The achievement rests on a technique that has become one of the most powerful control knobs in attosecond science: two-colour waveform synthesis. Instead of hitting the tunnel junction with a single laser beam, the researchers combined two coherent light fields of different colours, a fundamental frequency and its second harmonic. When these two fields overlap, their sum is no longer a simple sinusoidal oscillation. Within each optical cycle, the combined electric field becomes an asymmetric waveform, with a strong half-cycle in one direction and a weaker, differently shaped excursion in the other. For an electron contemplating a quantum jump across the tunnel barrier, that asymmetry matters enormously. The instantaneous field tilts the barrier, lowers it on one side, and determines the precise instant, within the optical cycle, when tunnelling is most probable. By adjusting the relative phase between the two colours, the team could shift that favoured instant back and forth, and with it, flip the direction of the resulting current pulse.

What makes the result remarkable is the combination of timescale and spatial precision. The junction in a scanning tunnelling microscope is a few nanometres or less across, and the tunnelling process itself is confined to a region far smaller than that. The researchers demonstrated sub-ångström topographic sensitivity, meaning the laser-driven current responded to height changes on the surface at the level of individual atoms, while achieving a lateral spatial resolution of two nanometres. In other words, the attosecond current they generated and steered remained a genuinely microscopic probe, not a diffuse photoresponse smeared across the sample. And they did it under ambient conditions, at room temperature and in air, rather than in the cryogenic vacuum that most ultrafast microscopy experiments demand.

A persistent obstacle in laser-driven STM has been heat. When laser light strikes the tip or the sample, some of its energy is inevitably absorbed, raising the temperature of the junction and producing thermal expansion of the tip, thermally assisted currents and other artefacts that can masquerade as genuine ultrafast dynamics. Earlier femtosecond and picosecond experiments in laser-assisted STM were often haunted by exactly this problem, and distinguishing a true sub-cycle electronic response from a slow thermal signal became a central methodological challenge for the field. The Technion team addressed this directly. Because their current control depends on the sub-cycle waveform of the two-colour field, flipping the carrier-envelope phase flips the current direction, whereas a thermal effect would be completely indifferent to the sign of the electric field. The observed phase-dependent, direction-reversing current is therefore an unambiguous signature of coherent, lightwave-driven electron transport, and the experiment was explicitly designed to be free of thermal artefacts.

To understand the mechanism, the researchers turned to theory at two levels of description. On the one-electron side, they employed a strong-field model of tunnelling, in the spirit of the Keldysh framework that has underpinned strong-field physics since 1965, treating the electron as a wavepacket launched through a field-tilted barrier. On the many-body side, collaborators performed time-dependent density functional theory calculations of the full metal-insulator-metal junction, capturing the collective response of the electrons in the electrodes. Projecting the experimental data onto both theoretical descriptions revealed a consistent physical picture: a three-step transport process operating in the non-adiabatic tunnelling regime. In this regime, the optical field changes so quickly that the electron cannot follow it adiabatically; instead, the field launches the electron into the barrier, the wavepacket propagates across the gap while the field continues to evolve, and the electron is finally collected on the far side, with the whole sequence compressed into a fraction of an optical cycle.

From their theoretical analysis, the team extracted a concrete number for the duration of the current burst: 860 attoseconds. That figure, less than a single femtosecond, quantifies how briefly the electrons stream through the junction during each pulse. It also places the experiment firmly in the lightwave electronics regime, where the oscillating carrier wave of light, rather than a sequence of electronic switches, dictates the timing of current flow. Lightwave electronics has been advancing rapidly in recent years, with petahertz-scale field control demonstrated in nanotips, nanoantennas and solid-state devices, but the STM junction offers something none of those platforms can: the ability to position the driven current with atomic-scale precision on a surface and, in principle, to image ultrafast charge dynamics as they unfold at specific sites, such as a single molecule or a defect.

