Between a particle's position and its momentum, Heisenberg's uncertainty principle draws a famous line: the two cannot be pinned down with arbitrary precision at the same time. For the pair of position and time, that rule does not formally apply – and yet, in practice, experiments run into a barrier. At this space-time limit, an electron's spatial extent and the timing of its motion can no longer be captured independently of each other. A research team has now made that limit visible directly, for the first time, in an experiment.
The work brings together several groups at the Regensburg Center for Ultrafast Nanoscopy (RUN) alongside a team from the Max Planck Institute in Hamburg. The physicists watched individual electrons at the precise moment they tunnel through an energy barrier – a quantum process with no equivalent in the classical world. When electrons are subjected to such ultrafast changes, as happens during tunnelling or in chemical reactions, space and time become coupled in intricate ways. What that coupling actually looks like had been almost impossible to measure, because doing so demands a time resolution in the attosecond range.
The view became possible through lightwave-driven scanning tunnelling microscopy on the atomic scale. The researchers modulated the tunnelling barrier with two time-delayed infrared light pulses that merged into phase-controlled waveforms. In this way they resolved single tunnelling events lasting less than a femtosecond. The result, published in the journal Nature Photonics, shows that an electron's position and its evolution in time cannot be fixed with arbitrary precision at once.
Why it matters
The finding is more than a footnote to basic physics. Many future technologies – from quantum technology to energy-efficient electronics for artificial intelligence to green tech – depend on understanding how matter behaves at its smallest scales: how chemical reactions unfold and how electrons travel through components. Still images of the atomic world are not enough; what is needed are time-resolved slow-motion movies from the nanoscale. That is precisely the kind of tool the teams have now sharpened. Being able to follow the motion of electrons on their own natural timescale brings researchers closer to steering chemical reactions deliberately – for instance, to make catalysts or materials more efficient.