Researchers at EPFL have developed a method to measure the duration of ultrafast quantum events without using an external clock. By analyzing electron spin changes during photoemission, they found that transition times vary significantly based on a material's atomic structure. Simpler structures lead to longer delays, ranging from 26 to over 200 attoseconds.
Physicists have long grappled with measuring time at the quantum scale, where events like electron transitions occur in attoseconds—10^{-18} seconds. Traditional methods rely on external clocks, which can interfere with delicate quantum processes. As Professor Hugo Dil of EPFL notes, "The central problem is the general role of time in quantum mechanics, and especially the timescale associated with a quantum transition."
To address this, Dil's team employed quantum interference techniques, avoiding any external timing devices. They used spin- and angle-resolved photoemission spectroscopy (SARPES), where synchrotron light excites electrons in a material, causing them to escape while carrying spin information. This spin encodes the duration of the transition from initial to final energy states upon photon absorption.
First author Fei Guo explains, "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 researchers tested materials with varying atomic geometries: three-dimensional copper, layered titanium diselenide (TiSe₂) and titanium ditelluride (TiTe₂), and chain-like copper telluride (CuTe). In copper, the transition lasted about 26 attoseconds. Layered materials showed delays of 140 to 175 attoseconds, while CuTe exceeded 200 attoseconds. These results indicate that lower-symmetry structures prolong quantum transitions.
Dil highlights the broader impact: "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."
The study, published in Newton (DOI: 10.1016/j.newton.2025.100374), involved collaborators from institutions including the Paul Scherrer Institut and the University of Tokyo. This approach could aid in designing materials with precise quantum properties for future technologies.