The lineage of this work runs through several landmark experiments. Attosecond science itself was born from the recognition that strong laser fields can ionize atoms within a fraction of an optical cycle, and that the emitted electrons carry timing information about that process. In 2011, researchers at the Max Planck Institute of Quantum Optics showed attosecond control of electrons emitted from a nanoscale metal tip, extending strong-field physics to nanostructures. Later experiments traced attosecond electron emission from tungsten nanotips and demonstrated attosecond field emission from nanotips driven by terahertz-scale waveforms. In parallel, ultrafast STM developed along its own track, first with pulsed lasers that suffered from thermal complications, then with single-cycle terahertz pulses that enabled femtosecond imaging of molecular motion, and more recently with all-optical sub-cycle microscopy on atomic length scales. The new work synthesizes these threads, bringing waveform-level attosecond control, the hallmark of gas-phase strong-field physics, into the spatially resolved world of the STM.

The two-colour approach itself has a distinguished history. Phase-controlled ionization with two colours was explored in atoms in the 1990s, and coherent control of above-threshold photoemission from tungsten nanotips followed in 2016. What the Technion team added was the junction geometry and the diagnostic power of the STM platform. In a tunnel junction, the current direction is a measurable, meaningful quantity: electrons can flow from tip to sample or from sample to tip, and the net laser-induced current reflects the asymmetry of the driving waveform. By scanning the two-colour phase and recording how the current reverses, the researchers effectively performed attosecond streaking inside the junction, using the optical waveform itself as the clock that timestamps the tunnelling event.

The practical implications reach toward a long-sought goal: triggering and imaging ultrafast charge dynamics at specific locations on a surface. Many of the most important processes in chemistry, materials science and nanotechnology, from charge transfer in molecular junctions to carrier dynamics in photovoltaic materials, begin with electron motion on attosecond to femtosecond timescales localized to nanometre-scale regions. A microscope that can launch a directional attosecond current pulse at a chosen point and then read out the response could watch those processes unfold in real space and real time simultaneously. The two-nanometre lateral resolution and sub-ångström vertical sensitivity reported here, achieved under ambient conditions, suggest that such spatiotemporal microscopy need not be confined to specialized cryogenic laboratories.

There remain challenges to overcome before attosecond STM becomes a routine tool. The current bursts are generated by intense optical fields, and extending the technique to more delicate samples, to insulating substrates and to single-molecule junctions will require careful control of the near-field enhancement at the tip. The theoretical framework, spanning one-electron strong-field models and full many-body simulations, will need to keep pace as experiments move to more complex materials. But the conceptual barrier has now been crossed. Electrons tunnelling through the gap of a scanning tunnelling microscope have been caught in the act, timed to within a few hundred attoseconds, and steered at will by the waveform of light. The fastest event in electronics and the sharpest eye in microscopy are, at last, looking at the same thing at the same time.

Subject of Research: Attosecond control of laser-driven electron tunnelling currents in a scanning tunnelling microscope junction

Article Title: Attosecond current control and timing in a scanning tunnelling microscope

Article References: Davidovich, D., Ma, B., Goldner, A., Cohen, S., Chen, Z., Borisov, A. G., & Krüger, M. (2026). Attosecond current control and timing in a scanning tunnelling microscope. Nature Photonics. https://doi.org/10.1038/s41566-026-02021-y

Image Credits: AI Generated

DOI: 10.1038/s41566-026-02021-y

Keywords: attosecond science, scanning tunnelling microscopy, lightwave electronics, two-colour laser pulses, quantum tunnelling, ultrafast microscopy, strong-field physics, TDDFT, nanoscale junctions, carrier-envelope phase, electron transport, Nature Photonics

The study, published in Nature Photonics, reports angle-resolved attosecond streaking measurements on helium and neon atoms, the pristine quantum systems that serve as the hydrogen atoms of attosecond physics. By recording not just the energies of photoelectrons kicked out by an extreme ultraviolet attosecond pulse, but also the full angular distribution of their emission directions, the team gained access to a quantity that conventional streaking discards entirely: the angular momentum content of the freed electron waves.

