A research team from Uppsala University in Sweden recently proposed a new time measurement method. The biggest feature is that there is no need to know the starting moment of the event in advance in the experiment, which is the "zero point of time" in the traditional sense. This method relies on the quantum evolution behavior of helium atoms in a strongly excited state. By analyzing its characteristic "fingerprint" that changes with time after short pulse light irradiation, it can directly read the length of time that has passed, providing a new time scale tool for ultrafast physical and chemical processes that cannot accurately determine the starting moment.
In this work, the researchers first used short pulses of light to excite helium atoms into a set of so-called Rydberg states and put the atoms in a quantum "superposition state" in which multiple Rydberg states are superimposed. The Rydberg state is a type of atomic excited state with extremely high energy and electrons far away from the nucleus. It is extremely sensitive to the environment. Quantum superposition means that atoms exist in multiple quantum states at the same time, and their overall evolution over time will form a complex wave packet structure. The traditional method is to accurately time the time from the moment of excitation, but the starting point of this study is to apply a second light pulse after a certain time, measure the probability that the helium atoms are ionized, that is, lose electrons and become charged ions, and then compare these measurement results with the theoretical model to deduce the time that has passed since the formation of the Rydberg state.
Johan Söderström, the leader of the research team, vividly likens this process to "reading a tape measure": You don't have to see someone starting to measure the distance from the zero mark. Just look at the current reading and you can know whether the distance difference from the starting point is 5 centimeters or 4000 meters. In this method, the superposition of Rydberg states of helium atoms evolves over time, leaving a unique change pattern on the observables - the so-called time "fingerprint", which is equivalent to the projection of the evolution of quantum wave packets in the observation space. By analyzing this fingerprint and matching it with theoretical calculations, researchers can directly read out the specific "time distance" from the generation of the wave packet to the moment of observation simply by observing within a limited time window.
The paper points out that this quantum fingerprint itself also has a "self-checking" function: the detailed structure of the wave packet evolving over time provides an internal consistency check for the corresponding time scale, thus improving the reliability of the measurement results. In terms of specific experiments, the team combined theoretical simulation and time-resolved photoelectron spectroscopy technology, that is, using two beams of light pulses with precisely controlled time intervals. One beam is used to excite helium atoms to form Rydberg state wave packets, and the other beam is used to knock out electrons and record the evolution of the photoelectron signal over time. The experimental results are highly consistent with theoretical predictions, indicating that this method can not only obtain time information, but also infer subtle energy differences such as "quantum defects" in the Rydberg state of helium atoms, thereby helping to deepen the understanding of atomic structure.
The researchers again used the analogy of a tape measure: when recording short distances, only a small section of the tape scale needs to be read, while measuring long distances requires a longer scale range. Corresponding to time measurement, if the event is very close to the "unknown starting point", only observing the fingerprints in a shorter time interval is enough to restore the time; for evolutions farther away from the starting point, fingerprints in a longer time span must be recorded to ensure that the correct time scale is matched. Therefore, this method is not a single measurement process that is static, but dynamically adjusts the amount of data required according to the length of time to be measured, providing a flexible quantum timing solution for experiments on different time scales.
It is worth noting that most of the experimental work for this study was performed in the HELIOS facility of the Ångström laboratory during the coronavirus pandemic and in the context of the temporary closure of some facilities at Uppsala University. In a relatively closed environment, the team was able to focus on using experimental time to repeatedly verify and optimize the time fingerprint method. After initially proving that the method is feasible, the researchers further proposed that in the future, this method is expected to be extended to molecular systems, such as to study the molecular dissociation process and its impact on the Rydberg state, to evaluate the universal applicability of this technology in more complex physical systems.
Although this new approach is conceptually capable of providing an absolute time scale, it is not designed to replace the traditional clocks used in daily life. The research team made it clear that it is more suitable as a special tool in pump-probe spectroscopy experiments for scenarios where fast process evolution needs to be observed with extremely short time resolution. In such experiments, the first pulse triggers the process, and the second pulse is responsible for taking a "time snapshot". However, the starting moment is often difficult to accurately define or even directly observable. This set of quantum fingerprint methods is expected to provide an absolute time scale for these fast processes without first determining the "time zero point."
From a broader perspective, this research provides a new idea for measuring time under the condition of "no starting point information", that is, relying entirely on the evolution of the quantum state itself to encode and decode time information, rather than using traditional counting mechanisms. The researchers point out that this method is not suitable for all types of time measurements, but it may become an extremely precise and complementary tool with unique advantages in experimental fields where it is difficult for existing technologies to accurately lock the starting moment, or when studying ultrafast processes inside atoms and molecules. Relevant results have been published in academic journals and attracted the attention of institutions such as Uppsala University and the American Physical Society. It is regarded as an important exploration in the research path of quantum time measurement.