Near-equilibrium thermalised quantum light

自世纪最重要的发明之一--第一台激光器问世以来，已经过去了近60年，但仍在发现能够实现高度相干、定向光源的新机制。最近，光的玻色-爱因斯坦凝聚（BEC）使人们能够探索量子统计、相变和激光之间的联系，不仅扩大了我们的理解，而且激发了具有新功能的光源。这些光源将能够模拟量子过程，否则使用现代计算机是难以实现的，并且通过利用其独特的量子相干特性来成像和感知超越量子极限。光子可以在室温下热化并经历玻色-爱因斯坦凝聚（BEC），这不是一个微不足道的声明。光学谐振器中的荧光介质被光学激发。谐振器具有许多光学模式，但其中一个具有明确定义的基态。发射到谐振器模式中的光子通过荧光介质的吸收和再发射而经历热化。这是由介质的振动状态促进的，其快速弛豫以保持热平衡。量子统计学确保，有足够的光子，即使在室温下，BEC也会发生，导致基态谐振器模式的宏观种群。BEC是一个普遍的过程，因此光子BEC可以比作原子系统中的凝聚，或激子-极化子微腔。本项目使用四个成分将基于光子的BEC科学从基础研究转向应用研究：量子相关，半导体光子BEC，平面波导谐振器和理论基础。这个项目的组成部分是：（A）测量和控制光子之间的量子相关性。虽然激光具有明确的泊松数统计，但BEC的统计数受荧光介质的影响很大。我们将测量模式内和模式间的相关性。与激光相反，我们预期由有限数量的发射体构成的介质将产生亚泊松相关，例如相对数压缩。利用脉冲泵浦和时间分辨的非平稳统计测量，我们将揭示如何识别和利用这些高度非经典的光状态。(B)无机半导体器件中的光子热化和凝聚。到目前为止，用于光子BEC的介质一直是液体染料。通过以非常非标准的方式使用非常标准的无机半导体（GaAs）作为热化介质，我们将制造其特性（发射光谱，阈值泵浦功率，相关性）可以通过成熟的制造技术进行调整的器件，适用于稳健和商业可行的技术。(C)用于光子BEC控制的新型平面谐振器开放式微腔谐振器已被证明适用于光子BEC，并且在光子的势能景观方面是灵活的。我们将探讨凝聚的传播光子使用面内分布谐振腔的几何形状，其中时间可以映射到传播维度。有效地，我们将实现亚皮秒的时间控制BEC的空间变化的谐振器设计。(D)光子相关理论。整个项目将包括一个强大的理论分析和建模方案。使用的基本模型是基于量子主方程，适用于染料和半导体。它将被解决与强大的数值技术来预测量子关联的条件，很好地描述了实验。设备将由我们的项目支持者使用现有的合作方法制造。虽然这项研究主要是好奇心驱动的，但它将揭示新的光量子态，表征它们的方法以及利用它们的途径，这将有助于量子传感和模拟。

Almost 60 years have passed since the first laser, one of the most important inventions of the 20th century, yet new mechanisms enabling highly coherent, directional light sources are still being discovered. Very recently, Bose-Einstein Condensation (BEC) of light has enabled exploration of the links between quantum statistics, phase transitions and lasers, not only expanding our understanding, but inspiring light sources with new capabilities. Such sources will enable simulation of quantum processes, otherwise intractable using modern computers, and imaging and sensing beyond the quantum limit by exploiting their unique quantum coherence properties.It's not a trivial statement that photons can be made to thermalise and undergo Bose-Einstein condensation (BEC) at room temperature. A fluorescent medium in an optical resonator is optically excited. The resonator has many optical modes, but one has a well-defined ground state. Photons emitted into the resonator modes undergo thermalisation by absorption and re-emission with the fluorescent medium. This is facilitated by the vibrational states of the medium, which relax rapidly, to maintain thermal equilibrium. Quantum statistics ensure that, with enough photons, BEC will occur, even at room temperature, resulting in a macroscopic population of the ground-state resonator mode. BEC is a universal process, so photon BEC can be compared to condensation in atomic systems, or exciton-polariton microcavities.This project uses four ingredients to shift the science of photon-based BEC from fundamental to applied research: quantum correlations, semiconductor photon BEC, planar waveguide resonators, and theoretical underpinning. Those ingredients of this project are: (A) Measurement and control of the quantum correlations among photons.- While lasers have well-defined Poissonian number statistics, the number of statistics of BEC are greatly influenced by the the fluorescent medium. We will measure both intra- and inter-mode correlations. In contrast to lasers, we expect that media made of finite numbers of emitters will generate sub-Poissonian correlations, e.g. relative-number squeezing. Using pulsed pumping and time-resolved measurements of non-stationary statistics we will uncover how to characterise and exploit these highly non-classical states of light.(B) Photon thermalisation and condensation in an inorganic semiconductor device.- The media used for photon BEC so far have been liquid dyes. By using a very standard inorganic semiconductor (GaAs) in a very non-standard way as the thermalisation medium, we will make devices whose properties (emission spectrum, threshold pump power, correlations) can be tuned through well-established fabrication techniques, suitable for robust and commercially viable technology.(C) New planar resonators for photon BEC control.- Open microcavity resonators have proven suitable for photon BEC and are flexible in terms of the potential-energy landscape for photons. We will explore condensation of propagating photons using an in-plane distributed-resonator geometry, where time can be mapped to propagation dimension. Effectively, we will achieve sub-picosecond temporal control over BECs by spatially varying resonator designs. (D) Theory of photon correlations.- The whole project will include a strong theoretical analysis and modelling programme. The basic model to be used is based on quantum master equations, applicable to both dyes and semiconductors. It will be solved with powerful numerical techniques to predict quantum correlations for conditions that well describe the experiments. Devices will be fabricated using existing collaborations by our project supporters with established methods. While this research is primarily curiosity-driven, it will uncover new quantum states of light, methods for characterising them, and routes to exploiting them, which will be useful for quantum sensing and simulation.