Physicists using the Swiss Light Source SLS have found a way to measure the time involved in quantum events and found it depends on the symmetry of the material.
“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 Hugo Dil, a physicist at Paul Scherrer Institute PSI and professor 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.”
Measuring quantum time without an external clock
Dil and his team from EPFL have 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.
Read more on the PSI website
Image: Quantum events can unfold on attosecond timescales, making them notoriously difficult to measure. Researchers have now devised a way to measure the duration of quantum transitions without relying on an external clock.
Credit: © EPFL 2026/iStock (bymuratdeniz)