Physicists discover a new way to “see” things without looking at them

Normally, to measure something, we must react to it in some way. Whether it’s a pulsation or tingling, an echo of sound waves, or a barrage of light, it’s nearly impossible to look without touching.

In the realm of quantum physics, there are some exceptions to this rule.

Researchers from Aalto University in Finland propose a way to “see” a microwave pulse without absorbing and re-emitting any light waves. It is an example of a special interaction-free measurement, where something is observed without being vibrated by an intermediate particle.

The basic concept of “looking without touching” is not new. Physicists have shown that it is possible to use the wave-like nature of light to probe voids without triggering their particle-like behavior by precisely splitting aligned light waves along different paths and then comparing their travels.

instead of Lasers and mirrorsthe team used microwave ovens and semiconductors, which made it a separate achievement. The setup used what is known as a transmitter to detect the pulsed electromagnetic wave in a room.

While these devices are relatively large by quantum standards, they simulate the quantum behavior of individual particles on multiple scales using a superconducting circuit.

“An interaction-free measurement is a fundamental quantum effect in which the presence of a photosensitive object is determined without irreversible photon absorption,” the researchers wrote in their published paper.

“Here we propose and experimentally demonstrate the concept of interaction-free coherent detection using a three-level superconducting transmission circuit.”

The team relied on quantum coherence generated by their custom system — the ability of objects to occupy two different states at the same time, like Schrödinger’s cat — in order to make the complex setup work.

“We had to adapt the concept to the different experimental tools available for superconducting devices,” says quantum physicist Gheorghe Sorin Paraoanu, of Aalto University in Finland.

“Because of that, we also had to change the standard interaction-free protocol in a crucial way: We added another layer of quantum using a higher energy level for transmission. We then used the quantum coherence of the resulting three levels of the system as a resource.”

The team’s experiments were supported by theoretical models that confirm the results. It’s an example of what scientists call the quantum advantage, the ability of quantum devices to surpass what’s possible with classical devices.

In the delicate landscape of quantum physics, touching things is akin to breaking them. Nothing spoils a wonderful wave of possibility like the reality crisis. In cases where detection needs a gentle touch, alternative sensing methods — like this one — can come in handy.

Areas where this protocol can be applied include quantum computing, optical imaging, noise detection, and cryptographic key distribution. In each case, the efficiency of the systems involved will be greatly improved.

“In quantum computing, our method can be applied to characterize microphoton states in some memory elements,” Parawanu says. “This can be seen as a highly efficient method of extracting information without disturbing the function of the quantum processor.”

Research published in Nature Communications.

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