Schlawin Frank, Dorfman Konstantin E, Mukamel Shaul
Department of Physics , University of Oxford , Oxford OX1 1PU , United Kingdom.
State Key Laboratory of Precision Spectroscopy , East China Normal University , Shanghai 200062 , China.
Acc Chem Res. 2018 Sep 18;51(9):2207-2214. doi: 10.1021/acs.accounts.8b00173. Epub 2018 Sep 4.
The application of quantum states of light such as entangled photons, for example, created by parametric down conversion, has experienced tremendous progress in the almost 40 years since their first experimental realization. Initially, they were employed in the investigation of the foundations of quantum physics, such as the violation of Bell's inequalities and studies of quantum entanglement. They later emerged as basic platforms for quantum communication protocols and, in the recent experiments on single-photon interactions, in photonic quantum computation. These applications aim at the controlled manipulation of the photonic degrees of freedom, and therefore rely on simple models of matter, where the analysis is simpler. Furthermore, quantum imaging with entangled light can achieve enhanced resolution, and quantum metrology can overcome the shot noise limit for classical light. This Account focuses on an entirely different emerging class of applications using quantum light as a powerful spectroscopic tool to reveal novel information about complex molecules. These applications utilize two appealing properties of quantum light: its distinct intensity fluctuations and its nonclassical bandwidth properties. These give rise to new and surprising behavior of nonlinear optical signals. Nonclassical intensity fluctuations can enhance nonlinear optical signals relative to linear absorption. For instance, the two-photon absorption of entangled photon pairs scales linearly (rather than quadratically) in the photon flux, just like a single photon absorption. This enables nonlinear quantum spectroscopy of photosensitive, for example, biological, samples at low light intensities. We will discuss how the two-photon absorption cross section becomes a function of the photonic quantum state, which can be manipulated by properties of the entangled photon pairs. In addition, the quantum correlations in entangled photon states further influence the nonlinear signals in a variety of ways. Apart from affecting the signal's scaling with intensity, they also constitute an entirely new approach to shaping and controlling excitation pathways in molecular aggregates in a way that cannot be achieved with shaped classical pulses. This is because between the two absorption events in entangled two-photon absorption, the light and material system are entangled. Classical constraints for the simultaneous time and frequency resolution can thus be circumvented, since the two are not Fourier conjugates. Here we review the simplest manifestation of quantum light spectroscopy, two-photon absorption spectroscopy with entangled photons. This will allow us to discuss exemplarily the impact of quantum properties of light on a nonlinear optical signal and explore the opportunities for future applications.
例如，通过参量下转换产生的纠缠光子等光量子态的应用，自首次实验实现以来的近40年里取得了巨大进展。最初，它们被用于研究量子物理的基础，如贝尔不等式的违背以及量子纠缠的研究。后来，它们成为量子通信协议的基本平台，并在最近关于单光子相互作用的实验中用于光子量子计算。这些应用旨在对光子自由度进行可控操纵，因此依赖于更简单的物质模型，便于分析。此外，利用纠缠光的量子成像可以实现更高的分辨率，量子计量学可以克服经典光的散粒噪声极限。本综述聚焦于一类全新的应用，即利用量子光作为强大的光谱工具来揭示复杂分子的新信息。这些应用利用了量子光的两个吸引人的特性：其独特的强度涨落和非经典带宽特性。这些特性导致了非线性光学信号的新奇且令人惊讶的行为。非经典强度涨落相对于线性吸收可以增强非线性光学信号。例如，纠缠光子对的双光子吸收在光子通量中呈线性（而非二次方）比例缩放，就像单光子吸收一样。这使得在低光强下对例如生物等光敏样品进行非线性量子光谱分析成为可能。我们将讨论双光子吸收截面如何成为光量子态的函数，而光量子态可通过纠缠光子对的特性进行操纵。此外，纠缠光子态中的量子关联以多种方式进一步影响非线性信号。除了影响信号随强度的缩放外，它们还构成了一种全新的方法，以一种用整形经典脉冲无法实现的方式来塑造和控制分子聚集体中的激发路径。这是因为在纠缠双光子吸收的两个吸收事件之间，光与物质系统是纠缠的。由于时间和频率并非傅里叶共轭，因此可以规避同时实现时间和频率分辨率的经典限制。在这里，我们回顾量子光光谱最简单的表现形式，即纠缠光子双光子吸收光谱。这将使我们能够示例性地讨论光的量子特性对非线性光学信号的影响，并探索未来应用的机会。