To appreciate why this matters, it helps to revisit how attosecond streaking works. A burst of extreme ultraviolet light, lasting mere hundreds of attoseconds, tears an electron from an atom. Almost simultaneously, a strong infrared laser field, the streaking field, acts on the liberated electron, shifting its final kinetic energy by an amount that depends on the vector potential of the light wave at the moment of ionization. Sweeping the delay between the two pulses maps out that vector potential, and with it the timing of photoemission itself. This is the scheme that underpinned the birth of attosecond metrology, celebrated in the 2001 measurement of light pulses shorter than a femtosecond, and it has since been used to clock tunneling delays and inner-shell processes with astonishing precision.

The classical reading of streaking treats the electron as a point charge absorbing a momentum kick from the field. But quantum mechanics insists that the electron, once free, is a superposition of partial waves, spherical wave components each labeled by a definite angular momentum quantum number. When the infrared field shakes the freshly ionized electron, it drives so-called continuum-continuum transitions, absorbing and emitting infrared photons while the electron remains unbound. These transitions redistribute population among the partial waves, and the resulting interference patterns encode themselves in the angular distribution of the detected electrons. Angle-resolved streaking, the team realized, is therefore not merely a timing measurement but a movie of angular momentum in motion.

The experimental apparatus combined intense attosecond pulses with a high-resolution velocity-map imaging spectrometer of the thick-lens design, capable of recording photoelectron momentum distributions with fine angular resolution. By scanning the delay between the attosecond pulse and the infrared streaking field in steps far finer than the optical cycle, the researchers assembled two-dimensional movies in which the photoelectron angular distributions visibly morph from one delay to the next. Measurements were performed on helium, whose simple two-electron structure makes theoretical treatment tractable, and on neon, which adds the complexity of multiple ionization channels, including contributions from both the 2p and 2s orbitals.

Theoretical support came from two independent computational frameworks. The team solved the time-dependent Schrödinger equation, the gold standard of strong-field theory, which tracks the full wavefunction of the atom and electron without approximation, and they also applied the strong-field approximation, a simplified model that treats ionization and subsequent field interaction in a controlled analytic limit. The agreement between experiment and the TDSE simulations was striking, and the SFA analysis provided the interpretive lens through which the measured angular distributions could be decomposed into their constituent partial waves and their time-dependent weights.

What the movies revealed is genuinely surprising. When the attosecond pulse ionizes the atom near the crest of the electric field of the infrared wave, the escaping electron populates a richer set of partial waves than when ionization occurs near the crest of the vector potential, a quarter cycle away. Since these two crests define the classical and quantum pictures of the light wave, the finding demonstrates that the quantum character of the interaction depends on precisely which feature of the field the electron encounters first. Moreover, the dominant interference mechanism switches character over the course of the streaking cycle: near one phase, partial waves of the same parity interfere, while at another, opposite-parity channels take over. Parity, the symmetry of a wavefunction under inversion of coordinates, is one of the most fundamental labels in atomic physics, and watching the interference regime flip between same- and opposite-parity contributions amounts to watching quantum symmetry bookkeeping play out in real time.

Equally striking is the strength of the multiphoton coupling the team uncovered. The infrared dressing field in a streaking experiment is comparatively gentle by strong-field standards, and one might expect the freed electron to exchange at most a single infrared photon in a perturbative handshake. Instead, the angular distributions show that substantial multiphoton continuum-continuum coupling persists even at weak infrared intensities, meaning the liberated electron absorbs and emits multiple infrared photons during its flight. The relevant interaction strength in the experiment was characterized by the dimensionless parameter gamma, defined as the product of the electron momentum, the peak vector potential, and the infrared frequency, which reached approximately 3.3 in the measurements, well into the regime where multiphoton dynamics flourish. This non-perturbative character has been latent in every streaking measurement ever performed, subtly shaping the very energy shifts researchers have been interpreting with quasi-classical formulas.

