Attosecond pulse trains are now generated without precisely aligning emitted harmonics, overturning classical theory. Sándor Varró and Ákos Gombkötő at the Wigner Research Centre for Physics have shown these pulses can be created by utilising newly defined quantum states called “attoquants” which exhibit intermodal entanglement. This occurs through collective atomic effects where atoms radiate indistinguishable harmonics at pressures around 10−5 Torr used in ion and electron experiments.

A new method for creating attosecond pulse trains, extremely brief bursts of light, bypasses previously understood limitations within high-harmonic generation. Traditionally, generating these pulses required precise alignment of individual waves as harmonics; however, this research demonstrates creation via attoquants exhibiting intermodal entanglement, a phenomenon linking multiple wave components. The discovery relies on collective atomic effects wherein atoms radiate indistinguishable harmonics under typical experimental conditions.

Sándor Varró and Ákos Gombkötő from the Wigner Research Centre for Physics have overturned a long-held assumption about forming attosecond pulse trains, extremely short bursts of light used to study rapid atomic processes. Previously, generating these pulses relied on ‘mode-locking’, ensuring simultaneous arrival of all wave components. However, this team demonstrated that such precision isn’t always necessary, instead utilising attoquants which exhibit intermodal entanglement. The researchers discovered attoquants possess key robustness against phase variations, meaning a structured pulse train can form even with random timing differences between emitted waves.

Intermodal entanglement builds strong attosecond pulse generation via collective atomic harmonics

A technique centred on creating ‘attoquants’, new quantum states designed to circumvent traditional limitations in attosecond pulse generation, was employed; these are constructed from multiple light waves linked through intermodal entanglement, meaning the properties of each wave component correlate with others irrespective of distance. Coherent permanent states were analysed, built as superpositions where every possible combination of harmonic frequencies is equally weighted and combined constructively without precise phase alignment.

Collective atomic effects underpin this method; driving many atoms simultaneously enables indistinguishable harmonics to be emitted which then naturally form the entangled attoquant structure at typical experimental pressures used for ion and electron studies, approximately 10−5 Torr.

This development builds upon initial work by demonstrating how such quantum states enable attosecond pulse generation while avoiding stringent control over light waves. Utilising collective atomic effects allows multiple atoms to emit indistinguishable harmonics, resulting in a natural entanglement within these structures under standard conditions. This advancement moves scientists beyond conventional methods reliant on meticulous phase control during high-harmonic generation.

Entanglement surpasses phase locking in generation of high intensity attosecond pulses

Attosecond pulse trains generated with entanglement now exceed levels previously thought possible without phase locking, as demonstrated by researchers at HUN-REN Wigner Research Centre for Physics; traditional techniques required precise alignment of harmonic frequencies, but these pulses were created using quantum states exhibiting intermodal entanglement. The breakthrough circumvents the need for mode-locking, a process ensuring all light waves arrive simultaneously and considered indispensable for generating ultra-short bursts used to study rapid atomic processes. Analysing coherent permanent states alongside collective atomic effects where atoms radiate indistinguishable harmonics proved that a locked Fourier superposition can be achieved even when emitted waves have random timing differences.

Detailed calculations of photon statistics, specifically measuring logarithmic negativity, validated this approach by confirming genuine nonclassical entanglement within their generated attosecond pulse trains. Analyses also focused on the Wigner function which provides insight into the state’s quasi-probability distribution; these demonstrate fundamentally quantum behaviour beyond conventional mode-locking techniques and validate its unique characteristics as an ‘attoquant’, coined by the scientists to describe states insensitive to relative phases.

Multi-atom effects during high harmonic generation were identified, revealing collectively driven atoms radiating indistinguishable harmonics creating permutation symmetry in each attoquant’s entangled structure. Demonstrating strong pulse structures without phase locking is key but scaling up production or maintaining coherence over extended periods for practical applications remains a challenge.

Entangled attoquanta offer a new route towards coherent extreme ultraviolet pulses

Attosecond science promises increasingly detailed snapshots of electron motion within atoms and molecules, driving advances across materials science and biology; however, generating these incredibly brief light pulses traditionally demanded careful control over emitted harmonics to ensure all contributing waves arrive at precisely the same moment. The researchers now challenge this established model with their theoretical exploration of ‘attoquants’, entangled quantum states capable of producing structured pulse trains even without such precise alignment. Acknowledging that creating genuinely entangled states remains an experimental hurdle, this work offers an important shift in perspective regarding attosecond pulse generation.

The team demonstrated a pathway towards extremely short bursts of light used in studying atomic processes without relying on mode-locking, where wave components must arrive simultaneously. Instead they explored intermodal entanglement using these newly defined ‘attoquant’ states which exhibit correlated linked wave properties regardless of distance. Calculations confirmed the attoquants possess nonclassical characteristics and photon statistics distinct from simply mimicking short pulses through timing alone.

Researchers have shown it is possible to generate coherent extreme ultraviolet pulses via entangled quantum states called ‘attoquanta, circumventing the need for precise phase alignment traditionally required during high harmonic generation. This means attosecond pulse trains can be created even when emitted harmonics are not perfectly synchronised, offering an alternative approach to producing these brief light bursts.

The scientists analytically demonstrated this with a specific state, a symmetric superposition of products of coherent states, confirming its genuinely nonclassical and entangled nature. They propose multi-atom effects contribute to generating such linked states during high harmonic generation, although maintaining coherence remains a challenge.

👉 More information
🗞 A quantum optical concept of attosecond pulses: the attoquants
✍️ Sándor Varró and Ákos Gombkötő
🧠 ArXiv: https://arxiv.org/abs/2608.17854

More like this