Time-resolved detection of atoms diffracted from a standing light wave

We present a time-resolved experimental observation of the diffraction of metastable helium atoms from a nearly resonant standing light wave. The application of a time-resolved detection technique and a pulsed source allows to resolve high diffraction orders, which are populated in the atom-light interaction. Furthermore, the rms momentum transfer from the light field on the atom as a function of the interaction time is investigated. Future applications of this technique may be the detailed investigation of the motion of atoms in standing light waves and the detection of correlations between spontaneously emitted photons and atoms.Dedicated to H. Walter on the occasion of his 60th birthday     

Atomic beam splitter from a mixture of atoms in the Bragg regime

We investigate the interaction of a standing-wave light field with the beam of two-level atoms moving in the Bragg regime. The atomic beam consists of two different isotopes and the density is sufficiently small so that at most one atom is inside the cavity at any time. The experimental setup is such that both the isotopes have the same momenta. The momentum transfer between the atoms and photons in the process essentially effects the center-of-mass motion of the atoms, thus separating the isotopes in different directions after specific interaction times.     

Real time analysis of diffraction patterns, at extremely low light levels

By analyzing previous experiments with statistically independents photons, just one concerns the diffraction pattern; it was revealed with photographic technique. The present describes an experiment which produces a diffraction pattern at very low intensity radiation consisting of statistically independent photons. This pattern was revealed in real time by using image intensification. Several photodetection arrangements utilizing image intensifier tubes and image storage devices have been tested and the results presented. This study involved the evaluation of the error probability in photons overlap at any time. The linearity dependence between light intensity and bright pattern diffraction was verified.     

Experimental demonstration of sub-shot-noise intensity correlations in an intense twin beam

Twin-beam states of light produced by spontaneous parametric downconversion are endowed with sub-shot-noise photon-number correlations at any intensity regime. We have demonstrated sub-shot-noise photon-number correlations in a ps-pulsed intense (more than 1000 photons) twin beam by measuring the variance of the difference in the number of photons detected in the two outputs of the crystal. The variance is 3.25 dB below the shot-noise level. Measurements show that detection of sub-shot-noise correlation requires a careful selection of twin coherence areas and that these areas depend on the pump intensity.     

光在極低强度下的干涉现象

用Michelson干涉仪得出干涉图案;用光电倍增管计算光子率,由此测定该图案内的强度分布。证明了极低光强度下所得的图案与正常强度所得的图案,在实验误差范围内没有区别。在低光强度下,每秒钟约有10⁶光子进入干涉仪,因此,在这样的低光强度下,平均说来,在任何时间,干涉仪内具有的光子数还远不到一个。     

Quantum theory and the nature of interference

It is shown that many of the difficulties associated with the Copenhagen interpretation of quantum theory are resolved by a new interpretation of interference derived from solutions to Maxwell's equations. An infinite wave model of the photon based on these solutions is described and used to explain the interference of single photons as well as the corpuscular behavior evident in the Compton and photoelectric effects. The wave-particle duality and the uncertainty relations are also discussed. According to the new interpretation of interference in a Young's double-slit experiment, photons which pass through the left-hand slit always arrive in the left-hand part of the screen and no photons pass into this area via the right-hand slit. This conclusion is compared with the viewpoint of the Copenhagen school and an experiment to distinguish between them is suggested.     

Position sensor based on slit imaging

A position sensor based on slit imaging is proposed and its measurement principle is described.An imaging slit is illuminated by a collimated laser beam with square-wave modulation and imaged on a detection double slit through a 4f system.A magnified image of the detection double slit is formed on a bi-cell detector.The position of the imaging slit is obtained by detecting light intensity on two parts of the bi-cell detector.In experiments,the feasibility of the sensor was verified.The repeatability was less than 40 nm.     

