Researchers at the University of Warsaw have developed a method for detecting extremely weak microwave fields using cold Rydberg atoms. By carefully controlling interactions between the atoms, the team reduced the impact of information loss during measurement and obtained three times more Fisher information than a comparable measurement without atomic interactions.
The findings, published in Nature Communications, could contribute to the development of more sensitive quantum sensors for applications in communication, metrology and quantum technologies.
Rydberg atoms offer a sensitive way to detect microwaves
The research brings together scientists from the Faculty of Physics at the University of Warsaw and the university’s Center for Optical Quantum Technologies at the Center for New Technologies (CeNT UW).
Rydberg atoms are atoms in which an electron is excited to a very high-energy state using precisely tuned lasers. In this state, the atoms can become up to a thousand times larger than they are in their ground state, making them exceptionally sensitive to external electric fields.
These properties make Rydberg atoms promising candidates for detecting weak microwave signals. Such measurements are important for quantum technologies, including superconducting quantum computers, which use microwave photons to store information.
Rydberg-based detectors can also measure electromagnetic fields without requiring calibration and operate with sufficiently low noise to detect thermal radiation from their surroundings.
Controlled atomic interactions help counter measurement losses
Information can be lost at several stages of a measurement, including the preparation of the atomic state, its evolution and the detection process. These losses generally reduce the precision with which a signal can be measured.
The Warsaw team found that carefully engineered interactions between Rydberg atoms could help limit this problem. Although the interactions themselves involve losses, they transform the atomic state so that subsequent losses during detection remove less information about the microwave field.
The researchers demonstrated that this approach could substantially mitigate the effects of measurement losses, which have traditionally been considered a major obstacle to improving sensor precision.
The experimental work was conducted by members of the Quantum-Optical Devices Laboratory at CeNT UW: Stanisław Kurzyna, Bartosz Niewelt, Professor Michał Parniak, Professor Wojciech Wasilewski and Dr Mateusz Mazelanik. Most of the researchers are also affiliated with the University of Warsaw’s Faculty of Physics.
Professor Rafał Demkowicz-Dobrzański, an expert in estimation theory at the Faculty of Physics, collaborated with the team to develop a theoretical explanation for the observed behaviour. The researchers also used optimisation methods developed at the university to analyse and describe the effect.
Experiment records three times more Fisher information
To evaluate measurement precision, the researchers used Fisher information, a statistical quantity that measures how much information an experiment provides about an unknown parameter. In this case, the parameter was the strength of the microwave field.
Higher Fisher information means that the measurement can estimate the field strength with lower uncertainty. This relationship is described by the Cramér-Rao inequality, a mathematical limit on the precision of parameter estimation.
In the experiment, the system using interactions between Rydberg atoms recorded three times more Fisher information than the corresponding system without those interactions.
The result indicates that controlled atomic interactions can improve the amount of information extracted from microwave measurements, even when significant losses occur.
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