LIGHT hits copper, an electron escapes. The whole thing is over in 26 attoseconds: so fast that even light, travelling at its cosmic speed limit, wouldn’t cross the width of a small virus. We’ve known for decades that quantum events happen this quickly, but until now we haven’t known what makes some last longer than others.
It turns out that shape matters. The atomic architecture of a material (whether it forms tidy three-dimensional lattices or stacks into flat sheets or strings itself along chains) determines how quickly electrons can break free when struck by light. Materials with simpler, more constrained structures slow things down. In the most extreme case researchers tested, electrons in a chain-like material took eight times longer to escape than those in ordinary copper.
This isn’t just about speed. Time has been quantum mechanics’ awkward problem since the field began; we use it constantly but understand it poorly. “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 Hugo Dil, a physicist at Switzerland’s École Polytechnique Fédérale de Lausanne. His team’s measurements of how long quantum transitions actually last could help resolve this almost century-old conundrum.
The challenge with measuring quantum time is that any external clock risks disturbing what you’re trying to observe. It’s like trying to time how long it takes a soap bubble to pop by throwing stopwatches at it. The 2023 physics Nobel recognized techniques that can probe these attosecond timescales, but they require comparison with external reference points.
Dil’s group found a way round this by reading information already encoded in the electrons themselves. When light excites an electron and kicks it out of a material, that electron carries away a kind of quantum timestamp in its spin, the property that makes electrons behave like tiny magnets. The spin changes depending on how the quantum transition unfolds, and these changes reveal duration without needing any external clock.
The principle relies on quantum interference. Light hitting an electron can nudge it along several different quantum pathways at once, and these pathways interfere with each other like overlapping ripples on a pond. That interference shows up as a specific pattern in the emitted electron’s spin. By measuring how this spin pattern shifts with electron energy, the researchers could calculate how long the transition lasted.
They tested this technique on materials with increasingly constrained atomic structures. Ordinary copper is fully three-dimensional, its atoms arranged in an open lattice. Titanium diselenide and titanium ditelluride are layered materials: think of them as stacks of atomic paper, weakly bound together. Copper telluride is even more restricted, with atoms strung along chains.
The pattern was clear and striking. In three-dimensional copper, electrons escaped in roughly 26 attoseconds. In the layered materials, this stretched to 140-175 attoseconds. In chain-like copper telluride, transitions lasted beyond 200 attoseconds. Reduce the symmetry of a material’s structure, and you slow down its quantum transitions.
Previous work had hinted at something like this. A 2017 study found that the high-temperature superconductor BSCCO, a layered, quasi-two-dimensional material, showed transition times around 120 attoseconds, much longer than copper’s 26. Researchers had wondered whether this reflected the material’s strong electron interactions. But the new results suggest dimensionality might be the more fundamental factor.
The connection makes a kind of sense. In gas-phase physics, asymmetry in the environment left behind after an electron escapes can tug on the departing particle and slow it down. Something similar might be happening in solids, where the degree of symmetry shows up in how many dimensions the crystal structure inhabits. The more constrained the structure (fewer mirror planes, less freedom of movement), the more complex the departure.
These aren’t just abstract measurements. Knowing how long quantum transitions last could help design materials with specific quantum properties, or improve technologies that depend on precise control of quantum states. And they provide a new tool for probing electron interactions in complex materials, since the technique works without external timing references that could introduce distortion.
More fundamentally, the work chips away at one of physics’ oldest puzzles. Quantum mechanics uses time constantly (every equation depends on it) but treats it differently from other quantities like position or momentum. Understanding what makes some quantum events last longer than others might eventually clarify time’s peculiar role in the quantum world. Whether a quantum transition truly takes time, or whether duration emerges from something else entirely, remains open. But now we can at least measure the difference between fast and slow, and see what that tells us.
Study link: https://arxiv.org/abs/2506.01476
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