Tracking electrons at the space-time limit | S. Maier et al. | ResearchPod

Transcript: Tracking electrons at the space-time limit

Alex: Welcome to another episode of ResearchPod. Today, we're looking at a study about observing the smallest particles in the universe. Sam, what are we talking about?

Sam: We're discussing a paper on filming the motion of a single electron. We've known for a long time where atoms sit inside materials—we can map them out. But watching an individual electron actually move, in real time? That's never been done before.

Alex: So the challenge is basically building a camera fast enough to catch something that small?

Sam: That's exactly it. Think of photographing a hummingbird. To freeze those wings mid-beat, your camera shutter has to open and close faster than the wings move. Electrons move on what scientists call an "attosecond" timescale. To give you a sense of how short that is: one attosecond is to a single second what a second is to the entire age of the universe.

Alex: That's a genuinely difficult scale to work at. So why has the timing side of this been so hard to crack?

Sam: It comes down to the tools we've had available. The standard instrument for studying surfaces at this scale is called a Scanning Tunneling Microscope. Imagine a needle so sharp it ends in a single atom, hovering just above a surface, feeling out what's there. It's extraordinarily precise in space—it can locate atoms—but building one image takes time. It's like a long-exposure photograph. If something moves quickly while the shutter is open, you get blur. The flickering motion of electrons happens so fast that the microscope simply can't keep up.

Alex: So the microscope sees where things are, but not when they move. How does this research get around that?

Sam: The team created what you could call an "attosecond shutter." Instead of changing the microscope itself, they used precisely timed pulses of laser light to control the electric field right at the tip of the needle. That field is what drives electrons to move in the first place. By shaping that field very carefully, they could open and close a measurement window faster than any mechanical shutter ever could—lasting only a fraction of a femtosecond, which is itself already a thousand times shorter than a nanosecond.

Alex: And that window is short enough to catch the electron mid-motion?

Sam: That's the idea. In quantum physics, electrons have a behaviour called tunnelling. Classically, if you put a wall in front of a ball, the ball stops. But at the quantum scale, an electron can pass through a barrier it shouldn't have enough energy to cross—as if it walked through the wall. The team used two laser pulses with different colours of light. By overlapping them, they created a specific, lopsided electric field—one that only pushes strongly in one direction for a very brief moment. That acts like a turnstile: it lets the electron through at a precise, known instant, so the team can measure exactly when the tunnelling happens.

Tracking electrons at the space-time limit

S. Maier, R. Spachtholz, K. Glöckl, C. M. Bustamante, S. Lingl, M. Maczejka, J. Schön, A. Riedel, K. Richter, F. J. Giessibl, F. P. Bonafé, M. A. Huber, A. Rubio, J. Repp, R. Huber

Aug 13, 2026·5:13·user_upload

Attosecond scanning tunnelling microscopySpace-time limitQuantum tunnellingWavefunction dynamicsSpatio-temporal couplingKramers–Henneberger theorem

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5 min

Briefing

Research Question

Understanding how electronic wavefunctions evolve in space and time is fundamental to chemistry and solid-state physics, yet observing these dynamics requires simultaneous atomic-scale spatial resolution and attosecond temporal resolution. This study addresses the challenge of tracking electron motion at the so-called space-time limit, specifically during the process of tunneling through an energy barrier.

Approach

The researchers combined scanning tunneling microscopy (STM) with ultrafast laser technology to create a sub-femtosecond lightwave-driven STM. By using two phase-controlled near-infrared pulses, they modulated the tunneling barrier at the tip-sample junction. To isolate the light-driven current from thermal artifacts and background noise, they employed a carrier-envelope phase (CEP) modulation scheme. This allowed them to measure the tunneling current with sub-cycle precision, which they then compared against full real-space/time-domain quantum simulations using time-dependent density functional theory (TD-DFT).

Main Findings

The experiments revealed that electron tunneling at these timescales is no longer instantaneous. The researchers identified isolated tunneling transients shorter than 1 femtosecond, demonstrating that the electron's response is delayed relative to the driving electric field. This retardation is a hallmark of the intermediate light-matter interaction regime, where the tunneling process involves transient occupation of excited states. By mapping these attosecond-confined wave packets, the team successfully localized electron motion on the angstrom scale, providing a direct observation of how light-field modulation reshapes electronic wavefunctions during charge transfer.

Why It Matters

This work bridges the gap between attosecond science and atomic-scale microscopy. By enabling the direct observation of electron dynamics inside atoms and molecules, this technique provides a powerful new tool for studying fundamental quantum processes, such as chemical bond formation and the behavior of electrons in crystalline solids, at their intrinsic speed and size limits.

Key Terms and Definitions

Space-time limit — The intrinsic scale of electronic motion where spatial and temporal dynamics are coupled, requiring both angstrom-scale and attosecond-scale resolution to observe.
Carrier-envelope phase (CEP) — The phase relationship between the peak of the carrier wave of an optical pulse and the peak of its intensity envelope, which is critical for controlling sub-cycle electron dynamics.
Keldysh parameter — A dimensionless value used to characterize the regime of light-matter interaction; a value near 1 indicates an intermediate regime where both tunneling and multi-photon excitation processes contribute to charge transfer.
Stroboscopic measurement — A technique used here to extract the CEP-dependent part of the tunneling current by modulating the laser phase at a specific frequency and using lock-in detection to isolate the signal.
Born–Oppenheimer approximation — A standard assumption in molecular physics that electrons react instantaneously to nuclear motion, which this study shows is insufficient for describing electron dynamics at the attosecond scale.

Tracking electrons at the space-time limit: Research Paper Podcast