New Form of Time Measurement


Researchers at Uppsala University have developed a new form of time measurement, which does not require you to measure from a starting point that is zero, as on an ordinary clock. Instead, they can directly at the time point they observe see how much time that has passed.

It is with the help of short light pulses the researchers have been able to create so called superpositions of Rydberg states in helium atoms. By studying how these states develop over time, the researchers have been able to develop a form of time measurement which does not require you to explicitly measure time zero.

“You can compare it to how you can look at a measuring tape and see how far you are from the start. Regardless if it is 5 centimeters or 4000 meters, we could show that it is possible to look at the probability that these Rydberg states can be ionised by another light pulse. And by studying only a short time interval, we could by comparing with theoretical models, directly read off how much time had passed since the Rydberg states were created,” says Johan Söderström who leads the research group in the Division of X-ray Photon Science at the Department of Physics and Astronomy.

In a similar way like it takes a relatively short distance to write 4 mm and a longer distance to write 500 mm, the researchers need to study time intervals of different length to be able to define times the further away from the non-measured time when the Rydberg states were created they are.

The researchers are now planning additional theoretical studies and studiers of molecular systems where it might be possible to study how molecules are fragmented and what this would mean for these Rydberg states.

The research was largely carried out during the covid pandemic when large parts of the university were closed, which gave the researchers opportunity to spend longer periods in the HELIOS lab in the Ångström laboratory in Uppsala where they could test this somewhat more exotic experiment.

This research opens up the possibility to measure time in a way that has not been possible before. It is not a solution which will fit all types of time measurements, but in those cases when it works it is very precise and may sometimes be the only way to actually measure the time passed, since it is not always easy to say when time zero occurs.

The research was carried out by Johan Söderström’s research group by Marta Berholts, Ronny Knut, Robert Stefanuik, Hampus Wikmark and Susmita Saha.

Article reference

Marta Berholts et al, Quantum Watch and its Intrinsic Proof of Accuracy, Phys. Rev. Research 4, 043041 – Published October 18, 2022, DOI:

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Camilla Thulin

Translation: Johan Wall

Last modified: 2023-08-04