Physicists measure quantum events without needing a clock

For decades, physicists have faced a frustrating paradox: quantum events happen in attoseconds—billionths of a billionth of a second—but any clock you bring in to measure them risks contaminating the measurement itself. It's like trying to time how fast a soap bubble pops while holding a hammer.

Now researchers at EPFL have found a way around this. They measured how long a quantum transition takes without using an external timing device at all. Instead, they read the answer directly from the electrons themselves.

The Spin Tells the Story

When light hits a material and knocks electrons loose, those electrons carry hidden information in their spin—a quantum property with no classical equivalent. The spin pattern changes depending on how the quantum transition unfolds. By carefully analyzing these spin signatures across different electron energies, the team could calculate the duration of the transition.

As researcher Fei Guo describes it, the method "does not require an external reference, or clock, and yields 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." In other words: the electron itself becomes the clock.

The physics here hinges on quantum interference. When light excites an electron, the process doesn't follow one path—it explores multiple pathways simultaneously. These pathways interfere with each other, leaving a measurable signature in the electron's spin. Read that signature correctly, and you've got your timing.

Structure Changes Everything

The team tested three materials with radically different atomic arrangements. Copper is three-dimensional and tightly bonded. Titanium diselenide and titanium ditelluride are layered, with weak bonds between sheets. Copper telluride forms simple chains. The results revealed something striking: the simpler and less symmetric the structure, the longer the quantum transition took.

In copper, the transition lasted about 26 attoseconds—almost instantaneous. In the layered materials, it stretched to 140–175 attoseconds. In copper telluride's chains, it exceeded 200 attoseconds. Atomic geometry, it turns out, profoundly shapes how quickly quantum events unfold.

This finding does more than satisfy curiosity about the nature of time. It hands researchers a new tool for understanding electron behavior in complex materials, which could eventually help scientists engineer materials with specific quantum properties for future technologies that demand precise control of quantum states.