The concept of time has perplexed philosophers and physicists for millennia, and the emergence of quantum mechanics has only deepened the enigma. Professor Hugo Dil, a physicist at the École Polytechnique Fédérale de Lausanne (EPFL), highlights the core issue: “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 happen within mere attoseconds (10-18 seconds), a duration so brief that light would not even traverse the width of a small virus. However, measuring such minuscule time intervals presents significant challenges, primarily because any external timing tool can potentially distort the very phenomena being observed. “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,” Dil explains. “This challenge can be resolved by using quantum interference methods, based on the link between accumulated phase and time.”
Innovative Approach to Time Measurement
In a groundbreaking study led by Dil, researchers have developed a novel method to accurately measure time in quantum events. When electrons absorb a photon and depart from a material, they carry information in the form of their spin, which varies depending on the progression of the underlying quantum process. By analyzing these minuscule changes, the researchers were able to deduce the duration of the transition without relying on an external clock.
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.”
Understanding Quantum Routes
The principle underlying this research is that when light excites an electron, it can follow multiple quantum paths simultaneously. These paths interfere with one another, manifesting as a distinct pattern in the emitted electron’s spin. By examining how this spin pattern changes with the electron’s energy, the team could calculate the duration of the transition.
The researchers employed a technique known as “spin- and angle-resolved photoemission spectroscopy” (SARPES). This method involves directing intense synchrotron light onto a material, which elevates its electrons to a higher energy state, prompting them to exit the material’s structure. Subsequently, the energy, direction, and spin of the emitted electrons are measured.
Material Structure and Transition Times
The team tested materials with varying atomic-level “shapes.” Some, like ordinary copper, are fully three-dimensional. Others, such as titanium diselenide (TiSe2) and titanium ditelluride (TiTe2), consist of weakly connected layers, behaving more like flat sheets. Copper telluride (CuTe) features an even simpler, chain-like structure. These structural differences made them ideal for exploring 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 rapid, lasting about 26 attoseconds. In the two layered materials, TiSe2 and TiTe2, the process slowed significantly to around 140-175 attoseconds. In CuTe, with its chain-like structure, the transition extended beyond 200 attoseconds. This indicates that the atomic-scale “shape” of the material greatly influences the speed of the quantum event, with lower-symmetry structures resulting in longer transition times.
Implications and Future Applications
Dil emphasizes the broader significance of these findings: “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 study offers physicists a new lens through which to view time’s behavior in quantum processes. It also provides a valuable tool for investigating electron interactions in complex materials. Understanding the duration of quantum transitions can aid scientists in designing materials with specific quantum characteristics, potentially enhancing future technologies that depend on precise quantum state control.
As research continues, the implications of these discoveries could extend beyond theoretical physics, influencing practical applications in fields ranging from material science to quantum computing. The ability to measure and manipulate quantum time with such precision may unlock new possibilities in technological innovation and deepen our understanding of the universe’s fundamental workings.
