(Press-News.org)
“The concept of time has troubled philosophers and physicists for thousands of years, and the advent of quantum mechanics has not simplified the problem,” says Professor Hugo Dil, a physicist at EPFL. “The central problem is the general role of time in quantum mechanics, and especially the timescale associated with a quantum transition.”
Quantum events, like tunnelling, or an electron changing its state by absorbing a photon, happen at mind‑bending speeds. Some take only a few tens of attoseconds (10-18 seconds), which is so short that light would not even cross the width of a small virus.
But measuring time intervals this small is notoriously difficult, also because any external timing tool can distort the very thing we want to observe. “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,” says Dil. “This challenge can be resolved by using quantum interference methods, based on the link between accumulated phase and time.”
Dil has now led research that has developed a way to accurately measure time in quantum events. When electrons absorb a photon and leave a material, they carry information in the form of their spin, which changes depending on how the underlying quantum process unfolds. By reading these tiny changes, the researchers could infer how long the transition takes, without ever using an external clock.
As first author of the study Fei Guo says: “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.”
The principle is this: When light excites an electron, it can follow several different quantum routes at once. These routes interfere with each other, and this interference shows up as a specific pattern in the emitted electron’s spin. By studying how that spin pattern changes with the electron’s energy, the team could calculate the duration of the transition.
For the study, the researchers used a technique called “spin- and angle-resolved photoemission spectroscopy” (SARPES). SARPES involves shining intense synchrotron light on a material, which pushes its electrons to a higher energy forcing them to exit the material's structure, and then measuring the energy, direction, and spin of the electrons that come out.
They tested materials with different “shapes” at the atomic level. Some are fully three‑dimensional, like ordinary copper. Others, like titanium diselenide (TiSe₂) and titanium ditelluride (TiTe₂), are built from weakly connected layers and behave more like flat sheets. Copper telluride (CuTe) has an even simpler, chain‑like structure. These differences make them ideal for testing how geometry affects timing.
The results showed a clear pattern: the simpler and more reduced the structure of the material, the longer the quantum transition lasted. In ordinary 3D copper, the transition was extremely fast, lasting about 26 attoseconds.
In the two layered materials, TiSe₂ and TiTe₂, the process slowed down noticeably to around 140–175 attoseconds. And in CuTe, which has a chain‑like structure, the transition stretched beyond 200 attoseconds. What this means is that the atomic‑scale “shape” of the material strongly influences how quickly the quantum event unfolds, with lower‑symmetry structures leading to longer transition times.
Dil explains: “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 findings give physicists a new way to understand how time behaves in quantum processes. They also provide a tool for probing how electrons interact in complex materials. Knowing how long a quantum transition lasts can help scientists design materials with specific quantum features and improve future technologies that rely on precise control of quantum states.
Other contributors
Lausanne Centre for Ultrafast Science (LACUS)
Paul Scherrer Institut
CY Cergy Paris Université
Université Paris-Saclay
University of West Bohemia
The University of Tokyo
Reference
Fei Guo, Dmitry Usanov, Eduardo B. Guedes, Mauro Fanciulli, Kaishu Kawaguchi, Ryo Mori, Takeshi Kondo, Arnaud Magrez, Michele Puppin, J. Hugo Dil. Dependency of quantum time scales on symmetry. Newton 06 February 2026.
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