Physicists Measure Time Inside Quantum Events Without a Clock
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Time at the quantum level may be far less mysterious than it seems. Physicists have found a way to measure how long ultra-fast quantum events actually take—without using a clock at all. Credit: Shutterstock Quantum events happen unimaginably fast, but measuring their timing without disturbing them has been a long-standing challenge. “The concept of time has troubled philosophers and physicists for thousands of years, and the advent of quantum mechanics has not simplified the problem,” says Professor Hugo Dil, a physicist at EPFL. “The central problem is the general role of time in quantum mechanics, and especially the timescale associated with a quantum transition.” At the quantum scale, events unfold at speeds that are hard to comprehend. Processes such as tunneling or an electron jumping to a new energy state after absorbing light can occur in just a few tens of attoseconds (10-18 seconds). That span is so brief that even light would not be able to cross the width of a small virus in the same amount of time. The Challenge of Measuring Ultrafast Quantum Events Capturing time intervals this short has long been a major obstacle. Any external timing device can interfere with the delicate quantum process being measured, potentially altering the result. “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 artifacts,” Dil says. “This challenge can be resolved by using quantum interference methods, based on the link between accumulated phase and time.” A Clock-Free Way to Measure Quantum Time Dil and his colleagues have now demonstrated a method that sidesteps the need for an external clock altogether. When electrons absorb a photon and are emitted from a material, they carry subtle information in their spin. That spin changes depending on how the quantum transition unfolds. By analyzing these changes, the team was able to determine how long the transition lasts. As first author of the study Fei Guo explains, “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.” How Quantum Interference Reveals Duration When light excites an electron, the process does not follow a single path. Instead, multiple quantum pathways are possible at the same time. These paths interfere with one another, and that interference leaves a distinct signature in the spin of the emitted electron. By tracking how this spin pattern changes with the electron’s energy, the researchers could calculate the duration of the transition itself.
Probing Electrons With Advanced Spectroscopy To carry out the measurements, the team relied on a method known as “spin- and angle-resolved photoemission spectroscopy” (SARPES). In this approach, powerful synchrotron light is directed at a material, boosting its electrons to higher energies and forcing them out of the material. Scientists then measure the energy, direction, and spin of the escaping electrons.
How Atomic Structure Shapes Quantum Timing The researchers examined materials with very different atomic arrangements. Some were fully three dimensional, such as ordinary copper. Others, including titanium diselenide (TiSe2) and titanium ditelluride (TiTe2), consist of weakly bonded layers that behave more like flat sheets. Copper telluride (CuTe) has an even simpler structure, forming chains of atoms. These contrasts allowed the team to test how geometry affects the timing of quantum transitions. Clear Patterns in Attosecond Measurements The measurements revealed a striking trend. The more reduced and less symmetric the structure of a material, the longer the quantum transition took. In three dimensional copper, the transition was extremely rapid, lasting about 26 attoseconds. In the layered materials TiSe2 and TiTe2, the same process slowed to roughly 140-175 attoseconds. In CuTe, with its chain-like structure, the transition exceeded 200 attoseconds. These results show that the atomic scale shape of a material plays a major role in how quickly a quantum event unfolds, with simpler structures leading to longer transition times.
What This Means for Quantum Physics and Technology “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,” Dil explains. Beyond addressing a deep scientific question, the findings give researchers a new tool for exploring how electrons behave in complex materials. Knowing exactly how long a quantum transition lasts could help scientists tailor materials with specific quantum properties and support future technologies that depend on precise control of quantum states. Reference: “Dependency of quantum time scales on symmetry” by Fei Guo, Dmitry Usanov, Eduardo B. Guedes, Mauro Fanciulli, Kaishu Kawaguchi, Ryo Mori, Takeshi Kondo, Arnaud Magrez, Michele Puppin and J. Hugo Dil, 6 February 2026, Newton.