We see the world around us because light is being absorbed by specialized cells in our retina. But can vision happen without any absorption at all – without even a single particle of light? Sur­prisingly, the answer is yes. 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. Now, in a study exploring the connection between the quantum and classical worlds, Shruti Dogra, John J. McCord, and Gheorghe Sorin Paraoanu of Aalto University have discovered a new and much more effective way to carry out interaction-free experiments. The team used transmon devices –super­conducting circuits that are relatively large but still show quantum behaviour– to detect the presence of microwave pulses generated by classical instruments. 

Although Dogra and Paraoanu were fascinated by the work done by Zeilinger’s research group, their lab is centred around microwaves and super­conductors instead of 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 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,” Paraoanu says. Quantum coherence refers to the possi­bility that an object can occupy two different states at the same time – something that quantum physics allows for. However, quantum coherence is delicate and easily collapses, so it wasn’t immediately obvious that the new protocol would work.

To the team’s pleasant surprise, the first runs of the experiment showed a marked increase in detection effi­ciency. They went back to the drawing board several times, ran theo­retical models confirming their results, and double-checked everything. The effect was definitely there. “We also demons­trated that even very low-power microwave pulses can be detected efficiently using our protocol,” says Dogra. The experiment also showed a new way in which quantum devices can achieve results that are impossible for classical devices, a quantum advantage. Researchers generally believe that achieving quantum advantage will require quantum computers with many qubits, but this experiment demons­trated genuine quantum advantage using a relatively simpler setup.

Interaction-free measure­ments based on the less effective older metho­dology have already found applications in specialized processes such as optical imaging, noise-detection, and crypto­graphic key distribution. The new and improved method could increase the effi­ciency of these processes dramati­cally. “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 says.

The group led by Paraoanu is also exploring other exotic forms of information processing using their new approach, such as 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. (Source: Aalto U.)

Reference: S. Dogra et al.: Coherent interaction-free detection of microwave pulses with a superconducting circuit, Nat. Commun. 13, 7528 (2022); DOI: 10.1038/s41467-022-35049-z

Link: QTF Centre of Excellence, Dept. of Applied Physics, Aalto University, Aalto, Finland