Quantum Optics and Quantum Electrodynamics of Strong Field Processes

Abstract: In its beginnings, the physics of intense laser-matter interactions was the physics of multiphoton processes. The theory was reduced then to high-order perturbation theory, while treating matter and light in a quantum manner. With the advent of chirped pulse amplification developed by D. Strickland and G. Mourou, which enabled generation of ultra-intense, ultra-short, coherent laser pulses, the need for a quantum electrodynamics description of electromagnetic (EM) fields practically ceased to exist and lost relevance. Contemporary attoscience (AS), and more generally ultrafast laser physics, awarded the Nobel Prize in 2023 to P. Agostini, F. Krausz, and A. L'Huillier, commonly uses the classical description of EM fields while keeping a fully quantum description of matter. The progress and successes of AS in the last 40 years have been spectacular, with an enormous amount of fascinating investigations in basic research and technology. Yet a central question remains: can ultrafast laser physics continue to advance without reintroducing quantum electrodynamics and quantum optics into its description of light-matter interactions? This article discusses future perspectives at the intersection of strong-field physics and quantum optics.

• The paper introduces fully quantized models that unify quantum optics, QED, and strong-field dynamics for ultrafast laser processes.
• It details experimental breakthroughs like bright squeezed states and optical Schrödinger cat states to probe high-harmonic generation and above-threshold ionization.
• The study outlines novel platforms and advanced measurement techniques for exploring light-matter entanglement and quantum correlations in extreme regimes.

Quantum Optics and Quantum Electrodynamics of Strong Field Processes

Introduction

This paper presents a comprehensive analysis of the intersection between quantum optics (QO), quantum electrodynamics (QED), and strong-field laser-matter interactions, with a particular focus on ultrafast processes such as high-harmonic generation (HHG) and above-threshold ionization (ATI). The authors argue for a paradigm shift in ultrafast laser physics (ULP), advocating for the reintroduction of fully quantum descriptions of electromagnetic fields, which have been largely neglected since the advent of chirped pulse amplification. The work synthesizes recent theoretical and experimental advances, highlighting the emergence of high-photon-number non-classical light states and their implications for quantum technologies.

Historical Context and Motivation

Traditionally, strong-field physics has relied on classical descriptions of electromagnetic fields, with quantum mechanics reserved for the treatment of matter. This approach has been sufficient for many applications in attoscience and ultrafast laser physics. However, the development of quantum technologies and the ability to generate and manipulate non-classical states of light in low-photon-number regimes have exposed limitations in this classical treatment. The paper contends that a fully quantized approach to both light and matter is essential for further progress, particularly in the context of high-intensity laser fields and their interaction with complex materials.

Recent Advances in Quantum Optics of Strong Fields

The authors detail several key breakthroughs that have enabled the generation and control of non-classical light states in strong-field regimes:

• Optical Schrödinger Cat States and Multimode Squeezed Fields: Conditioning on measurable observables in HHG and ATI allows the engineering of high-photon-number non-classical states, including cat states and multimode squeezed fields.
• Bright Squeezed States: Experimental demonstrations have shown that bright squeezed light can drive or perturb HHG and strong-field ionization, moving quantum optics into the high-intensity domain.
• Light-Matter Entanglement: Conditioning on ATI events or HHG recombination paths enables the generation of entangled states between quantized light fields and electronic states, with recent work on photoelectron density matrix reconstruction providing new avenues for characterization.

Current and Future Challenges

The paper identifies four principal challenges for the field:

1. Generation of Massively Quantum States of Light: Systematic methods for producing large-photon-number entangled states, extending post-selection techniques to correlated solids and resonant media, and leveraging structured light to probe topological and chiral states.
2. Generation of Massively Quantum States of Light and Matter: Exploration of light-matter entanglement in complex materials, with a focus on ultrafast processes and the impact of photon entanglement on observable quantum correlations.
3. Characterization and Exploitation of Generated States: Development of ultrafast quantum-optical methods for entanglement verification and quantification, enabling applications in nonlinear optics, precision metrology, and quantum information.
4. Simulation of QED of Attoscience with Quantum Platforms: Design of quantum simulators using cold atoms and ions to study strong-field dynamics under controlled conditions, facilitating benchmarking and extension of theoretical models.

Technological and Theoretical Innovations

The authors highlight several technological and theoretical advances that are driving progress in the field:

• Fully Quantized Theoretical Models: Quantum field-theoretical treatments of strong-field processes, including hybrid approaches that balance semiclassical dynamics for matter with quantum descriptions of light.
• Advanced Experimental Techniques: Attosecond metrology capable of characterizing photon statistics, coherence functions, and Wigner functions, as well as time-resolved studies of quantum correlations.
• Structured and Quantum Light Sources: Integration of structured driving fields and bright squeezed light sources with strong-field platforms, enabling high-fidelity quantum control.
• Novel Material Platforms: Utilization of 2D materials, correlated systems, and topological insulators as testbeds for quantum optics, with HHG revealing phase-sensitive emission and quantum pathways.
• Quantum Simulators: Use of cold atom and trapped ion systems to mimic strong-field interactions, providing clean, tunable platforms for testing QED-QO principles.
• Measurement and Characterization Tools: Extension of quantum tomography, homodyne detection, and entanglement witnessing to the ultrafast domain, with machine-learning-based inversion methods for real-time diagnosis.

Implications and Future Directions

The unification of QED, QO, and strong-field physics has significant implications for both fundamental science and quantum technology. The ability to generate and manipulate high-photon-number non-classical states opens new possibilities for scalable quantum light sources, quantum simulation platforms, and ultrafast quantum processors. The paper emphasizes the need for increased complexity in materials, light sources, and measurement techniques to fully exploit the potential of quantum light-matter interactions at extreme scales.

Theoretical implications include the necessity of quantum field-theoretical treatments for accurate modeling of strong-field processes, particularly in correlated and topological materials. Practically, the advances described pave the way for new applications in precision metrology, nonlinear optics, and quantum information science, with attosecond-scale entanglement offering unique capabilities.

Conclusion

This work delineates a roadmap for the future of ultrafast science, where the quantum properties of both light and matter are central to the description and exploitation of strong-field processes. The convergence of QED, QO, and ULP is enabling the generation of complex non-classical states and the exploration of new regimes in quantum technology. Continued progress will depend on the development of fully quantized models, advanced experimental platforms, and sophisticated measurement techniques, with the ultimate goal of harnessing quantum coherence and entanglement for next-generation applications.