Quantum sensing in open quantum systems

Measuring extremely small signals, such as tiny forces, magnetic fields, or temperature changes , is at the heart of modern technology, from medical diagnostics to navigation and fundamental physics experiments. Scientists are increasingly turning to quantum systems to push measurement precision beyond classical limits. However, real devices are never perfectly isolated, they are open: they interact with their environment, constantly exchanging energy. Understanding how to exploit this openness rather than fight it has become one of the central challenges in quantum sensing.

A promising route involves operating sensors near so-called exceptional points, special conditions where the behaviour of a system changes dramatically and its response to external disturbances can become strongly amplified. These effects have been demonstrated in classical systems, but whether similar advantages can be achieved in truly quantum devices has remained unclear.

Researchers from the Joint Laboratory of Optics, working with collaborators in Japan and the United States, have now developed a new theoretical approach that answers this question. Their work published recently in a prestigious journal Physical Review Letters shows that quantum sensors based on a class of systems known as pseudo-Hermitian systems can reach the ultimate precision limits allowed by such open quantum systems.

"The results clarify when operating near exceptional points can genuinely improve measurement sensitivity, when it cannot, and how quantum effects modify the picture known from classical physics," says the main author of the paper Ievgen Arkhipov from the Joint Laboratory of Optics, and he continues: "By establishing these principles, the study opens the door to new designs of precise quantum sensors that take advantage of environmental interactions rather than being degraded by them."

This research brings scientists a step closer to practical quantum technologies capable of detecting signals in open quantum systems.

Citation: I. I. Arkhipov, F. Nori, Ş. K. Özdemir: Achieving the Quantum Fisher Information Bound in Pseudo-Hermitian Sensors, Phys. Rev. Lett. 136 (2026), 080802 DOI: 10.1103/qk5r-h851.