Physicists have made a groundbreaking discovery about the speed of quantum time, shedding light on a concept that has puzzled scientists for millennia. Professor Hugo Dil, a renowned physicist at EPFL, explains that the enigma of time persists even with the advent of quantum mechanics, particularly regarding the timescale of quantum transitions.
At the quantum level, processes occur at astonishing speeds. For instance, tunneling or an electron transitioning to a new energy state after absorbing light can happen in just a few tens of attoseconds (10^-18 seconds). This is so rapid that light itself wouldn't travel the width of a virus during that time.
Measuring quantum time is an incredibly challenging task. Any external timing device risks disrupting the delicate quantum process being studied, altering its behavior. Professor Dil highlights the 2023 Nobel Prize in Physics as a testament to our ability to access these short times, but he also emphasizes the risk of introducing artifacts through external time scales.
To overcome this challenge, Dil and his team have developed a novel method that eliminates the need for external clocks. When electrons absorb a photon and are emitted from a material, they carry information encoded in their spin. This spin changes depending on the quantum transition's progression. By carefully analyzing these changes, the researchers can determine the duration of the transition.
Fei Guo, the study's first author, explains that these experiments don't require an external reference or clock. They provide the timescale necessary for the electron's wavefunction to evolve from an initial to a final state at a higher energy upon photon absorption.
The key to this discovery lies in quantum interference. When light excites an electron, multiple quantum pathways are possible simultaneously, and they interfere with each other. This interference leaves a distinct signature in the spin of the emitted electron. By examining how this spin pattern changes as the electron's energy varies, the team can calculate the transition's duration.
To conduct these measurements, the researchers employed a technique called 'spin- and angle-resolved photoemission spectroscopy' (SARPES). In this method, intense synchrotron light strikes a material, boosting its electrons to higher energy and forcing them to escape the material's structure. Scientists then measure the energy, direction, and spin of the emitted electrons.
The team tested materials with diverse atomic arrangements, including fully three-dimensional copper, layered titanium diselenide (TiSe2) and titanium ditelluride (TiTe2), and chain-like copper telluride (CuTe). These materials offered a unique opportunity to study the influence of geometry on timing.
The results revealed a consistent pattern. The simpler and more reduced the atomic structure of a material, the longer the quantum transition lasted. In three-dimensional copper, the transition was incredibly fast, taking about 26 attoseconds. In the layered materials TiSe2 and TiTe2, the process slowed to between 140 and 175 attoseconds. CuTe, with its chain-like structure, saw the transition exceed 200 attoseconds.
These findings demonstrate that the atomic-scale shape of a material significantly affects the speed of quantum events, with lower symmetry structures leading to longer transition times.
This discovery has profound implications for our understanding of quantum time. It provides fundamental insights into what determines time delays in photoemission and offers a new tool for studying electron behavior in complex materials. Knowing the duration of a quantum transition could enable scientists to design materials with tailored quantum properties and support future technologies that rely on precise quantum state control.
Other contributors to this research include the Lausanne Centre for Ultrafast Science (LACUS), Paul Scherrer Institut, CY Cergy Paris Université, Université Paris-Saclay, University of West Bohemia, and The University of Tokyo.
