The wave-particle duality is arguably one of the most intriguing concepts of quantum mechanics. Depending on the circumstances of the observation, a fundamental particle–like an electron–shows either wave- or particle-like behavior. One typical wave phenomenon is interference, where crests and troughs of two or more waves overlap and reinforce (or eliminate) each other. For example, two stones dropped in a pond at two positions simultaneously create such an interference pattern on the surface, where the ripples from both overlap.

Both particle and wave-behavior play a role in the famous double-slit experiment: A single electron comes upon a wall with two small slits. because of the electron-wave manifestation, the electron passes through both slits simultaneously and the two partial waves interfere like the surface waves on the pond. When we measure where the electron-particle emerges behind the wall, we find that this is determined by the interaction of the two partial electron-waves. Where they reinforce each other, the chance of finding the electron is increased, and where they eliminate one another, the chance is zero.

Nature's simplest manifestation of the double-slit experiment is electron emission from a hydrogen molecule. The hydrogen molecule consists of two protons and two electrons, which do not have a well-defined position inside the molecule. The electrons are better imagined as clouds that cover the molecule. The density of the clouds and the chance to locate an electron is highest at the two centers (determined by the positions of the protons). When a photon (a light particle) is absorbed by the molecule, an electron is emitted. Both centers release their part of the electron cloud and the wave-properties of the emitted electron result in an interference pattern.

In a recent experiment, we utilized this simplest double-slit setup to investigate the movement of a photon inside the hydrogen molecule. We shined photons on hydrogen molecules and, with a special measuring device called reaction microscope, we observed the angle under which the electron emerged from the molecule. By recording and analyzing hundreds of thousands such events, we could visualize the interference created by the electron waves.

Our data shows that when the light is parallel to the hydrogen molecule, the resulting interference pattern is asymmetric. If the two parts of the electron waves had formed simultaneously at the two protons, the pattern should have been symmetric around the center of the molecule. Since this is not the case, we can conclude that one part of the electron wave must form slightly before the other and has more time to spread out. This is a result of the photon travel time across the molecule at 247 zeptoseconds (1 zeptosecond is equal 10^-21 seconds).

The stone-and-pond analogy provides a tangible comparison: Like a skipping flat pebble that jumps off the water surface once before it sinks at a position further away, the photon touches the electron cloud twice: at the positions of the two protons. Due to the delay between the birth of the two waves on the pond's surface, the resulting interference pattern is shifted towards the position where the stone sunk.

The wave-properties of electrons emitted from the double-slit-like hydrogen molecule showed how the finite speed of light is relevant even in the atomic microcosm. The measurement of the photon travel time through the molecule constitutes one of the fastest time measurements in atomic physics up to this day. This achievement contributes to the ongoing effort to study ultrafast processes in real-time.

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