Using quantum mechanics to see objects without looking at them

The new method bridges the quantum and classical worlds.


Since the inception 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 whereby the presence of a photosensitive object is determined without irreversible photon absorption.

In a study exploring the connection between the quantum and classical worlds, scientists from Aalto University have discovered a new and much more effective way to carry out interaction-free experiments. They proposed the concept of coherent interaction-free detection 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 but still show quantum behavior.

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

Gheorghe Sorin Paraoanu of Aalto University said, “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 crucially: 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.”

Quantum coherence- the possibility that an object can occupy two different states simultaneously- is delicate and easily collapses. Hence, it wasn’t immediately obvious that the new protocol would work.

It is surprising for scientists that- in their protocol, quantum coherence serves as a resource, yielding a significantly high detection success probability. The first demonstration of the experiment showed a marked 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 double-check everything and ran theoretical models. The models confirmed their results- The effect was there, indeed.

Shruti Dogra of Aalto University said, “We also demonstrated that even very low-power microwave pulses can be detected efficiently using our protocol.”

The experiment also demonstrated a novel method for using quantum devices to gain an advantage over classical ones- a quantum advantage. The conventional consensus among scientists is that attaining a quantum advantage will require quantum computers with numerous qubits. Yet, this experiment proved a real quantum advantage with a relatively simple setup.

Paraoanu said“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.”

Using their new approach, scientists are now exploring other exotic forms of information processing like counterfactual communication (communication between two parties without any physical particles being transferred) and counterfactual quantum computing (where the result of a computation is obtained without, in fact, running the computer).

Journal Reference:

  1. Dogra, S., McCord, J.J. & 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
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