The concept of time has perplexed philosophers and physicists for millennia. With the advent of quantum mechanics, the enigma has only deepened. “The central problem is the general role of time in quantum mechanics, and especially the timescale associated with a quantum transition,” says Professor Hugo Dil, a physicist at the École Polytechnique Fédérale de Lausanne (EPFL).
Quantum events, such as electron tunneling or state changes triggered by photon absorption, occur at astonishingly brief intervals—some lasting mere attoseconds (10-18 seconds). This duration is so fleeting that light itself would not traverse the width of a small virus in that time. Measuring such minuscule intervals poses significant challenges, primarily because external timing tools can alter the phenomena they aim to observe.
Breaking New Ground in Quantum Time Measurement
Despite these hurdles, recent advancements have offered promising solutions. “Although the 2023 Nobel Prize in Physics shows we can access such short times, the use of an external time scale risks inducing artefacts,” Dil explains. “This challenge can be resolved by using quantum interference methods, based on the link between accumulated phase and time.”
Professor Dil has spearheaded research that introduces a novel method to measure time in quantum events with unprecedented accuracy. This technique leverages the information carried by electrons in the form of their spin when they absorb photons and exit a material. The changes in spin, reflective of the underlying quantum processes, allow researchers to deduce the duration of these transitions without relying on an external clock.
Innovative Techniques and Their Implications
Fei Guo, the study’s first author, elaborates: “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 straightforward yet profound. When light excites an electron, it can simultaneously embark on multiple quantum paths. These paths interfere, manifesting as a distinct pattern in the emitted electron’s spin. By analyzing how this spin pattern varies with the electron’s energy, the research team can calculate the transition’s duration.
Exploring Material Structures and Quantum Timing
The researchers employed a technique known as “spin- and angle-resolved photoemission spectroscopy” (SARPES). This method involves directing intense synchrotron light at a material, elevating its electrons to higher energy levels and causing them to exit the material’s structure. The energy, direction, and spin of the emitted electrons are then measured.
Various materials with distinct atomic-level “shapes” were tested. Ordinary copper, a fully three-dimensional material, was compared to layered materials like titanium diselenide (TiSe₂) and titanium ditelluride (TiTe₂), which behave more like flat sheets. Copper telluride (CuTe), with its chain-like structure, was also examined. These structural differences proved ideal for assessing how geometry impacts 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, transitions occurred in about 26 attoseconds. For the layered TiSe₂ and TiTe₂, the process slowed to approximately 140-175 attoseconds. In CuTe, with its chain-like structure, the transition extended beyond 200 attoseconds. This indicates that the atomic-scale “shape” of a material significantly influences the speed of quantum events, with lower-symmetry structures leading to longer transition times.
Implications for Future Technologies and Quantum Understanding
“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,” Dil notes. “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 offer physicists a fresh perspective on time’s behavior in quantum processes and a new tool for probing electron interactions in complex materials. Understanding the duration of quantum transitions can aid in designing materials with specific quantum properties, enhancing future technologies reliant on precise quantum state control.
As quantum mechanics continues to evolve, these insights could be pivotal in unraveling the mysteries of time and advancing technological innovations. The research not only opens new avenues for exploration but also challenges existing paradigms about the nature of time itself.
