Coherence time measurements using a single detector with variable time resolution

We present a simple technique for measuring coherence times for stationary light fields using a single detector with tunable time resolution. By measuring the equal-time second-order correlation function at varying instrument response functions it is possible to determine the coherence time and also the shape of the temporal decay without the need to record time-resolved data. The technique is demonstrated for pseudothermal light. Possible applications for dynamic light scattering and photon statistics measurements are discussed.     

混沌光场光子统计分布及二阶相干度的分析与测量

利用通信波段双通道单光子探测器,采用Hanbury Brown-Twiss关联测量方案,理论分析并实验测量了光反馈半导体激光器产生的混沌光场的光子统计分布及不同混沌状态光场的二阶相干度.通过对混沌光场二阶相干度g~((2))(τ)的理论分析,得出随着延迟时间和相干时间的变化,其与相干光、热光及单光子态的二阶相干度可明显区分并呈现出不同分布.同时实验上产生了频谱宽度6.7 GHz的混沌光场,测量了不同光子数分布的结果,并用高斯随机分布、泊松分布、玻色-爱因斯坦分布对光子数分布进行理论拟合,发现随着入射平均光子数的增加,光子数分布从玻色-爱因斯坦分布过渡到泊松分布,但整个过程都与高斯随机分布符合较好,且光场的二阶相干度g~((2))(0)由2降至1.通过改变偏置电流(I=1.0Ith-2.0Ith)和反馈强度(0—10%),实验上研究了混沌光场由低频起伏到相干塌陷的过程中不同状态宏观动力学特性与二阶相干度的对应关系.结果表明:混沌光场在此过程中始终呈现出明显的聚束效应,并在频谱宽度最大时达到最强;同时给出了光子计数测量中聚束效应减弱的物理原因.实验表明该系统及方法能很好地揭示不同状态混沌光场的光子统计特性.     

计数率和分辨时间对光场统计性质测量的影响——单探测器直接测量的实验分析

利用工作在盖革(Geiger)模式下的单光子探测器(single-photon-counting module, SPCM)，采取单个探测器直接对光子计数的方法，对相干光场及热光场的光子数统计性质进行了测量和分析. 通过改变计数率，调整分辨时间，系统地研究了光场二阶相干度测量值受实验条件的影响. 结果表明，在综合考虑系统中的各种因素对测量影响的情况下，通过选择合适的测试条件，可以利用单个单光子探测器直接探测的方法快速确定一个待测光场的二阶相干度. 实验表明在实测计数率为109 kc/s，分辨时间范围为2⁸ns—2¹²ns的条件下，该系统能很好地揭示相干光场和热光场的光子统计性质. 关键词： 光子统计 单光子探测器 二阶相干度     

压缩态光场在双光子吸收介质中的演化

采用生成函数方法,由光场密度算符主方程,求得光场压缩度s,二阶相干度g^(x)和光子统计分布的严格解析表达式,同时讨论它们随标度时间τ的演化。计算表明,在双光子吸收过程中,虽然压缩度s逐渐减弱,但相当长的时间内光场仍呈现压缩特性;该过程有利于呈现和增强光子反聚束效应和光子亚泊松分布特性;光子分布的振荡特征维持较短一段时间后很快消失。 关键词：     

Photon statistics of amplified spontaneous emission in a dense wavelength-division multiplexing regime

The photon statistics of amplified spontaneous emission in the few-modes regime and in single mode, conditions that are typical of dense wavelength-division multiplexing transmission, have been experimentally proved by direct detection. The dependence of the degeneracy factor for the Bose-Einstein distribution on the degree of second-order coherence of light is stated. The theoretical dependence of the number of amplified spontaneous emission modes on the ratio between the optical channel and the detector bandwidths has also been confirmed by experiments, thus quantifying the loss of validity of asymptotic approximations when they are extended to the few-modes regime.     

与运动二能级原子作用的双模光场的量子特性

用全量子化理论,研究了与运动二能级原子相互作用的双模量子化光场的量子特性,分析了场模结构对平均光子数,单模二阶相干度,两模间的相关性的影响. 研究结果表明,随着场模结构参量的增大,平均光子数回复周期变短,振荡加剧,在崩塌区域内振荡出现分裂, 同时原子运动对光场的二阶相干性有明显的影响.     