The implications ripple outward across attosecond science. Streaking underlies measurements of photoemission delays that probe electron correlation, Wigner-like time lags in atomic and molecular ionization, and the characterization of ever-shorter light pulses, including isolated attosecond and even attosecond-scale X-ray pulses from free-electron lasers. A quantum-state-resolved understanding of the streaking mechanism itself gives experimentalists a sharper tool for separating intrinsic atomic delays from measurement-induced ones, a distinction that has occupied the field since delays in photoemission were first clocked in helium in 2010. The results also connect to a broader frontier in which angular distributions of photoelectrons serve as reporters of wave-packet structure, from Fano resonance phase measurements to attosecond chronoscopy of electron vortices and chiral photoionization dynamics.

For a technique so mature, the discovery carries a certain humility: one of attosecond science’s most reliable instruments has been quietly performing quantum acrobatics all along. The team has made its experimental data publicly available through Zenodo, and the delay-resolved photoelectron movies of helium and neon are available as supplementary material, allowing anyone to watch partial waves rise and fall across an optical cycle. As attosecond physics pushes toward ever more complex targets, from molecules to surfaces to liquids, the lesson of this work is clear: the measuring light does not merely nudge electrons along classical trajectories, it reshapes their quantum identity, and only by reading the angular momentum of the liberated waves can the full story of ultrafast dynamics be told. The stopwatch, it turns out, has been keeping quantum time.

Subject of Research: Angle-resolved attosecond streaking of photoelectron angular momentum in helium and neon

Article Title: Attosecond streaking of photoelectron angular momentum

Article References: Gao, J., Liang, H., Hasan, M., Yuan, Y., Eisenhutt, Z., Tsai, M.-S., Chen, M.-C., & Han, M. (2026). Attosecond streaking of photoelectron angular momentum. Nature Photonics. https://doi.org/10.1038/s41566-026-02020-z

Image Credits: AI Generated

DOI: 10.1038/s41566-026-02020-z

Keywords: attosecond science, attosecond streaking, photoelectron angular distributions, angular momentum, continuum-continuum transitions, helium, neon, strong-field approximation, time-dependent Schrödinger equation, Nature Photonics, ultrafast photonics, multiphoton coupling

Chirality has traditionally been considered a geometric or structural property of molecules. The distinct left- or right-handed configurations of molecules such as amino acids and sugars dictate their biological functions and interactions. For instance, the chirality of a drug molecule determines whether it will be beneficial, inert, or even toxic within the human body. Despite this well-established paradigm, the static view of chirality neglects the ultrafast dynamics of the electron cloud that envelops these molecules, which can also exhibit handedness in their motion and interactions.

Addressing this limitation, Professor Hans Jakob Wörner and his research team have taken chirality research into entirely new territory by examining how electrons themselves behave differently when ejected from chiral molecules. Their approach harnesses an advanced technique that uses ultra-short bursts of circularly polarized light—attosecond pulses that last only a billionth of a billionth of a second—to probe electron ejection dynamics with extraordinary temporal precision. This innovation reveals for the first time that the electrons stripped from chiral molecules do not just reflect the structural handedness but possess their own directional handedness tied intimately to the chirality of the molecule and the light’s rotation.

The central phenomenon explored in the study is photoelectron circular dichroism (PECD), a quantum effect where an electron’s emission direction depends on the interplay between the molecule’s chirality and the helicity of the circularly polarized light used to excite it. Remarkably, the electrons do not eject symmetrically but preferentially along or opposite to the propagation direction of the light beam, depending on their mirror-image configurations. Observing PECD has so far been limited by technological constraints, but the newly developed attosecond pulse setup surmounts these barriers, allowing not only detection but also temporal manipulation of this effect.

Wörner’s team employed a sophisticated experimental arrangement combining circularly polarized attosecond pulses in the extreme ultraviolet (XUV) spectral range with a synchronized circularly polarized infrared pulse. This dual-pulse approach confers remarkable control: by adjusting the relative phase between the two pulses, the researchers can modulate the timing and direction of electron emission from chiral molecules. This technique unveils the ultrafast electron dynamics underlying PECD at their natural attosecond timescale and demonstrates that chirality manifests not just in static spatial arrangements but also in fleeting electron motions.

