Since the dawn of quantum mechanics, the quest to understand measurements has been a rich source of intellectual fascination. The interaction-free measurement is a fundamental quantum effect where the presence of a light-sensitive object is determined without irreversible photon absorption.

In an investigation into the connection between the quantum world and the classical world, scientists at Aalto University have discovered a new and much more effective way of conducting interaction-free experiments. They proposed the concept of coherent interaction-free sensing and demonstrated it experimentally.

They used a three-level superconducting transmon device to detect the presence of microwave pulses generated by classical instruments. The transmon devices are superconducting circuits that are relatively large yet exhibit quantum behavior.

Anton Zeilinger, one of the winners of the 2022 Nobel Prize in Physics, was the first to experimentally implement the idea of ​​an interaction-free experiment using optics.

Gheorghe Sorin Paraoanu from Aalto University said: “We had to adapt the concept to the different experimental tools available for superconducting devices. Therefore, we also had to crucially change the standard interaction-free protocol: we added another layer of “quantum” by using a higher energy level of the transmon. We then used the quantum coherence of the resulting three-level system as a tool.”

Quantum coherence – the possibility that an object can occupy two different states at the same time – is delicate and easily collapses. Therefore, it was not immediately clear that the new protocol would work.

It is surprising to scientists that in their protocol quantum coherence serves as a tool, which gives a considerably high detection probability. The first demonstration of the experiment showed a clear increase in detection efficiency.

experiment protocol
The experiment protocol achieved much higher efficiency than previous methods. Image: John J. McCord/Aalto University.

They went back to the drawing board several times to check everything and run theoretical models. The models confirmed their results. The effect was indeed there.

Shruti Dogra from Aalto University said: “We have also shown that even very low power microwave pulses can be efficiently detected using our protocol.”

The experiment also demonstrated a new method of using quantum devices to gain an advantage over classical ones – a quantum advantage. The conventional consensus among scientists is that achieving a quantum advantage requires quantum computers with countless qubits. Still, this experiment turned out to be a real quantum advantage with a relatively simple setup.

Paraoanu said, “In quantum computing, our method could be applied to diagnose microwave photon states in certain memory elements. This can be considered a very efficient way to extract information without disrupting the operation of the quantum processor.”

With their new approach, scientists are now exploring other exotic forms of information processing, such as counterfactual communication (communication between two parties without the transfer of physical particles) and counterfactual quantum computing (where the result of a computation is obtained without, in fact, the computer).

Magazine reference:

  1. Dogra, S., McCord, JJ & Paraoanu, G.S. Coherent interaction-free detection of microwave pulses with a superconducting circuit. Nat Commun 13, 7528 (2022). DOI: 10.1038/s41467-022-35049-z