Kerr介质中纠缠原子与非简并双光子相互作用的动力学

考虑了在位于充满Kerr介质的腔中,一对纠缠的二能级原子之一与非简并双光子的相互作用,当该原子离开腔被作选测性测量时,分析测量后光场的量子性质,研究结果表明,在纠缠度一定时,Kerr效应使光场平均光子数的崩塌回复周期减小,光场的二阶相干度减弱,使Cauchy-Schwartz不等式关联程度减弱.     

Thermal vacuum state corresponding to squeezed chaotic light and its application

For the density operator(mixed state) describing squeezed chaotic light(SCL) we search for its thermal vacuum state(a pure state) in the real-fictitious space. Using the method of integration within ordered product(IWOP) of operators we find that it is a kind of one- and two-mode combinatorial squeezed state. Its application in evaluating the quantum fluctuation of photon number reveals: the stronger the squeezing is, the larger a fluctuation appears. The second-order degree of coherence of SCL is also deduced which shows that SCL is classic. The new thermal vacuum state also helps to derive the Wigner function of SCL.     

Experimental Study on Coherence Time of a Light Field with Single Photon Counting

The second-order degree of coherence of pseudo-thermal light and coherence time are experimentally studied via the Hanbruy-Brown-Twiss （HBT） scheme. The system consists of two non-photon-number-resolving single- photon-counting modules （SPCMs） operating in the Geiger mode. We investigate the coherence time of the incident beam for different spot sizes on a ground glass a~d speeds of a rotating ground glass. The corresponding coherence time can be obtained from Gaussian fitting for the measured second-order degree of coherence. The results show that the coherence time of measured pseudo-thermal light depends on the spot sizes and the rotating speeds of the ground glass. The maximum value of the second-order degree of coherence is reduced as the rotating speed decreases. This result can be well explained by the model of mixed thermal and coherent fields with different ratios.     

三能级电磁感应透明中辐射场的量子统计特性

对Λ型三能级原子电磁感应透明（EIT）过程中辐射场的二阶相干度进行了研究。理论分析表明，在电磁感应透明系统中，由于原子的相干效应导致其上能级共振荧光场的二阶相干度将呈现单光子场的量子统计特性。并对其随耦合场强度和探测光失谐的变化进行了详细的分析和讨论，结果发现：在｜Ω｜〉（Γ2＋Γ3）／2情况下．采用较弱的耦合光功率（由托比频率Ω表征）及较大的探测光失谐，在较长时间延迟范围内，二阶相十度保持小于1，更利于实现非经典场的量子统计行为；相反，在｜Ω｜≤（Γ2＋Γ3）／2情况下，探测光的失谐量越小，越利于获得二阶相干度小于1的量子统计光场。南此可见选取合适的参量可优化电磁感应透明过程中单光子场的量子统计特性。     

Measuring the photon distribution with ON/OFF photodetectors

Reconstruction of photon statistics of optical states provides fundamental information on the nature of any optical field and helps with various relevant applications. Nevertheless, no detector that can reliably discriminate the number of incident photons is available. On the other hand, the alternative of reconstructing the density matrix by quantum tomography leads to various technical difficulties that are particularly severe in the pulsed regime (where mode matching between a signal and local oscillator is very challenging). Even if on/off detectors, as usual avalanche photodiodes operating in Geiger mode, seem useless as photo-counters, it was recently shown how reconstruction of photon statistics is possible by considering a variable quantum efficiency. Here we present experimental reconstructions of photon number distributions of both continuous-wave and pulsed light beams in a scheme based on on/off avalanche photodetection assisted by maximum-likelihood estimation. Reconstructions of the distribution for both semiclassical and quantum states of light (as single photon, coherent, pseudothermal, and multithermal states) are reported for single-mode and multimode beams. The stability and good accuracy obtained in the reconstruction of these states clearly demonstrate the interesting potentialities of this simple technique.     

