Time is one of the most profound and perplexing concepts in both philosophy and physics, a topic that has sparked debates for millennia. "The arrival of quantum mechanics has only complicated our understanding,” states Professor Hugo Dil from EPFL. The core issue lies in how time is integrated into quantum mechanics, especially regarding the timescale of quantum transitions.
In the realm of quantum physics, processes occur at astonishingly high speeds. For instance, phenomena like tunneling or an electron transitioning to a different energy level after absorbing light can occur in mere attoseconds (10^-18 seconds). To put this in perspective, during that minute span, light wouldn’t even traverse the width of a tiny virus.
The Challenge of Measuring Quantum Time
Measuring such fleeting moments is incredibly difficult. Any external timing mechanism can disrupt the delicate quantum process being observed, potentially altering its behavior. Professor Dil notes, "While the Nobel Prize in Physics awarded in 2023 indicates that we can access these ultra-short times, using an external time reference can introduce inaccuracies.” Fortunately, researchers have found a way around this by employing quantum interference techniques, which rely on the relationship between accumulated phase and time.
Innovative Methods for Time Measurement Without Clocks
Dil and his research team have pioneered a groundbreaking approach that eliminates the need for external clocks. When electrons absorb photons and are emitted from a material, their spins carry vital information. This spin shifts based on how the quantum transition occurs. By meticulously analyzing these spin changes, the researchers could determine the duration of the transition.
As Fei Guo, the study's lead author, explains, "Our experiments do not rely on external references or clocks; they directly provide the timescale needed for the electron's wavefunction to evolve from an initial state to a final one at a higher energy after photon absorption."
Understanding Quantum Duration Through Interference
The process of light exciting an electron doesn't follow just one path; instead, multiple quantum pathways coexist simultaneously, leading to interference. This interference creates a distinct signature in the emitted electron's spin. By scrutinizing how these spin patterns change with variations in the electron's energy, the team was able to deduce the length of the transition.
To perform these measurements, the researchers utilized a technique called "spin- and angle-resolved photoemission spectroscopy" (SARPES). In this method, powerful synchrotron light is directed at a material, energizing its electrons and causing them to escape. Scientists then measure the emitted electrons' energy, direction, and spin to gather data.
How Atomic Structure Influences Quantum Timing
The researchers investigated materials with varying atomic configurations. Some were fully three-dimensional, like copper, while others, such as titanium diselenide (TiSe₂) and titanium ditelluride (TiTe₂), consist of loosely connected layers resembling flat sheets. Copper telluride (CuTe) features an even simpler, chain-like atomic arrangement. These variations provided an excellent opportunity to explore how atomic geometry impacts timing.
The results indicated a consistent trend: the more simplistic the atomic structure of a material, the longer the quantum transition duration. In the case of three-dimensional copper, the transition was remarkably swift, occurring in approximately 26 attoseconds. However, in the layered materials TiSe₂ and TiTe₂, the transition slowed to between 140 and 175 attoseconds. For CuTe, with its chain-like configuration, the transition extended beyond 200 attoseconds. These discoveries demonstrate that a material's atomic architecture significantly influences the speed of quantum events, with less symmetrical structures resulting in longer transition times.
Implications for Understanding Quantum Time
Professor Dil elaborates, "Our findings not only contribute fundamental insights into what dictates the time delays in photoemission but also enhance our understanding of the factors that affect time at the quantum level. This could lead us closer to comprehending the role of time in quantum mechanics as a whole."
By advancing our grasp of time in the context of quantum physics, this research opens new avenues for investigating electron behavior in complex materials. Understanding the duration of quantum transitions could aid scientists in designing materials with specific quantum characteristics and supporting future technologies that require precise manipulation of quantum states.
Other collaborators on 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.
