[Paper Review] Quantum coherence in the dynamical excitation, ionization, and decaying of neon gas induced by X-ray laser

Yongqiang Li, Cheng Gao+3 more|arXiv (Cornell University)|Jan 12, 2015

Laser-Matter Interactions and Applications66 references6 citations

TL;DR

This paper develops a large-scale quantum master equation approach to model coherent dynamics in neon gas under ultra-intense, fully coherent X-ray laser irradiation, incorporating both coherent excitation and incoherent decay processes. It demonstrates that quantum coherence suppresses multiphoton absorption via Rabi oscillations and power broadening, with single-photon ionization dominating at ~10^18 W/cm², challenging rate equation assumptions in high-intensity X-ray experiments.

ABSTRACT

We develop a large scale quantum master equation approach to describe dynamical processes of practical open quantum systems driven by both coherent and stochastic interactions by including more than one thousand true states of the systems, motivated by the development of highly bright and fully coherent lasers in the X-ray wavelength regime. The method combines the processes of coherent dynamics induced by the X-ray laser and incoherent relaxations due to spontaneous emissions, Auger decays, and electronic collisions. As examples, theoretical investigation of {\it real} coherent dynamics of inner-shell electrons of a neon gas, irradiated by a high-intensity X-ray laser with a full temporal coherence, is carried out with the approach. In contrast to the rate equation treatment, we find that coherence can suppress the multiphoton absorptions of a neon gas in the ultra-intense X-ray pulse, due to coherence-induced Rabi oscillations and power broadening effects. We study the influence of coherence on ionization processes of neon, and directly prove that sequential single-photon processes for both outer- and inner-shell electrons dominate the ionizations for the recently typical experiments with a laser intensity of $\approx10^{18}$ ${ m W/cm^2}$. We discuss possible experimental implementations such as signatures for coherent evolution of inner-shell electrons via resonance fluorescence processes. The approach can also be applied to many different practical open quantum systems in atomic, quantum optical, and cold matter systems, which are treated qualitatively by a few-level master equation model before.

Motivation & Objective

• To address the lack of a systematic method for simulating coherent dynamics in complex open quantum systems under intense, coherent X-ray fields.
• To overcome the limitations of traditional rate equations, which neglect coherence effects like Rabi oscillations and power broadening.
• To investigate how quantum coherence influences excitation, ionization, and decay in neon gas under ultra-intense X-ray laser pulses.
• To provide a theoretical framework applicable to atomic, molecular, and quantum optical systems with strong dissipation and coherence.
• To identify experimentally accessible signatures of inner-shell electron coherence, such as resonance fluorescence spectra.

Proposed method

• Develops a large-scale quantum master equation approach including over 1,000 true electronic states of neon.
• Simultaneously models coherent dynamics driven by the X-ray laser and incoherent relaxation via spontaneous emission, Auger decay, and electron collisions.
• Solves the master equation numerically to track time evolution of density matrix elements, capturing both coherence and dissipation.
• Compares results with Einstein’s rate equation approach to isolate coherence effects.
• Incorporates power broadening effects naturally through the master equation formalism, absent in rate equations.
• Uses Gaussian X-ray pulses with specified intensity (2.5×10¹⁷ W/cm²) and duration (10 fs) for simulation.

Experimental results

Research questions

• RQ1 How does quantum coherence affect multiphoton absorption in neon under ultra-intense X-ray laser irradiation?
• RQ2 To what extent do Rabi oscillations and power broadening suppress ionization compared to rate equation predictions?
• RQ3 Which ionization mechanism—single-photon or multiphoton—dominates at intensities of ~10¹⁸ W/cm²?
• RQ4 What experimental signatures can reveal coherent dynamics of inner-shell electrons in neon?
• RQ5 How does the inclusion of coherence alter the population dynamics of excited and ionized states?

Key findings

• Coherence suppresses multiphoton absorption in neon due to Rabi oscillations and power broadening, contrary to rate equation predictions.
• Power broadening (~1 eV) from the intense X-ray field dominates over Auger and spontaneous decay broadening (~0.1 eV).
• The oscillatory structure in resonant excitation from rate equations is smoothed out in the master equation results due to power broadening.
• Single-photon ionization of both outer- and inner-shell electrons dominates at ~10¹⁸ W/cm², regardless of coherence effects.
• Rabi oscillations of inner-shell electrons occur at ~10¹⁵ Hz, too fast for current time-domain detection, but resonance fluorescence may serve as an observable signature.
• The master equation method provides a general framework for studying coherent dynamics in complex open quantum systems beyond few-level models.