A groundbreaking investigation has unveiled how bright squeezed vacuum (BSV) light sources profoundly amplify nonlinear atomic tunneling processes, presenting a new frontier in quantum-optical phenomena. This pioneering study harnesses the intrinsic quantum statistical nature of BSV—characterized by pronounced photon number fluctuations—to elevate ionization rates well beyond classical expectations, signaling a paradigm shift in our understanding of light–matter interactions at the quantum level.
Central to this research is the innovative use of a quantum-adapted Ammosov–Delone–Krainov (QADK) theory, a sophisticated extension of the conventional semiclassical ADK model. While the ADK theory treats the electromagnetic field as a classical wave, the QADK framework fully incorporates quantum fluctuations and light–electron entanglement, allowing it to faithfully capture how BSV statistics imprint onto the ionization dynamics of atoms. This theoretical leap offers a nuanced, quantum-optical description of tunneling ionization, explaining the enhanced electron emission yields observed experimentally with BSV fields.
Experimentally, BSV light was generated by pumping cascaded nonlinear beta barium borate (BBO) crystals with ultrafast femtosecond pulses near 790 nm. Unlike traditional coherent sources produced by optical parametric amplifiers, the BSV state exhibited a multi-mode spectral structure with strong photon bunching signatures, characterized by second-order correlation functions ( g^{(2)} ) exceeding unity, and reaching values as high as 2.6 under spectral filtering conditions. Critically, the unfiltered BSV retained its broadband multi-frequency profile to preserve peak intensity essential for inducing atomic ionization in sodium vapor targets.
The ionization process itself was interrogated using an ultrahigh-vacuum ion momentum spectrometer capable of coincident detection of photoelectrons and parent ions, driven by intense elliptically polarized BSV pulses. Elliptical polarization was methodically chosen to facilitate angular streaking, a technique that deciphers the momenta of emitted electrons with high fidelity and minimal rescattering artifacts, enabling precise calibration of the effective electric field intensity and the resultant electron kinetic energy distributions. The Keldysh parameter, lying near 1.48, situates the ionization regime firmly within non-adiabatic tunneling, affirming the process is dominated by direct tunneling rather than multiphoton or resonance-assisted pathways.
Harnessing the QADK model, the research elucidates how the quantum statistical distribution of BSV light results in an effective vector potential represented as an operator weighted by the BSV field’s momentum quadrature. This formulation demonstrates that the classical intensity is replaced by an effective quantum field, dispersing ionization probabilities across a spectrum of fluctuating instantaneous field strengths. Such fluctuations cause the photoelectron momentum distributions to broaden, enhancing the ionization yield beyond classical predictions.
In this scenario, the multi-frequency nature of real-world BSV is accounted for by modeling the light field as an ensemble of independent modes each described by quantum squeezing parameters. The resultant photoelectron spectra are derived as convolutions over these modal distributions, yielding momentum and energy profiles that reconcile well with experimental measurements. The theory predicts and experiments confirm a 20-fold quantum enhancement in the ionization rate relative to coherent light with equivalent classical intensity—testament to the profound effect of quantum photon statistics on strong-field atomic processes.
Furthermore, detailed analysis of photon number statistics revealed a direct quantitative relationship between the effective ionizing intensity and the second-order photon correlation function ( g^{(2)} ). The work derives a linear scaling law demonstrating that greater photon bunching inherent to BSV states directly translates to heightened effective intensity experienced by the atom. This finding underscores the role of photon statistics rather than mere mean photon number in dictating strong-field phenomena, highlighting the critical nature of quantum coherence and squeezing in nonlinear light-matter interactions.
An important experimental control involved comparing ionization yields from pulses of different durations; coherent pulses of 70 fs and BSV pulses of approximately 150 fs were studied to ascertain temporal effects. Computational simulations showed negligible differences in electron kinetic energy distributions across this range, reinforcing that observed enhancements were attributable primarily to quantum statistical properties rather than temporal pulse shape or length.
Moreover, multi-mode analysis conducted via spectral filtering and Hanbury Brown–Twiss techniques demonstrated the presence and controllability of multiple frequency modes within the BSV source. By adjusting the pump power and spectral bandwidth, the researchers could systematically tune the mode number and ( g^{(2)} ), verifying the theoretical treatment of the BSV light as a multimode squeezed vacuum and underlining the consistency of the experimental and theoretical frameworks.
The choice of elliptically polarized light was pivotal, balancing sufficient peak intensity along the major axis to drive robust ionization with an angular streaking signal amenable to accurate momentum measurements. This strategy circumvented the intensity reduction pitfalls of circular polarization while minimizing complex rescattering features typical of linear polarization, thereby yielding clean electron momentum maps reflective of the underlying quantum statistics of the field.
Calibration of effective intensity through electron momentum peak measurements distinguished this approach from traditional energy-scaling methodologies. The photoelectron momentum distributions reliably retained Gaussian profiles despite increased photon bunching, validating their use as precise indicators of the instantaneous ionizing field strength. Notably, the prominence of heavy tails in these distributions was recognized as a hallmark of the quantum character of the BSV field.
Collectively, this research opens new avenues for utilizing quantum light sources to manipulate and control ultrafast, strong-field atomic processes beyond classical limits. The demonstrated ability to imprint photon statistics onto electron emission characteristics via light–matter entanglement provides a fresh platform for exploring quantum-enhanced nonlinear optics, with potential applications spanning attosecond science, quantum information processing, and precision spectroscopy.
This intersection of quantum-optical theory and cutting-edge experimental techniques offers an unprecedented window into the complex dynamics of tunneling ionization in the presence of nonclassical light. As the field advances, exploiting such quantum statistical enhancements may enable revolutionary control over a wide range of photonic and electronic phenomena, transcending the conventional boundaries dictated by classical electromagnetic fields.
Subject of Research: Quantum-enhanced nonlinear atomic tunneling induced by bright squeezed vacuum light.
Article Title: Nonlinear atomic tunnelling boosted by bright squeezed vacuum.
Article References:
Jiang, Z., Pan, S., Chen, J. et al. Nonlinear atomic tunnelling boosted by bright squeezed vacuum. Nature (2026). https://doi.org/10.1038/s41586-026-10485-9
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s41586-026-10485-9
Tags: beta barium borate nonlinear crystalsbright squeezed vacuum quantum light sourcesionization rate amplificationlight–electron quantum entanglementmulti-mode spectral photon bunchingnonlinear atomic tunneling enhancementquantum statistical photon fluctuationsquantum-adapted Ammosov–Delone–Krainov theoryquantum-optical light–matter interactionssemiclassical ADK model extensiontunneling ionization dynamicsultrafast femtosecond pulse pumping
