New Method Quantifies the Duration of Quantum Tunneling

EPFL physicists have developed a way to quantify the duration of certain quantum events, and they report that the timescale depends on a material’s symmetry. The study was published in Newton.

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The concept of time has troubled philosophers and physicists for thousands of years, and the advent of quantum mechanics has not simplified the problem. The central problem is the general role of time in quantum mechanics, and especially the timescale associated with a quantum transition.

Hugo Dil, Professor and Physicist, EPFL

Quantum phenomena, such as tunnelling or an electron transitioning its state through the absorption of a photon, occur at astonishing speeds. Some events transpire in mere tens of attoseconds (10^-18 seconds), a duration so brief that light would not even traverse the width of a virus.

However, measuring time intervals of this magnitude is notoriously challenging, particularly because any external timing device can interfere with the very phenomenon one aims 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. This challenge can be resolved by using quantum interference methods, based on the link between accumulated phase and time.

Hugo Dil, Professor and Physicist, EPFL

Dil has recently conducted research that has created a method to precisely measure time in quantum events. When electrons absorb a photon and exit a material, they transmit information through their spin, which varies based on the progression of the underlying quantum process. By analyzing these minute changes, the researchers were able to deduce the duration of the transition, all without the need for an external clock.

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.

Fei Guo, Study First Author, EPFL

The principle is as follows: When light interacts with an electron, it can take multiple quantum paths simultaneously. These paths interfere with one another, and this interference manifests as a distinct pattern in the spin of the emitted electron. By examining how this spin pattern varies with the electron's energy, the research team was able to determine the duration of the transition.

The researchers employed a method known as "spin- and angle-resolved photoemission spectroscopy" (SARPES). This technique entails directing intense synchrotron light onto a material, which elevates its electrons to a higher energy state, compelling them to leave the material's structure, followed by measuring the energy, direction, and spin of the ejected electrons.

The researchers examined materials with various atomic-level "shapes". Some are entirely three-dimensional, such as standard copper. Others, like titanium diselenide (TiSe₂) and titanium ditelluride (TiTe₂), consist of loosely connected layers and exhibit behavior more akin to flat sheets. Copper telluride (CuTe) features an even more straightforward, chain-like configuration. These variations render them suitable for investigating how geometry influences timing.

The findings revealed a distinct trend: the simpler and more reduced the material's structure, the longer the quantum transition persisted. In typical 3D copper, the transition occurred extremely rapidly, lasting approximately 26 attoseconds.

In the bilayer materials, TiSe₂ and TiTe₂, the process significantly decelerated to approximately 140–175 attoseconds. In contrast, CuTe, which possesses a chain-like configuration, exhibited a transition that extended beyond 200 attoseconds. This indicates that the atomic-scale "shape" of the material has a profound impact on the speed at which the quantum event occurs, with structures of lower symmetry resulting in prolonged transition durations.

“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,” explained Dil.

The results offer physicists a novel perspective on the behavior of time within quantum processes.

Understanding the duration of a quantum transition can assist researchers in creating materials with targeted quantum characteristics and enhance forthcoming technologies that depend on the accurate manipulation of quantum states.

The study received funding from the Swiss National Science Foundation (SNSF) and the Program ERC CZ (TWISTnSHINE),