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arXiv · 2608.18773

Electron Correlation Enables Phase-Coherent One-Attosecond Pulse Trains

Andrés Marchisio·Isobel McSweeney·Paraskevas Tzallas·Maciej Lewenstein·Marcelo F. Ciappina
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Abstract

Attosecond synthesis is ultimately a phase problem: a broad spectrum produces an ultrashort waveform only if its harmonics remain phase-locked. We show theoretically that correlated two-electron high-harmonic generation in helium driven by a long, multicycle laser pulse supports a train of soft-x-ray bursts with durations approaching 1 as. Using a two-electron strong-field approximation, we calculate the complex harmonic spectrum and reconstruct the temporal emission while retaining its full intrinsic spectral phase, rather than imposing a flat-phase approximation. Despite the trajectory-dependent phase accumulated by two continuum electrons, the correlation-extended plateau contains a broad phase-coherent region reaching the keV range. Its superposition produces reproducible bursts separated by one half-cycle of the driving field. These results identify electron correlation not only as a mechanism for extending the high-harmonic cutoff, but also as a route toward phase-coherent x-ray waveforms on the zeptosecond timescale.

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Andrés Marchisio, Isobel McSweeney, Paraskevas Tzallas, Maciej Lewenstein, Marcelo F. Ciappina. 2026-08-19. Electron Correlation Enables Phase-Coherent One-Attosecond Pulse Trains. https://arxiv.org/abs/2608.18773

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