Scientists have developed a novel method for measuring time at the quantum scale, a process that has proven notoriously difficult due to the extremely short intervals involved.
The research team behind the accomplishment suggests that tools capable of measuring quantum-scale events in attoseconds (10^-18 seconds) could provide further insight into the factors that influence the time delay associated with quantum transitions.
They also suggest that this quantum-scale time-measurement capability might help physicists finally quantify the role of time in quantum mechanics.
The Central Problem with Measuring Time at the Quantum Scale
In a statement, Professor Hugo Dil, a physicist at Ecole Polytechnique Fédérale de Lausanne (EPFL), noted that the mere concept of time has troubled physicists and philosophers for millennia, and the sometimes ‘spooky’ science of quantum mechanics “has not simplified the problem.”
“The central problem is the general role of time in quantum mechanics, and especially the timescale associated with a quantum transition,” the professor explained.
For example, quantum scale events like tunneling (the mysterious process where a quantum particle passes through a seemingly impassable barrier), or an electron absorbing a photon (which changes its state) can occur in as few as tens of attoseconds. For comparison, the researchers said, quantum-scale events that occur at such “mind-bending seeds” are so short that light could not completely traverse the width of a small virus.
Unfortunately, Professor Dil notes, tools designed to measure such briefly occurring phenomena can distort the event in the process.
“Although the 2023 Nobel prize in physics shows we can access such short times, the use of such an external time scale risks to induce artefacts,” he explained.
Instead of using external time-tracking approaches, the EPFL research team wondered whether they could leverage the ‘internal time scales’ generated by quantum processes as they occurred. For example, when an electron absorbs a photon, it leaves a material and carries information about the absorption in the form of the resulting spin.
The researchers said measuring these changes would reveal information about “how the underlying quantum process unfolds. They could also use this information to infer the time required for the transition to occur without an external timekeeping device.
“The principle is this: When light excites an electron, it can follow several different quantum routes at once,” they explained. “These routes interfere with each other, and this interference shows up as a specific pattern in the emitted electron’s spin.”
In theory, monitoring these quantum-scale changes to a particle’s spin pattern could reveal how long the process takes, even in attoseconds.
“This challenge can be resolved by using quantum interference methods, based on the link between accumulated phase and time,” Dil explained.
Experiments Reveal Atomic-Level ‘Shape’ Affects Attosecond-Scale Transition
To test the concept, the team used a technique called “spin- and angle-resolved photoemission spectroscopy” (SARPES). According to their statement, this process involves shining light on a material to ‘push’ its electrons to a higher energy state. This increased energy forces the electrons to leave the material’s physical structure. Most importantly, the process enables the measurement of the energy direction and spin of the electrons as they exit the base material.
The first experiment involved testing materials with different atomic level ‘shapes.” The research team notes that some of the selected materials, such as copper, are “dully three-dimensional.” Conversely, other tested materials, like titanium diselenide (TiSe₂) and titanium ditelluride (TiTe₂), consist of ‘weakly connected layers,” which causes them to act more like two-dimensional flat sheets. They also used the SARPES technique to test Copper telluride (CuTe), which they describe as having an “even simpler, chain‑like structure.”
After running several tests, the team found a consistent pattern. The simpler the material’s structure, the longer the quantum transition of the electrons lasted. For example, the transition in ordinary copper lasted around 26 attoseconds. When testing the somewhat simpler, two-layered materials, TiSe₂ and TiTe₂, the transition process slowed down to 140-175 attoseconds. Finally, the transition in the chain-like material CuTe lasted “well beyond” 200 attoseconds.
According to the researchers, this result indicates that the material’s atomic structure influences the overall duration of the quantum-scale event, with lower-symmetry structures leading to longer transition times.
Solving Other Mysteries About the Quantum Realm
When discussing the implications of their research, the study’s first author, Fei Guo, said their approach provides physicists with a new way to understand how time behaves in quantum-scale processes.
“These experiments do not require an external reference, or clock, and yield the time scale required for the wavefunction of the electron to evolve from an initial to a final state at a higher energy upon photon absorption,” Guo explained.
Another potential application of the team’s technique involves material design. According to the team’s statement, the ability to measure such ultra-short events “can help scientists design materials with specific quantum features and improve future technologies that rely on precise control of quantum states.”
When discussing other potential applications, Dil said their findings might help scientists unravel several other longstanding mysteries about quantum mechanics.
“Besides yielding fundamental information for understanding what determines the time delay in photoemission, our experimental results provide further insight into what factors influence time on the quantum level, to what extent quantum transitions can be considered instantaneous, and might pave the way to finally understand the role of time in quantum mechanics,” the professor explained.
The study “Dependency of quantum time scales on symmetry” was published in the Cell Press journal Newton.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.