Calculation of Photon Path Distribution Based on Photon Behavior Analysis in a Scattering Medium

We calculated the photon path distribution (PPD) in a scattering medium based on a theoretical analysis, which utilizes the relationship between the photon intensity and photon pathlength. This PPD is defined by local photon pathlengths for photons having total pathlengths of l between the light input points and detection points. The PPD of photons that consist of the impulse response at time t (= l/c) was calculated for a 2-D model. Precise analysis of photon migration in the scattering medium is essential in order to carry out image reconstruction of diffuse optical tomography (DOT). We show the PPD at time t (the total pathlength l = ct) and demonstrate its effectiveness. Our method for describing photon migration is intuitive and allows finding weight functions in DOT.     

Experimental study of the dependences of retrieval efficiencies on time delay between magneto-optical-trap being turned off and optical storage

We report an experimental study of electromagnetically induced transparency(EIT)-based light storage in a cloud of cold atoms loaded into a magneto-optical-trap(MOT). After the MOT is turned off, the retrieval efficiencies of rightand left-circularly polarized signal light fields each as a function of storage time are measured for different time delays between MOT off and the storage event, respectively. The results show that in the delay ranging from 0.015 ms to 3.5 ms,the retrieval efficiency for a zero-storage time(0.2 μs) and the storage lifetime can exceed 15% and 1.4 ms, respectively.The measured results will provide important help for optimizing the storage of the polarized entanglement photons in cold atomic ensembles.     

狭缝衍射中的偏振效应

在狭缝的夫琅和费衍射中,由振动平行和垂直于缝边的偏振光的衍射条纹得到的缝宽值不同。本文介绍这种偏振效应的实验结果,并用边界衍射波对其物理原因进行分析,理论与实验结果符合很好。 关键词：     

Wave-particle duality using the Compton effect

Thought experiments based on the double-slit interferometer had a crucial role to develop ideas concerning the wave-particle duality and the Bohr's complementarity principle. Ideally, a slit with a sufficiently low mass recoils due to the passage of the photon. This motion denounces the path taken by the light and suppresses any attempt to observe an interference pattern. In real life, however, available which-way information in such a setup is significantly impaired by the typical magnitudes of photons and slits, making the verification of the effect almost impossible. Here, we extend this discussion by applying similar ideas to the Mach-Zehnder interferometer. That is, we study the consequences of the beam-splitter recoil, during the passage of the photon, over the interference pattern produced by the device. Unlike the double-slit experiment, this recoil can now be encoded in the wavelength of the photon itself, which, in principle, is more easily accessed. Fortuitously, the model used to describe the interaction between the idealized beam-splitter and the photon clearly indicates that an interferometer based on Compton's effect could be build to study wave-particle duality. We follow this hint, finding realistic experimental parameters needed to observe the trade-off between wave and corpuscular behaviors in such a modified interferometer.     

Detection of VUV photons with large-area avalanche photodiodes

The room-temperature response of large-area avalanche photodiodes (LAAPDs) to 128- and 172-nm light pulses is investigated. The minimum detectable number of photons, which can produce a signal just above the noise level, is found to be around 1300 and 600 photons, respectively. The LAAPD relative statistical fluctuations in the detection of 15 000 photons of 128 nm and 25 500 photons of 172 nm were found to be about 3.9% and 2.2%, respectively. Both the minimum detectable number of photons and statistical fluctuations do not depend on the photon wavelength, but rather on the number of charge carriers produced by the light pulse in the LAAPD. For these light levels, good LAAPD performance is already achieved for gains as low as 30 to 60.     

Superluminal propagation of evanescent modes as a quantum effect

Contrary to mechanical waves, the two‐slit interference experiment of single photons shows that the behavior of classical electromagnetic waves corresponds to the quantum mechanical one of single photons, which is also different from the quantum‐field‐theory behavior such as the creations and annihilations of photons, the vacuum fluctuations, etc. Owing to a purely quantum effect, quantum tunneling particles including tunneling photons (evanescent modes) can propagate over a spacelike interval. With this picture we conclude that the superluminality of evanescent modes is a quantum mechanical rather than a classical phenomenon.     

