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Artist’s rendering of an extremely short electron wave packet (blue) at the boundary between space and time. Credit: Brad Baxley (parttowhole.com)
Werner Heisenberg’s uncertainty principle describes one of the most intriguing features of quantum physics: certain pairs of physical quantities describing a particle, such as position and momentum, cannot simultaneously be determined with arbitrary precision—not because of imprecise measuring instruments, but because nature forbids it. Between position and time, however, there is no Heisenberg uncertainty principle.
A research team comprising several groups at RUN led by Profs. Jascha Repp, Rupert Huber, Franz Giessibl, and Klaus Richter, as well as a team from the Max Planck Institute in Hamburg led by Angel Rubio, has now observed for the first time that the location and time evolution of an electron cannot be measured with arbitrary precision simultaneously. This so-called space-time limit has important implications for future applications. The work is published in the journal Nature Photonics.
Many future technologies, from green tech and quantum technologies to high-performance electronics for artificial intelligence, require a precise understanding of how matter functions at the microscopic level: how chemical reactions occur, how light interacts with matter, and how electrons move through electronic components. High-resolution still images of the microscopic building blocks of matter are not sufficient for this; rather, time-resolved slow-motion movies from the nanocosmos are needed.
At the Regensburg Center for Ultrafast Nanoscopy (RUN), ultrafast microscopes are developed and used to directly capture the motion of electrons, atoms, and molecules in microscopic slow-motion movies with the highest possible spatial and temporal resolution. Ten years ago in Regensburg, the motion of a single molecule in space and time was resolved for the first time using ultrafast scanning tunneling microscopy. Compared to atoms and molecules, on this length scale, electrons move a thousand times faster—namely, on time scales of attoseconds.
The orders of magnitude are extreme: An atom is about ten million times smaller than a millimeter, and an attosecond is one-billionth of a billionth of a second. Thus, an attosecond relates to a second as a second relates to the age of the universe. What is particularly fascinating is that electron motion does not obey the laws of classical mechanics, but rather the strange rules of quantum physics.
To achieve a corresponding increase in temporal resolution compared to previous experiments and to directly image and control the quantum dynamics of individual electrons, the researchers developed a new laser system. Using its laser pulses they control electron motion on these extreme time scales in such a way that the electrons transfer from an atomically sharp metal tip to a silver surface over a distance of only a few atomic diameters. These electron movements are measured as current, and the temporal information is obtained by using two pulses of light.
Simon Maier, the lead author of the paper, explains: “By varying the time interval between the two laser pulses, we can directly observe how the electrons respond.” The electron motion observed in this way exhibits signatures on attosecond timescales—which means that the light pulses can transfer electrons on these timescales, and one can watch them do so.
What makes this special is that the electrons do not move like classical particles. Rather, as quantum mechanical waves, the electrons penetrate the energy barrier between the tip and the sample, for which they actually do not possess enough energy according to the laws of classical physics. They “tunnel” through it, as if they were passing through a massive wall without destroying it.
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“Our measurement can be understood as a high-speed camera for the electron wave packets, since you can see at what point in time the tunneling process takes place,” explains Katharina Glöckl, a doctoral student and co-author of the publication.
To gain a better understanding of microscopic electron dynamics at the “space-time limit,” Prof. Angel Rubio’s group conducted complex quantum simulations. The calculations explain the experimental results with remarkable accuracy. They also show that the electron does not follow the light field immediately, but with a tiny delay of 500 attoseconds.
In this frontier region of the smallest spatial and temporal scales, the fundamental physical limits of quantum physics become apparent.
