A team of scientists have devised a means of using quantum mechanics to "view" objects indirectly. The new method could improve measurements for quantum computers and other systems. It brings together the quantum and classical worlds.

We "see" things via the complex interaction of light photons within specialized cells in the retina of our eyes. However, some scientists have speculated that a similar phenomenon could be replicated without photo-absorption or without any light.

If we could, we'd be able to study things indirectly without "contaminating" them with light photons or influencing a system while attempting to study it. Imagine using light to study a strip of a photosensitive film without "damaging" it.

As it turns out, according to this groundbreaking discovery, it seems we can.

As part of a study on the relationship between the quantum and classical worlds, Shruti Dogra, John J. McCord, and Gheorghe Sorin Paraoanu of Aalto University have found a new and much better way to do interaction-free tests.

The scientists employed transmon devices, relatively large superconducting circuits that exhibit quantum behavior, to find microwave pulses produced by conventional instrumentation.

Although the work of Zeilinger's research team interested Dogra and Paraoanu, their lab is focused on microwaves and superconductors rather than lasers and mirrors.

‘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,’ Paraoanu explained.

Quantum coherence is the ability of an object to exist in two separate states simultaneously, which is something that quantum physics permits. It wasn't immediately clear if the new protocol would function because quantum coherence is fragile and easily collapses.

To the team's surprise, the experiment's initial runs demonstrated a considerably higher detection efficiency. They repeatedly returned to the drawing board, tested their findings using theoretical models, and verified everything. There was undoubtedly an impact.

‘We also demonstrated that even very low-power microwave pulses can be detected efficiently using our protocol,’ says Dogra.

The experiment also demonstrated a novel method for using quantum devices to gain an advantage over classical ones, known as a quantum advantage. Researchers' consensus is that attaining a quantum advantage will necessitate quantum computers with numerous qubits.

Yet, this experiment proved an absolute quantum advantage with a relatively straightforward setup.

What kind of applications could this new technique yield?

Optical imaging, noise detection, and the distribution of cryptographic keys are just a few specialized processes where interaction-free measurements based on the less efficient older methods have already been used. The new and improved approach may significantly boost these processes' efficacy.

‘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,’ Paraoanu explained.

The team led by Paraoanu is also experimenting with other novel ways of processing information, such as counterfactual quantum computing and counterfactual communication, which involves two people communicating without exchanging any physical particles (where the result of a computation is obtained without running the computer).

You can view the study for yourself in the journal Nature Communications.

Study abstract:

"The interaction-free measurement is a fundamental quantum effect whereby the presence of a photosensitive object is determined without irreversible photon absorption. Here we propose the concept of coherent interaction-free detection and demonstrate it experimentally using a three-level superconducting transmon circuit. In contrast to standard interaction-free measurement setups, where the dynamics involves a series of projection operations, our protocol employs a fully coherent evolution that results, surprisingly, in a higher probability of success. We show that it is possible to ascertain the presence of a microwave pulse resonant with the second transition of the transmon, while at the same time avoid exciting the device onto the third level. Experimentally, this is done by using a series of Ramsey microwave pulses coupled into the first transition and monitoring the ground-state population."