Normally, we have to interact with an item in some way in order to measure it. It's almost impossible to see without touching, whether it's by a prod or a poke, an echo of sound waves, or a shower of light.
There are a few exceptions to this rule in the field of quantum physics, claims a study published in Nature Communications.
Aalto University researchers in Finland suggest a method for "seeing" a microwave pulse without any light waves being absorbed and reemitted. It is an illustration of a unique measurement type known as interaction-free measurement, in which the object being viewed is not disturbed by a mediating particle.
The basic idea of "seeing without touching" is not new. By splitting beautifully aligned waves of light down various paths and comparing their trips, physicists have demonstrated that it is possible to exploit the wave-like nature of light to traverse places without evoking its particle-like behaviour.
It is a distinct accomplishment because the team used microwaves and semiconductors rather than lasers and mirrors. A transmon device was utilised in the setup to find an electromagnetic wave blasted into a chamber.
These devices use a superconducting circuit to replicate the quantum behaviour of individual particles on many levels, despite being relatively massive by quantum standards.
"The interaction-free measurement is a fundamental quantum effect whereby the presence of a photosensitive object is determined without irreversible photon absorption," wrote the researchers in their published paper.
"Here we propose the concept of coherent interaction-free detection and demonstrate it experimentally using a three-level superconducting transmon circuit."
To make the complicated setup work, the scientists relied on the quantum coherence generated by their custom system, which allows items to existing in two states simultaneously, like Schrödinger's cat.
"We had to adapt the concept to the different experimental tools available for superconducting devices," Science Alert quoted quantum physicist Gheorghe Sorin Paraoanu, from Aalto University, as saying.
"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."
The team's experiments were supported by theoretical models that verified the findings. It is an illustration of the quantum advantage, which refers to the capacity of quantum devices to outperform those of classical devices.
Touching something in the fragile world of quantum physics is like breaking it. Nothing like the crunch of reality can destroy a tidy wave of possibility. Alternative sensing techniques, like this one, may be useful in situations where detection requires a softer touch.
Quantum computing, optical imaging, noise detection, and cryptographic key distribution are some areas in which this technique can be used. Each time, the mechanisms involved would be much more effective.
"In quantum computing, our method could be applied for diagnosing microwave-photon states in certain memory elements," says Paraoanu. "This can be regarded as a highly efficient way of extracting information without disturbing the functioning of the quantum processor."