A method for interaction-free measurement that uses quantum coherence to more effectively detect objects without “seeing” them

A new, more effective method has been discovered for interaction-free measurements that detect the presence of an object of interest without directly “seeing” it. This method uses quantum coherence. The study, by Finland’s Aalto University, was published in Nature Communications on December 7, 2022.

We can see the world around us because special cells in the retina of our eyes absorb light. So, is it possible to have “sight” without absorbing any light?

For example, let’s say you have a roll of film in a film camera cartridge. The film is so sensitive that even a single photon hitting the film can sensitize it and render it useless. You can’t know if there’s film in a cartridge in the usual classical way, but in the quantum world you can.

The idea of ​​non-interaction measurement that recognizes the existence and shape of an object without irradiating the target object with light appeared in the 1990s. He focused on his research and discovered a new method for more effective non-interaction experiments.

The new method uses a transmon device, a relatively large but quantum-behavior superconducting circuit, as a measuring instrument to detect the presence of microwave pulses generated by a classical device. A transmon is a type of superconducting qubit.

Because the experimental tools are different from conventional methods using lasers and mirrors, the standard interaction-free experimental protocol had to be modified for this study. So we added a layer of ‘quantumness’ using the high energy levels of the transmon, and used the resulting quantum coherence of the three-level system as a resource.

Quantum coherence is the possibility that an object can occupy two different states at the same time. But quantum coherence is delicate and fragile, so it wasn’t immediately clear whether the new protocol would work.

However, since the first experiments were performed, the detection efficiency has improved significantly. The research team went back and forth and double-checked everything, and the effect was definitely there. Furthermore, it was demonstrated that even very low-power microwave pulses can be efficiently detected using this protocol.

Interaction-free measurements with older, less effective techniques are already being applied to specialized tasks such as optical imaging, noise detection, and cryptographic key distribution. New and improved techniques could dramatically improve the efficiency of these tasks.

For example, in quantum computing, the technique can be applied to diagnose microwave photon states within specific memory devices. This could be a very efficient way of extracting information without interfering with the functioning of quantum processors.