen on the silver screen, these topics capture our imaginations because they dare to question the very nature of the universe and existence itself. Now, Finnish scientists have successfully used quantum mechanics to see objects without actually looking at them.
That last sentence may sound like a head-scratcher, but quantum physics is all about peeling back the layers of the seen and experienced world. Study authors posit this groundbreaking new work helps connect the “classical” and “quantum” worlds, potentially improving measurements in quantum computers and other applications.
To start, it helps to review human vision, in a nutshell. We see our surrounding environments thanks to light being absorbed by specialized cells in our retina. But, ask yourself, can vision happen without any light absorption whatsoever, not even a single particle of light? Interestingly, study authors say the answer to that question is yes.
Often linked to the famous Schrödinger’s cat thought experiment, quantum mechanics are usually best understood via analogy. Imagine for a moment that you possess a camera cartridge that may or may not contain a roll of photographic film. The supposed roll of film is super sensitive, and coming into contact with even a single photon would destroy it. Ordinarily, within our “classical world,” there would be no way of truly knowing if the film exists without ruining it. In the quantum world it’s possible to see the film without really looking at it.
Anton Zeilinger was one of the first scientists to experimentally implement the notion of an interaction-free experiment using optics. He won the 2022 Nobel Prize in Physics. Today, in continuation of that work, Shruti Dogra, John J. McCord, and Gheorghe Sorin Paraoanu of Aalto University have discovered a new and more effective way to carry out these interaction-free experiments.
How did they do it?
The research team used transmon devices, or superconducting circuits that while relatively large still exhibit quantum behavior, to identify the presence of microwave pulses generated by classical instruments. While Zeilinger’s research, of course, made a major impression on Dogra and Paraoanu, the pair of scientists explain that their lab is focused specifically on microwaves and superconductors, as opposed to lasers and mirrors.
“We had to adapt the concept to the different experimental tools available for superconducting devices. Because of that, we also 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. Then, we used the quantum coherence of the resulting three-level system as a resource,” Paraoanu says in a university release.
Quantum coherence refers to the possibility of an object occupying two different states simultaneously. While this complex idea is possible in the quantum world, it is still delicate and easily collapsable. Researchers were unsure if the new protocol would work. However, to their surprise, the very first tests conducted for the experiment showed a marked increase in detection efficiency.
Even after further theoretical models were put together, and everything was double checked, the findings remained consistent. The effect was definitely there. “We also demonstrated that even very low-power microwave pulses can be detected efficiently using our protocol,” Dogra adds.
Breakthrough useful in quantum computing, too
This experiment also demonstrated another new way in which quantum devices are capable of reaching results impossible for “classical” devices; a phenomenon called quantum advantage. Generally, researchers believe achieving quantum advantage will require quantum computers with tons of qubits, but this experiment actually showed quantum advantage via a relatively simpler setup.
Interaction-free measurements based on the earlier, less effective methodology had already noted applications in various specialized processes such as optical imaging, noise-detection, and cryptographic key distribution. This newer and better method may increase the efficiency of all those processes by a wide margin.
“In quantum computing, our method could be applied for diagnosing microwave-photon states in certain memory elements. This can be regarded as a highly efficient way of extracting information without disturbing the functioning of the quantum processor,” Paraoanu explains.
Paraoanu’s research team is also investigating other exotic forms of information processing with their new approach, including counterfactual communication (communications between two parties without any physical particles being transferred) and counterfactual quantum computing (where the result of a computation is gained without actually running the computer).
The study is published in Nature Communications.