Scientists Invent ‘Quantum Watch’, a Mind-Bending New Way to Measure Time

Scientists have invented a trippy new way to measure time by searching for eerie “fingerprints” in the quantum realm, which governs the universe at very small scales, reports a new study. The novel technique differs from the most familiar ways of keeping time because it is not anchored to a “time zero” that marks the start of a recorded period.

Our human compulsion to tell time has manifested in ingenious mechanisms over the millennia, including sundials, stopwatches, and hyper-precise atomic clocks. All of these devices measure time by clocking the period between two intervals, whether that is the back-and-forth swing of a pendulum in a grandfather clock, or the time between the starting shot of a race and the winner crossing the finish line. 

Now, a team led by Marta Berholts, an experimental physicist at Tartu University, have created a very different type of “quantum watch” that "does not require an initial “time zero” reference point to make its time measurements, according to a recent study in Physical Review Research. 

“To our knowledge, the concept of obtaining time fingerprints, and therefore avoiding the need to measure time zero, is completely novel,” Berholts said in an email. She added that the new invention is a watch, not a clock, because “a clock requires keeping track of time” whereas “a watch simply provides the time.” 

“The quantum watch provides a fingerprint representing a specific time, and hence only requires interaction when initiating and reading out the time,” she explained. “All other devices require keeping track of time. This differentiation comes from the fact that the quantum watch, unlike all the other clocks, measures times in a different way.”

Berholts stumbled upon this mind-boggling concept while during her postdoctoral project at Uppsala University, which coincided with the COVID-19 pandemic. The outbreak left her with “a lot of time to spend in a laser lab in a foreign country,” she noted.

"In the beginning, nobody was even thinking about such a cool concept as the quantum watch,” Berholts recalled, because the project was simply focused on studying the dynamics of electrons triggered by ultrashort laser pulses. 

“We knew that we would see quantum beats but we did not think of the fact that it could be a quantum watch,” she continued. “The idea came after the experiments were done at the stage of data analysis. When we compared the results of the experiment with the simulations we were surprised to see that they match extremely well. We were even more surprised when we realized that the theory was able to find flaws in the experiment.” which is referred to in the study as the “drift of the delay stage.” 

The team created this quantum watch by shooting lasers at helium atoms until they reached an excited “Rydberg'' state with special properties. Scientists use Rydberg atoms to study all kinds of interesting problems in physics, but the new study focuses on the unpredictable signatures, called “wave packets,” produced by excited electrons that orbit the atoms. 

These packets emerge from a quantum phenomenon called superposition, in which an object can occupy two states of reality at the same time. In the macro-scale world that humans experience, it’s impossible to be in two places at once, or to simultaneously exist in two different realities, but such mind-boggling feats are possible under the weird quantum rules that exist on the very small scales of atoms and subatomic particles.

When the wave packets of multiple Rydberg atoms interact, complex “fingerprints” emerge that are somewhat analogous to the choppy waters of colliding ripples in a pond. These patterns, known as quasiunique beat signatures (QUBS), are so idiosyncratic that they can be used as timestamps that measure the evolution of the wave packets relative to each other. 

“We show that the oscillations resulting from an ensemble of highly excited Rydberg states” can “give rise to a unique interference pattern that does not repeat during the lifetime of the wave packet,” the team explained in their study. “These fingerprints determine how much time has passed since the wave packet was formed and provide an assurance that the measured time is correct.” 

“Unlike any other clock, this quantum watch does not utilize a counter and is fully quantum mechanical in its nature,” the researchers added.

In other words, the patterns created by the interacting atoms are so distinct that they carry an innate record of their lifespans. In addition to the novelty of this timekeeping approach, the team notes that the watch is highly accurate, with a margin of error of just eight femtoseconds (one femtosecond is a millionth of a billionth of a second). For this reason, the new technique could be extremely useful for so-called “pump-probe experiments” that record ultrafast chemical reactions.

“Pump-probe experiments, also called time-resolved experiments, could be compared to a movie,” Berholts said. “However, this movie is not about people but about molecules, atoms, or electrons that live in their quantum mechanical world, moving around at ultrafast speeds. In our pump-probe experiment, we were observing electrons’ movement on a femtosecond timescale. This timescale is even hard to imagine as a femtosecond is to a second as a second to about 32 million years!” 

“Keeping accurate track of time at such timescales becomes complicated,” she added. “A quantum watch is a tool that has to be used whenever a pump-probe experiment is running on a femtosecond timescale to make sure that the time is accurate. The higher temporal accuracy of the processes we observe in quantum mechanical systems could be of great importance for future progress in quantum technology.”

To that end, the researchers suggest many tweaks to their technique that could make it widely applicable for a host of experiments that use different atomic elements or photon energies, which could provide new glimpses of the bizarre quantum domain.

“In this study, we focused on the temporal domain, in our next study we are investigating the spatial domain or, in other words, the distances from the atomic core that electrons can travel and how we can control them,” Berholts concluded. “Hence, more intriguing findings are coming soon!”

Update: This article has been updated to include comments from lead author Marta Berholts.