Quantum 'watch' as fingerprint, a way to tell time without counting

Unlike the usual clocks such as mechanical, quartz crystal or atomic, the new
Unlike the usual clocks such as mechanical, quartz crystal or atomic, the new "quantum watch" can accurately show time without counting it. Source: Siim Lõvi/ERR

The new technique, which was developed with the assistance of an Estonian scientist, enables precise time measurement without actually counting time. Despite the quantum clock's lack of familiar time intervals, it can be used in foundational research required to develop quantum technologies.

"Every conventional clock has a counter and a zero point from which time is measured," Marta Berholts, a researcher at the University of Tartu and Uppsala University, explained. "The quantum 'clock', on the other hand, measures the interval of time as such — here we measure time in terms of the unique nodes in the temporal pattern of the process we are observing, and from the structure of this trace we can accurately determine the time."

The usual atomic clocks that constitute the basis of modern civilization are essentially the same as century-old wall clocks; however, instead of visible pendulums, they are based on the natural oscillations of atoms, primarily between the two fundamental elements of cesium-133. One second, one minute or one hour are passed after a certain number of oscillations have been recorded.

Unlike these usual clocks such as mechanical, quartz crystal or atomic, which work by counting the number of well-defined oscillations, the new "quantum timer" does not use any counter all, the researcher explained.

Instead, it provides a "fingerprint" that represents a specific time interval. Therefore, to separate it from the functionality of conventional clocks, the researchers called it a "watch."

The important achievement of the team is that such a quantum watch offers an opportunity to have "an absolute timestamp without the necessity to measure time zero."

The new approach is particularly useful for measuring ultra-short intervals of time on the femtosecond scale. Although one femtosecond, or 0.000,000,000,000,001 seconds, may seem as extremely short to humans, electrons can travel a significant distance in that amount of time.

This means, among other things, that faster chemical reactions occur on the same timescale. "If we can attain more precision on such a small duration, this could stimulate the development of quantum technology. The more precisely we can record the behavior of electrons, the better," Berholts explained.

The charms and travails of a quantum watch

The technique developed by Marta Berholz and her colleagues is based on a previously proven pump-and-probe spectroscopy experiment. To test the properties of a material used, for instance, in a solar panel it is first bombarded with an ultrashort laser pulse (USPL), and then the effect of the initial excitation on the material is measured with a delayed second laser pulse.

"We only wanted to observe how atoms and electrons react to laser radiation; we did not expect it to become a kind of quantum clock," the researcher said.

The team created their quantum "watch" by simultaneously exciting several energy levels of the helium atom with a powerful laser (an ultrashort XUV pulse). This resulted in electron superposition (a superposition of Rydberg states) — essentially electrons being in multiple places at the same time — and this connection shifting over time.

Phase differences between states generate interference that can either increase or weaken the signal. Similarly, as waves that are formed by tossing two stones into a pond can join together, the waves here amplify each other in some places and cancel each other out in others. This is reflected in the number and location of electrons measured by the detector.

"The picture we observed was extremely interesting due to the interference's incredibly complex structure. We saw distinctive traces that were precisely related to the elapsed time since the atom's excitation," Berholts said.

Moreover, they were noticeably more structured than the interference patterns seen in general physics textbooks. Given that instead of two waves about 40 electron waves interacted simultaneously, this was to be expected; the complexity ensures that patterns do not repeat during the measurement period.

"Quantum physics captures the world so accurately that, by inputting the initial conditions of the experiment into the model, we can predict with great precision what the pattern will look like at any given time," the researcher explained.

To determine how much time had passed since the beginning of the experiment, Berholts compared the results of the experiment with what she saw in the computer simulation.

The duration of excitation states restricts the length of time that can be measured. This watch cannot be used on a scale of seconds, but rather from femtoseconds to microseconds, the scientist explained.

For the published study that appeared in Physical Review Research, Berholts' own 12-inch computer was enough to provide evidence that the technique works.

Results from an experiment (above) and from a computer simulation (below). Source: Marta Berholts/Physical Review Research

The researcher believes that the new approach to time measurement could be used in many contexts. Currently, pump-probe techniques create a delay between two laser pulses by moving mirrors. Due to the short time periods, however, even smallest uncertainties associated with their implementation could have a significant impact on the outcomes.

"In the future, our technique will be fairly simple to implement in any pump-probe experiments. Maybe not in every lab, but it's not that difficult. Due to its simplicity and precision, the quantum watch has the potential to become an indispensable tool in any experiment," she said.

"As this quantum 'watch' is the only one capable of detecting extremely small time intervals with such a great precision, it could also be used in fundamental research where zero time is not as trivial to identify," Berholts added. For example, this new quantum watch could help us better understand how molecules decay or how light particles or magnetic fields affect matter.


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Editor: Kristina Kersa

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