强场量子光学中近似相空间描述的局限性
Limitations of an approximative phase-space description in strong-field quantum optics
AI总结：
本文研究了强场过程中近似相空间描述在预测高阶谐波生成和阈上电离中的局限性，指出其在量子光学可观测量如压缩度和光子统计上的不准确性，并通过单带模型验证了近似相空间描述的误差。
摘要：
近年来，强场过程如高阶谐波生成（HHG）和由非经典光态驱动的阈上电离已成为研究热点。这些过程的理论建模通常采用非经典驱动场的近似相空间展开，以相干态为基，已被证明能准确预测谐波光谱。然而，其在计算量子光学可观测量如压缩度和光子统计方面的准确性尚未充分考虑。本文引入了这种近似相空间描述并讨论其准确性，发现其通过将驱动激光视为经典态的不可信混合物，错误地表征了驱动激光的量子光学性质。我们进一步表明，这种驱动场描述的误差映射到HHG发射的光上，既无法捕捉到亚泊松光子统计，也无法捕捉到低于真空波动的四阶压缩。最后，为了验证近似相空间描述，我们考虑了单带模型下的量子HHG，该模型给出了精确的解析解。使用该特定模型的近似相空间表示，我们发现发射场的四阶方差存在与脉冲持续时间和发射体密度成比例的小量误差。我们的结果表明，使用这种近似相空间描述会错误地表征量子光学可观测量。因此，将此类结果赋予物理意义时，应伴随对误差的定量分析。
英文摘要：
In recent years, strong-field processes such as high-order harmonic generation (HHG) and above-threshold ionization driven by nonclassical states of light have become an increasingly popular field of study. The theoretical modeling of these processes often applies an approximate phase-space expansion of the nonclassical driving field in terms of coherent states, which has been shown to accurately predict the harmonic spectrum. However, its accuracy for the computation of quantum optical observables like the degree of squeezing and photon statistics has not been thoroughly considered. In this work, we introduce this approximative phase-space description and discuss its accuracy, and we find that it mischaracterizes the quantum optical properties of the driving laser by making it an incoherent mixture of classical states. We further show that this error in the driving field description maps onto the light emitted from HHG, as neither sub-Poissonian photon statistics nor quadrature squeezing below vacuum fluctuations can be captured by the approximative phase-space description. Lastly, to benchmark the approximative phase-space description, we consider the quantum HHG from a one-band model, which yields an exact analytical solution. Using the approximative phase-space representation with this specific model, we find a small quantitative error in the quadrature variance of the emitted field that scales with pulse duration and emitter density. Our results show that using this approximative phase-space description can mischaracterize quantum optical observables. Attributing physical meaning to such results should therefore be accompanied by a quantitative analysis of the error.
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