Researchers at Uppsala University developed a new way to measure time that does not require a known starting point, or "time zero," as conventional clocks do. Instead of measuring from the exact moment an event begins, the method works by examining how helium atoms change over time after they are exposed to short pulses of light. The approach could be useful in experiments where it is difficult or even impossible to identify when the process being studied actually started.
The method uses helium atoms that are placed into what are known as superpositions of Rydberg states using short light pulses. Rydberg states are highly excited atomic states in which an electron has absorbed enough energy to move far from the atom"s nucleus, making the atom especially sensitive to its surroundings. A superposition means the atom exists in a combination of several quantum states at the same time, allowing researchers to study how those states evolve together. Instead of measuring from the instant these states are created, the researchers study how likely the atoms are to be ionised by a second pulse of light. Ionisation is the process of removing one or more electrons from an atom, turning it into a charged particle called an ion. By comparing those results with theoretical models, they can work out how much time has passed since the Rydberg states were formed.
“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.
The researchers say the changing Rydberg states create a unique pattern, or "fingerprint," as time passes. This pattern comes from the evolution of an atomic wave packet, which is a group of overlapping quantum states that change together over time. By reading this fingerprint, they can determine how much time has elapsed since the wave packet was created. Unlike a conventional clock, which counts time from a fixed starting point, this method identifies the elapsed time directly from the current state of the atoms. According to the researchers, the fingerprint also provides a built-in way to confirm that the measured time is correct.
The study combined theoretical modelling with time-resolved photoelectron spectroscopy experiments, a technique that uses two carefully timed light pulses. The first pulse excites the atoms, while the second removes electrons from them so researchers can observe how the atoms have changed over extremely short timescales. The experimental results closely matched the theoretical predictions. The researchers also found that the changing behavior of the atomic states can be used to study properties of helium atoms known as quantum defects. These are small differences between the expected and actual energy of Rydberg states that help scientists better understand atomic structure.
To explain the idea, the researchers compare it to reading a measuring tape. You do not need to watch someone begin measuring to know the distance. Looking at the current position on the tape is enough to tell how far it is from the starting point. In the same way, their method does not need to observe "time zero" to determine how much time has passed.
The team also found that the amount of data needed depends on how much time has passed. Short observation periods are enough for events that happened recently, while longer periods are needed to measure events further away from the unknown starting point. The researchers compare this to using a measuring tape, where writing down a distance of 4 millimeters requires only a small section, while measuring 500 millimeters needs a much longer section.
Much of the research was carried out during the COVID-19 pandemic. With many parts of Uppsala University temporarily closed, the researchers were able to spend extended periods in the HELIOS laboratory at the Ångström Laboratory in Uppsala, where they tested the experimental approach.
The team is now planning further theoretical studies involving molecular systems. They want to investigate whether the method can be used to study how molecules break apart and how that process affects the Rydberg states. If successful, the technique could be applied to a wider range of physical systems.
The researchers say the method is not intended to replace conventional clocks. Instead, it could become a useful tool for pump-probe spectroscopy, a technique that uses one pulse of light to trigger a process and a second pulse to observe how it changes over time. This allows scientists to study physical and chemical processes that happen extremely quickly. In these experiments, identifying the exact starting time is often difficult. The new approach could provide an absolute timestamp without first having to find time zero, while relying entirely on quantum mechanical behavior rather than a counting mechanism.
The findings offer a new way to measure elapsed time in experiments where the starting point cannot be directly observed. While the method will not be suitable for every type of time measurement, the researchers say it could provide a highly precise solution in situations where existing methods are difficult or impossible to use.
Source: Uppsala University, APS
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