Entanglement and Coherence in Attosecond Science Experiments

阿秒科学是超快激光物理学的一个分支，旨在通过泵浦探测实验研究并可能控制电子在自然时间尺度上的运动。高谐波产生过程形成的阿秒脉冲的波长在极紫外（XUV）到软x射线光谱范围内。因此，阿秒脉冲对任何介质（固体、液体或气体）都是电离辐射。正如我课课组最近在初步的理论和实验工作中所表明的那样，离子和光电子通常会表现出量子力学纠缠，这影响了阿秒泵浦探测实验所依赖的相干性。在这里，我提议执行一个项目，通过执行一系列实验，将利用孤立的阿秒激光脉冲的阿秒泵浦探测光谱与运动完全离子-光电子重合检测相结合，来澄清阿秒分子科学中纠缠和相干之间的关系。据我所知，我在MBI的实验室是目前世界上唯一一个可以进行分离阿秒脉冲和重合检测实验的实验室。更具体地说，将在提案中进行两个实验。一方面，实验将在H2中进行，在H2中，被隔离的阿秒激光脉冲形成最大的离子+光电子纠缠态。我将研究离子或光电子与近红外探测激光的相互作用如何诱导离子中的电子相干性，这在实验上表现为当阳离子离解时实验室框架H+喷射的不对称性。我将探讨电子相干性如何依赖于分子相对于激光偏振轴和光电子弹射角的排列，并将实验研究如何平均光电子自由度导致从“封闭”到“开放”量子系统的转变。另一方面，实验将在N2中进行，在N2中，被隔离的阿秒激光脉冲电离将导致几种不同阳离子态的形成。这些状态在相干叠加中的程度将通过超短（<2fs）紫外激光脉冲的解离来探测，作为分子相对于激光偏振轴和光电子弹射角的排列函数。预计阳离子中的电子相干度，或者相反，离子与光电子之间的纠缠度，将强烈地依赖于这些参数。我期望这一建议将导致对纠缠在阿秒科学中的作用的重大澄清，从而有助于阿秒科学的进一步发展，以充分实现其承诺。

Attosecond science is a branch of ultrafast laser physics that aims to investigate and possibly control electronic motion on its natural timescale by means of pump-probe experiments. Attosecond pulses formed by the process of high-harmonic generation have wavelengths in the extreme ultra-violet (XUV) to soft X-ray spectral range. Accordingly, attosecond pulses are ionizing radiation for any medium (solid, liquid or gaseous) that is placed in its path. As my research group has recently shown in preliminary theoretical and experimental work, the ion and photoelectron will commonly display quantum-mechanical entanglement, which influences the coherence that attosecond pump-probe experiments rely on. Here I propose to perform a project that will clarify the relation between entanglement and coherence in attosecond molecular science, by performing a series of experiments that combine attosecond pump-probe spectroscopy utilizing isolated attosecond laser pulses with kinematically complete ion-photoelectron coincidence detection. To the best of my knowledge, my laboratory at MBI is the only laboratory worldwide where experiments combining isolated attosecond pulses and coincidence detection can currently be performed. More specifically, two experiments will be performed within the proposal. On the one hand, experiments will be performed in H2, where the isolated attosecond laser pulse forms a maximally entangled ion+photoelectron state. I will investigate how interaction of the ion or the photoelectron with a near-infrared probe laser induces electronic coherence in the ion, which experimentally manifests as an asymmetry in the laboratory-frame H+ ejection when the cation dissociates. I will explore how the electronic coherence depends on the alignment of the molecule with respect to the laser polarization axis and the ejection angle of the photoelectron, and will experimentally investigate how averaging over the photoelectron degrees of freedom leads to a transition from a “closed” to an “open” quantum system. On the other hand, experiments will be performed in N2, where ionization by the isolated attosecond laser pulse will lead to the formation of several different cationic states. The degree to which these states are in a coherent superposition will be probed using dissociation by an ultrashort (<2fs) UV laser pulse, both as a function of the alignment of the molecule with respect to the laser polarization axis and the ejection angle of the photoelectron. It is expected that the degree of electronic coherence in the cation, or, conversely, the degree of entanglement between the ion and the photoelectron, will strongly depend on these parameters. I expect that this proposal will lead to significant clarification of the role of entanglement in attosecond science, and thus will contribute to the further development of attosecond science towards full realization of its promise.