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arXiv:2604.13485

Attosecond Access to the Quantum Noise of Light

En-Rui Zhou, Yi-Jia Mao, Pei-Lun He, Feng He

quant-phphysics.atom-phSubmitted Apr 15, 2026• Announced Apr 16, 2026• 0 citations• Revised Aug 18, 2026 (v2)
v1 · Apr 15, 2026v2 · Aug 18, 2026
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Abstract

Bright squeezed light has entered strong-field atomic physics, creating an immediate need to characterize the quantum field delivered to the target on subcycle timescales. Here we show that attosecond streaking maps the coherent displacement and phase-sensitive covariance of a displaced squeezed field onto distinct harmonics: the mean photoelectron momentum follows an $ω$-periodic shift of the spectral center, whereas the momentum variance exhibits a $2ω$-periodic breathing. A Feynman--Vernon formulation represents Gaussian quantum light by stochastic vector-potential trajectories, enabling Coulomb-resolved TDSE simulations with a finite XUV gate and without explicit photon-state propagation. A coherent-state reference calibrates the streaking phase and variance background, enabling retrieval of the coherent amplitude and phase together with the squeezed-noise amplitude and squeezing phase. This establishes attosecond streaking as an in situ, gas-phase diagnostic of bright squeezed light.

Comments
8 pages, 3 figures

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Abstract Correlations between optical modes, ranging from classical fluctuations to quantum entanglement, are a cornerstone of modern optics and emerging quantum technologies. However, probing these correlations on the natural timescale of electronic motion - the attosecond regime - remains a major challenge. Here, we generate and characterize correlated attosecond extreme-ultraviolet (XUV) emission by perturbing gas-phase high-harmonic generation with two-mode bright squeezed vacuum. The signal and idler fields imprint their correlated amplitude and phase fluctuations onto two families of harmonics, thereby transferring these correlations from the infrared to broadband XUV modes. Even harmonics serve as an intrinsic attosecond phase-sensitive probe of these correlations through interference between indistinguishable XUV pathways. Shot-resolved covariance measurements reveal the amplitude correlations, while even-harmonics resolve the phase correlations with sub-cycle precision. Time-domain reconstruction of the correlated emission reveals a transition from maximal fluctuations to near-silence within only 30 as. Whereas the signal and idler driven attosecond pulse trains exhibit excess fluctuations individually, their joint reconstruction shows strongly suppressed fluctuations, reflecting their shared correlations. These results establish a framework for generating and probing quantum correlations on attosecond timescales, opening the door to quantum attosecond science.
Until recently, attosecond optical spectroscopy and quantum optics evolved along non-overlapping directions. In attosecond science, attosecond pulses have been regarded as classical waves, applied to probe electron dynamics on their natural time scale. Here, we transfer fundamental concepts of quantum optics into attosecond physics, enabling control of both the properties of the XUV attosecond pulses and the quantum fluctuations of matter on attosecond time scales. By combining bright squeezed vacuum (BSV) with a strong laser field to drive high-harmonic generation, we transfer the quantum properties of the BSV onto the resulting XUV attosecond pulses. Applying advanced attosecond interferometry, we reconstruct the quantum state of the XUV high harmonics and their associated attosecond pulses with attosecond precision. Finally, we resolve the squeezing of the electron's wavepacket during one of the most fundamental strong-field phenomena - field induced tunneling. The ability to measure and control quantum correlations in both electrons and XUV attosecond pulses establishes a foundation for attosecond quantum electrodynamics, manipulating the quantum state of electrons and photons with sub-cycle precision.
The variance of bright squeezed vacuum (BSV) is not only strong but also phase dependent, yet recent strong-field experiments with BSV have recorded only its magnitude. A coherent field of the same color provides the phase reference that resolves this phase-dependent noise. Strong-field ionization of xenon by this light shows that the photoelectron yield follows the BSV quadrature along the coherent field, and that the nonlinearity of ionization amplifies the quadrature noise many times over in the electron yield. The noise is moreover distributed unevenly over the photoelectron momentum distribution, differing by an order of magnitude between ionization channels. Strong-field ionization thus offers a phase-sensitive, channel-resolved detector of quantum light, while the quadrature noise of quantum light offers a probe of the intensity dependence of individual ionization pathways.