Optimal Photon Blockade on the Maximal Atomic Coherence

There is generally no obvious evidence in any direct relation between photon blockade and atomic coherence. Here instead of only illustrating the photon statistics, we show an interesting relation between the steady-state photon blockade and the atomic coherence by designing a weakly driven cavity QED system with a two-level atom trapped. It is shown for the first time that the maximal atomic coherence has a perfect correspondence with the optimal photon blockade. The negative effects of the strong dissipations on photon statistics, atomic coherence and their correspondence are also addressed. The numerical simulation is also given to support all of our results.     

Kerr 介质中三能级原子与双模场非共振相互作用的量子统计性质

研究了Kerr介质中双模场与V型三能级原子的非共振相互作用. 对Cauchy-Schwartz不等式的研究表明,在两模场失谐量相等(δ₁=δ₂)条件下,当初始场强ξ一定而两模场的光子数差q减小时,场的非经典相关程度加强;q一定而ξ增大时,场的非经典相关程度减弱. 当Δδ=|δ₂-δ₁|≠0且q和ξ的取值一定时,随介质常量x的增大,光场两模间的非经典相关程度减弱,且Δδ的大小对这种非经典相关程度的减弱有重要影响. 对场的 关键词： Kerr介质 非共振相互作用 Cauchy-Schwartz不等式 场的二阶关联函数     

高Q Kerr介质腔中非简并双光子Jaynes-Cummings模型中光场的性质

研究了一位于高Q Kerr介质腔中的二能级原子与双模光场相互作用系统中,Kerr效应对光场单模平均光子数、单模二阶相干度、两模间的相关性,以及Cauchy-Schwartz不等式的影响。 关键词：     

Resolution and visibility of two-color pseudothermal lensless ghost imaging

Two-color (or nondegenerate-wavelength) lensless ghost imaging using pseudothermal light source is investigated theoretically by use of classical optical coherence theory. We find that for two-color pseudothermal lensless ghost imaging the visibility and resolution is determined by (Δ) the product of the wavelength and the corresponding path length rather than the wavelength for each path or the path length. We also confirm our theoretical conclusion by numerical simulations. The result shows we must make a trade-off between resolution and visibility in devise practical experiments, as the resolution of the imaging can be improved by decreasing Δ but with visibility getting poor, and vice versa.     

Coherence and the statistics of the phase difference between partially polarized electromagnetic waves

Coherence between two vectorial harmonic light vibrations is analyzed in terms of the statistics of their phase difference. This provides a natural and simple extension of second-order coherence to cover more complicate situations. In particular this assigns large coherence to quantum light states providing the most accurate interferometric measurements allowed by the quantum theory, even if they are incoherent according to the standard second-order approach.     

Photon statistics of semiconductor microcavity lasers

We present measurements of first- and second-order coherence of quantum-dot micropillar lasers together with a semiconductor laser theory. Our results show a broad threshold region for the observed high-beta microcavities. The intensity jump is accompanied by both pronounced photon intensity fluctuations and strong coherence length changes. The investigations clearly visualize a smooth transition from spontaneous to predominantly stimulated emission which becomes harder to determine for high beta. In our theory, a microscopic approach is used to incorporate the semiconductor nature of quantum dots. The results are in agreement with the experimental intensity traces and the photon statistics measurements.     

Enhancement of spatial coherence during light propagation in bounded media

The spatial coherence of thermal light propagated through passive bounded media is measured. It is found that specular reflection at the boundaries enhances the degree of coherence. This result has considerable implications for the spatial coherence of light from laser and amplified spontaneous emission (A.S.E.) systems.     

量子拍激光：双模亚泊松光

在Λ型量子拍系统中再引入原子相干，可以产生明亮的双模亚泊松光.运用Scully-Lamb理论计算了MandelQ因子.当相对相位和相对振幅的量子噪声被抑制到真空噪声水平时，平均模呈现亚泊松光子分布；或者当平均相位和平均振幅的量子噪声被抑制到真空噪声水平时，相对模呈现亚泊松光子分布，MandelQ因子的最佳值均可达到-0.5；并且，在上述两种情况下，每个关联自发辐射激光模呈现亚泊松光子分布，MandelQ因子的最佳值均可达到-0.25. 关键词：