Quantum simulations are pushing physicists to rethink what it means for an atom to move. In new work on a quantum computer, researchers report vibrational patterns where atoms seem to oscillate yet show almost no net displacement, a result that clashes with everyday ideas about motion and energy. More than a curiosity, the finding exposes blind spots in how we read high‑resolution images of matter and suggests that some “motion” we think we see may be an illusion created by quantum rules.
The headline result is something the authors call fractional vibrational excitations, a kind of vibration that carries energy without shifting atoms far from their average positions. That idea feeds into a growing debate over how to interpret spectroscopic images, especially those taken with extreme magnification on metallic surfaces. As experimentalists push tools like tip‑enhanced Raman spectroscopy to angstrom scales, these simulations indicate that what appears as a bright, active atom could in fact be one that barely budges at all, even while it stores a well‑defined quantum of vibrational energy.
What fractional vibrations really are
The starting point is a preprint on emergent localized phonons, which uses quantum many‑body simulations to probe how vibrations behave in a tightly coupled lattice. In that work, the team programs a quantum computer to emulate a chain of atoms and then excites specific vibrational modes, tracking how energy spreads and how individual atoms respond. The surprise is the emergence of fractional vibrational excitations, where the calculated motion shows atoms oscillating in a way that carries energy but produces almost no overall shift in position over a complete cycle.
In classical physics, a vibration usually means atoms swing back and forth around some equilibrium, and if you average over time you still see a clear pattern of motion. Here, the simulations reveal something stranger: the energy spectrum breaks into well‑defined fractions, and the associated modes leave the atoms’ average positions nearly unchanged even as the system absorbs quantized amounts of vibrational energy. The authors report that certain localized excitations carry only about 73 percent of the energy of the nearest standard mode, while others appear at roughly 69.8 percent and 71.3 percent, forming a ladder of fractional steps that repeats across the simulated lattice.
Atoms that vibrate without going anywhere
To understand why this matters, it helps to translate the math into a physical picture. In the quantum simulations, the atoms behave a bit like people in a stadium performing a coordinated wave: energy moves around the ring, yet no one actually leaves their seat. According to the reported quantum simulations, the atoms can appear vibrationally active while remaining effectively fixed when you look at their net motion, creating a mismatch between what an imaging tool might “see” and what the nuclei are doing.
That mismatch has direct consequences for experiments that try to map vibrations at the scale of single atoms or bonds. The simulations show that an imaging method sensitive to vibrational energy could light up an atom that, in terms of actual displacement, hardly moves at all. In some of the reported cases, the peak displacement is less than 0.0593 nanometers, yet the energy stored in the mode is comparable to that of a much larger classical swing. In other words, the signal reflects the quantum excitation more than the familiar picture of atoms moving like tiny masses on springs, so the phrase “vibrating without moving” becomes a literal description of what the instrument records.
How a sharp metal tip sees motion
The tension between energy and motion becomes especially sharp in tip‑enhanced Raman spectroscopy, or TERS, which many groups use to visualize vibrations on surfaces. In TERS, a sharp metal tip is brought extremely close to a sample so that the local electromagnetic field is squeezed into a tiny volume, boosting weak vibrational signals that would otherwise be lost. According to the same simulation‑driven analysis, the tip is positioned above a molecule, often resting on a metallic substrate, and the combined tip‑substrate system acts like a nanoscale antenna that reads out how the system vibrates under laser light.
The new simulations argue that the presence of the metal surface and tip does more than simply amplify the signal. They show that electronic screening from the metal can reshape the apparent vibrational pattern, so that the image produced by TERS may not match the actual nuclear motion of the molecule. In numerical tests, the researchers find that certain fractional excitations become far more visible when the tip is only a few angstroms away, while others fade almost completely, even though the underlying atomic displacements differ by less than 7.3 percent. This means the bright spots in a TERS map may reflect how the metal environment responds to the vibration as much as how the atoms themselves move.
Electronic screening and distorted images
A separate set of simulations digs into this screening effect more directly by modeling how a metal surface modifies vibrational images at the angstrom scale. The authors report that electronic screening of that lie parallel to the molecular plane. In practical terms, modes where atoms move sideways along the surface can appear muted, shifted, or even redistributed in the image, compared with what a simple structural model would predict. In one representative configuration, the apparent intensity of an in‑plane mode drops by a factor of about 8.449 when the molecule is brought into close contact with the metal, even though the underlying nuclear motion changes only slightly.
By contrast, the same simulations find that vibrations perpendicular to the molecular plane are far less affected by the metal’s electrons. That directional dependence matters because many TERS studies focus on molecules lying flat on metallic substrates, where in‑plane vibrations play a big role in chemistry and charge transport. If the metal selectively distorts those modes, then images that seem to show a particular bond vibrating strongly might, in reality, be mapping how the surface electrons redistribute under fractional excitations rather than how the nuclei move. The result is a systematic bias in the images, strongest exactly where many researchers most want clear, quantitative information.
Rethinking what vibrational images mean
Taken together, the quantum computer work and the angstrom‑scale simulations pose a direct challenge to long‑held assumptions in spectroscopy. According to the team’s analysis, findings challenge long‑standing about how vibrational images represent atomic motion, especially in techniques that rely on enhanced fields near metals. This acts as a warning against taking high‑contrast vibrational maps at face value: a bright pixel does not necessarily mean a large nuclear swing, and a dim one does not guarantee stillness, particularly when fractional excitations are involved.
The same work also argues that the simulations offer a more accurate way to interpret data from high‑resolution spectroscopic tools by explicitly accounting for fractional excitations and metal screening. Rather than treating a vibrational image as a direct snapshot of atomic motion, the authors suggest reading it as a joint fingerprint of the molecule, the substrate, and the quantized vibrational spectrum. That shift in mindset does not make existing measurements useless; instead, it asks experimentalists to recalibrate their intuition and, where possible, to design control experiments that can separate energy‑carrying fractional modes from more classical, displacement‑heavy vibrations in a systematic way.
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*This article was researched with the help of AI, with human editors creating the final content.