The experimental breakthrough is as much a technological feat as a conceptual advance. Generating circularly polarized attosecond pulses requires precision engineering of high-harmonic generation processes under carefully controlled conditions. By producing these tailored light flashes and synchronously overlaying them with infrared pulses, the team could visualize and actively steer chiral electron emissions in real time. This attosecond precision pushes the boundaries of chiral spectroscopy and electron dynamics, marking a new era where electron flow itself is explored as an intrinsic chiral property.

From a fundamental science perspective, the implications of this work extend far beyond the immediate ability to measure PECD. It challenges the long-held notion that chirality is exclusively a spatial qualifier by establishing chirality as a fundamentally dynamic electronic property. According to Meng Han, the study’s first author, the discovery that electron behavior in chiral molecules can be directly controlled on attosecond time scales invites rethinking of chiral phenomena, laying a foundation for manipulating molecular processes with unparalleled finesse.

The potential practical applications of these insights are equally profound. Chirality plays a defining role in the pharmaceutical industry, where the wrong enantiomer of a drug can cause adverse effects. Enhancing the sensitivity and specificity of chiral analysis through attosecond techniques could revolutionize drug design and safety testing. The attosecond flash spectroscopy and coherent control of electron emission dynamics might also facilitate novel synthetic pathways, enabling selective manipulation of chemical reactions based on molecular handedness.

Moreover, the new approach promises to provide answers to long-standing questions about the origins of molecular chirality in biological systems—a puzzle that touches on the fundamental nature of life itself. By observing how electronic motion evolves and is controlled in chiral molecules, scientists could gain fresh perspectives on the emergence and evolution of homochirality, a key feature of biochemical systems where only one handedness predominates.

Beyond chemistry and biology, the ability to control chirality at the electronic level heralds innovative possibilities in emerging fields such as spintronics, where electron spin and its manipulation underpin next-generation information processing technologies. The precise control over electron emission directionality might be harnessed to develop molecular-scale electronic devices and sensors that exploit chiral-induced spin selectivity for enhanced performance.

This breakthrough further aligns with the development of molecular machines and biosensors, as controlling electron dynamics with attosecond resolution and chiral specificity could enable intricate mechanical and sensing functions at the nanoscale. The fusion of attosecond physics with molecular chirality thus offers a powerful toolkit for advancing nanotechnology platforms that rely on dynamic electronic interactions.

In essence, this pioneering research by Wörner and his collaborators transcends the classical boundaries of chirality science. By illustrating that chirality is as much about the ultrafast behavior of electrons as it is about the arrangement of atoms, they have opened the door to a new understanding of how molecular asymmetry shapes the quantum world. This could transform theoretical and applied sciences, providing researchers with unprecedented control over the fundamental processes that govern molecular functionality.

The study epitomizes the convergence of quantum physics, chemistry, and ultrafast laser technology, demonstrating how cutting-edge experimental techniques can unravel phenomena previously hidden due to temporal or spatial constraints. As attosecond methodologies continue to mature, their integration into the study of chirality promises a rich harvest of insights, from elucidating complex biomolecular mechanisms to enabling novel technological applications rooted in the quantum characteristics of matter.

This research not only enriches our comprehension of the intricate dance of electrons within chiral molecules but also sets a compelling example of how probing the fastest processes in nature can reveal entirely new scientific vistas. The dynamic nature of chirality, as revealed through attosecond control of photoelectron emissions, beckons researchers to rethink conventional concepts and explore the full potential of chiral electronic phenomena in diverse scientific and technological domains.

Subject of Research:
Attosecond-scale electron dynamics in chiral molecules and their control via circularly polarized light.

Article Title:
Attosecond control and measurement of chiral photoionization dynamics.

Web References:
http://dx.doi.org/10.1038/s41586-025-09455-4

References:
Published in Nature; authors include Hans Jakob Wörner and Meng Han et al.

Keywords

Chirality, Photoelectron Circular Dichroism, Attosecond Pulses, Circularly Polarized Light, Electron Dynamics, Quantum Control, Ultrafast Spectroscopy, Molecular Asymmetry, Spintronics, Molecular Machines, Biosensors, High-Harmonic Generation