Bright squeezed vacuum reveals hidden quantum effects in strong-field physics

November 20, 2025 feature Bright squeezed vacuum reveals hidden quantum effects in strong-field physics by Tejasri Gururaj, Phys.org edited by Gaby Clark, reviewed by Robert Egan Editors' notes This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: fact-checked peer-reviewed publication trusted source proofread The GIST Add as preferred source An illustration of the setup used to generate BSV and measure strong-field energy spectra. Credit: J. Heimerl et al/Nature Physics. https://doi.org/10.1038/s41567-025-03087-1. In a new study published in Nature Physics, researchers have demonstrated that quantum light, particularly bright squeezed vacuum (BSV), can drive strong-field photoemission at metal needle tips. Attosecond science—the study of electron behavior on timescales of 10⁻¹⁸ seconds—has traditionally relied on intense laser pulses that correspond to "coherent states" of light. They function as classical electromagnetic waves with predictable, oscillating electric fields that push electrons to high energies. When electrons rescatter from surfaces under this intense illumination, they produce characteristic signatures: a plateau in their energy spectrum followed by a sharp cut-off. These features have become central to probing matter with attosecond precision. The new research explores whether genuinely quantum light, with fundamentally different properties than classical laser pulses, can still drive these same strong-field dynamics. "Our motivation came from the long-standing separation between strong-field physics and quantum optics," co-author Dr. Jonas Heimerl from the Friedrich-Alexander University Erlangen-Nürnberg (FAU) told Phys.org. "These two areas traditionally deal with vastly different light regimes—one with extremely intense, classical light fields and the other with the quantum properties of light, typically involving only a few photons. We were fascinated by the idea of bridging this divide." A counterintuitive quantum phenomenon Unlike the traditional coherent laser, BSV represents a quantum state of light with unusual properties. In classical laser pulses, the oscillating electric field follows a predictable sinusoidal curve, with a well-defined amplitude. BSV, by contrast, consists entirely of intense quantum fluctuations that oscillate at twice the carrier frequency, with no coherent wave component at all. "Classically, strong-field effects require a strong, oscillating electric field that accelerates electrons," explained co-author Jonathan Pölloth, from FAU. "In BSV, however, the average field is zero—meaning that, on average, there should be no force acting on the electrons. The counterintuitive part is that amplified quantum fluctuations alone, without a mean field, are sufficient to drive electrons to high energies." Despite having zero mean field, BSV can still be extremely intense because its intensity depends on the square of the electric field. In collaboration with Andrei Rasputnyi and Maria Chekhova, the team generated BSV using an unseeded optical parametric amplifier, creating pulses centered at 1,600 nm wavelength with durations of 25 femtoseconds. These pulses exhibited enormous photon-number fluctuations from shot to shot, ranging from nearly zero to approximately 1012 photons per pulse.

The team selected BSV specifically to test whether quantum light could drive strong-field physics without a classical field component. Experimental setup and methodology To investigate this puzzle, the team focused their BSV pulses onto a tungsten needle tip with a radius of just a few tens of nanometers, held inside an ultrahigh vacuum chamber. Tungsten is widely used for such experiments due to its suitable work function and stability. The experimental setup requires two critical conditions. First, the nanoscale sharpness of the tip concentrates and amplifies the optical field at its apex, creating the intense conditions needed for strong-field photoemission. Second, clean, reproducible measurements require ultrahigh vacuum conditions to prevent tip surface contamination. When the quantum light struck the tip, electrons were emitted through nonlinear photoemission—a process where multiple photons must work together to liberate an electron from the metal surface. The researchers used a custom-built electron spectrometer that could measure not only the total number of electrons, but also the energy of each individual electron. Crucially, photon number measurements for each light pulse were synchronized using a photodiode. This shot-resolved detection was necessary for the analysis. For each of approximately 600,000 BSV pulses, the team recorded both the photon number and the energies of the emitted electrons, allowing them to correlate the two measurements on a pulse-by-pulse basis. Emergence of hidden patterns When the researchers first analyzed their data by averaging over many pulses, they observed broad electron energy spectra extending beyond 60 electronvolts, far exceeding the energies expected for classical light of comparable mean intensity. However, these averaged spectra showed no clear plateau or cut-off, the signature features expected when electrons rescatter from surfaces under intense light. The breakthrough came when they post-selected their data, sorting electron spectra based on the number of photons detected in each individual pulse. "Initially, when we averaged over many BSV pulses, the characteristic plateau feature of strong-field physics disappeared because squeezed vacuum light has strong photon number fluctuations from pulse to pulse, leading to 'averaging' and smearing-out of the underlying structure," explained co-author Dr. Peter Hommelhoff, Professor of Physics at FAU. "To counter this, we realized we could post-select spectra according to the photon number of each individual pulse. This way, we recovered the hidden plateau and cut-off features." For pulses with higher photon numbers, the electron energy spectra showed clear plateaus ending in sharp cut-offs at well-defined energies. These features matched those seen with classical coherent light and increased proportionally with photon number—exactly as expected from the "10-Up law" that governs strong-field rescattering. From the cut-off energies, the team extracted an optical field enhancement factor of 3.4±0.6 at the needle tip, consistent with values obtained using coherent light. The results indicate that electrons driven by BSV behave as if driven by an ensemble of coherent light pulses, each with a different intensity. "Even though the squeezed vacuum has no classical field, its quantum state can be understood as a superposition of many coherent field components—each capable of driving the electrons as a classical pulse would," explained Dr. Heimerl. "The electrons effectively sample this ensemble of coherent states." Computer simulations supported this interpretation, reproducing the experimental results when the team modeled BSV as a weighted sum of different classical field intensities according to the quantum state's probability distribution. Beyond theory The work opens new possibilities for using strongly driven electrons as quantum light sensors. "Strongly driven electrons are extremely sensitive to the instantaneous electric field of light," explained Pölloth. "Unlike conventional photon-counting techniques, this method directly measures how quantum light can affect matter on ultrafast timescales and could potentially reveal spatial and temporal features of quantum fields with attosecond resolution." The work advances strong-field quantum optics, an emerging field that seeks to understand how quantum properties of light influence extreme light-matter interactions. "Future studies could explore how different types of quantum light influence strong-field processes like high-harmonic generation and above-threshold ionization," added Hommelhoff. The researchers note that adding a second classical light field to the BSV could enable full quantum state tomography, highlighting specific quantum features depending on the relative phase between the two fields. Written for you by our author Tejasri Gururaj, edited by Gaby Clark, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you. More information: Heimerl et al, Quantum light drives electrons strongly at metal needle tips, Nature Physics (2025). DOI: 10.1038/s41567-025-03087-1. Journal information: Nature Physics © 2025 Science X Network Citation: Bright squeezed vacuum reveals hidden quantum effects in strong-field physics (2025, November 20) retrieved 7 January 2026 from https://phys.org/news/2025-11-bright-vacuum-reveals-hidden-quantum.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.