“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, such as tunneling or an electron changing its state by absorbing a photon, occur at astonishing speeds. Some of these transitions take only a few tens of attoseconds (10-18 seconds), a duration so brief that light would not even traverse the width of a small virus. Measuring such minuscule time intervals, however, poses a significant challenge, as any external timing mechanism can interfere with the observation itself. “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,” adds Dil. “This challenge can be resolved by using quantum interference methods, based on the link between accumulated phase and time.”
Innovative Approach to Quantum Time Measurement
Professor Dil has spearheaded research that introduces a novel method to measure time in quantum events with precision. When electrons absorb a photon and exit a material, they retain information in the form of their spin, which varies depending on the quantum process’s progression. By analyzing these minute changes, researchers can deduce the duration of the transition without relying on an external clock.
Fei Guo, the study’s first author, 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.”
Understanding Quantum Routes
The principle behind this breakthrough is that when light excites an electron, it can simultaneously follow multiple quantum paths. These paths interfere with each other, creating a distinctive pattern in the emitted electron’s spin. By examining how this spin pattern shifts with the electron’s energy, the team was able to calculate the transition duration.
Experimental Techniques and Findings
The researchers employed a technique known as “spin- and angle-resolved photoemission spectroscopy” (SARPES). This involves directing intense synchrotron light onto a material, elevating its electrons to a higher energy level and causing them to exit the material’s structure. The energy, direction, and spin of the emitted electrons are then measured.
Various materials with different atomic-level “shapes” were tested. Some, like ordinary copper, are fully three-dimensional. Others, such as titanium diselenide (TiSe2) and titanium ditelluride (TiTe2), consist of weakly connected layers and behave more like flat sheets. Copper telluride (CuTe) has an even simpler, chain-like structure. These structural differences made them ideal for examining 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.
Implications for Quantum Mechanics and Material Science
These findings underscore the significant impact of a material’s atomic-scale “shape” on the speed of quantum events, with lower-symmetry structures leading to longer transition times. “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,” Dil elaborates.
The implications of this research extend beyond theoretical physics, offering a new lens through which physicists can comprehend time’s behavior in quantum processes. This understanding also provides a valuable tool for probing electron interactions in complex materials. By knowing the duration of quantum transitions, scientists can design materials with specific quantum properties and enhance future technologies that depend on precise quantum state control.
As the field of quantum mechanics continues to evolve, this breakthrough in time measurement represents a significant step forward, promising to unlock new possibilities in both scientific understanding and technological advancement.