门控下InGaAs/InP单光子探测器用于符合测量的时域滤波特性研究

基于砷化镓/磷化铟雪崩光电二极管(InGaAs/InP APD)的半导体单光子探测器因工作在通信波段,且具有体积小、成本低、操作方便等优势,在实用化量子通信技术中发挥了重要作用.为尽可能避免暗计数和后脉冲对单光子探测的影响,InGaAs/InP单光子探测器广泛采用门控技术来快速触发和淬灭雪崩效应,有效门宽通常在纳秒量级.本文研究揭示了门控下单光子探测器可测量的最大符合时间宽度受限于门控脉冲的宽度,理论分析与实验结果良好拟合.该研究表明,门控下InGaAs/InP单光子探测器用于双光子符合测量具有显著的时域滤波特性,限制了其在基于双光子时间关联测量的量子信息技术中的应用.     

Quantum improvement of time transfer between remote clocks

Exchanging light pulses to perform accurate space-time positioning is a paradigmatic issue of physics. It is ultimately limited by the quantum nature of light, which introduces fluctuations in the optical measurements and leads to the so-called standard quantum limit (SQL). We propose a new scheme combining homodyne detection and mode-locked femtosecond lasers that lead to a new SQL in time transfer, potentially reaching the yoctosecond range (10(-21)-10(-24) s). We demonstrate that this already very low SQL can be overcome using appropriately multimode squeezed light. Benefitting from the large number of photons and from the optimal choice of both the detection strategy and of the quantum resource, the proposed scheme represents a significant potential improvement in space-time positioning.     

Imaging in turbid media using quasi-ballistic photons

We study by means of experiments and Monte Carlo simulations, the scattering of light in random media, to determine the distance up to which photons travel along almost undeviated paths within a scattering medium, and are therefore capable of casting a shadow of an opaque inclusion embedded within the medium. Such photons are isolated by polarisation discrimination wherein the plane of linear polarisation of the input light is continuously rotated and the polarisation preserving component of the emerging light is extracted by means of a Fourier transform. This technique is a software implementation of lock-in detection. We find that images may be recovered to a depth far in excess of that predicted by the diffusion theory of photon propagation. To understand our experimental results, we perform Monte Carlo simulations to model the random walk behaviour of the multiply scattered photons. We present a new definition of a diffusing photon in terms of the memory of its initial direction of propagation, which we then quantify in terms of an angular correlation function. This redefinition yields the penetration depth of the polarisation preserving photons. Based on these results, we have formulated a model to understand shadow formation in a turbid medium, the predictions of which are in good agreement with our experimental results.     

Effect of the photon's Brownian Doppler shift on the weak-localization coherent-backscattering cone

We report the first observation of the dependence of the coherent-backscattering (CBS) enhanced cone with the frequency of the backscattered photon. The experiment is performed on a diffusing liquid suspension and the Doppler broadening of light is induced by the Brownian motion of the scatterers. Heterodyne detection on a CCD camera is used to measure the complex field (i.e., the hologram) of the light that is backscattered at a given frequency. The analysis of the holograms yield the frequency and the propagation direction of the backscattered photons. We observe that the angular CBS cone becomes more narrow in the tail of the Brownian spectrum. The experimental results are in good agreement with a simple theoretical model.     

Time-resolved single-photon detection by femtosecond upconversion

We demonstrate a time-resolved single-photon detection technique based on ultrafast sum-frequency generation, providing femtosecond measurement capability for single photons in photonic quantum information processing. Noncollinear broadband upconversion in periodically poled MgO-doped stoichiometric lithium tantalate with an ultrafast pump and detection with a Si single-photon counter enable efficient detection of IR photons and temporal resolution of ~150 fs. We utilize the timing resolution to map the generation efficiency profile along the propagation axis of a periodically poled KTiOPO(4) crystal, revealing its local grating quality with millimeter resolution. We also apply the technique to two-photon coincidence measurements and directly demonstrate time anticorrelation between coincident-frequency entangled photons that are parametrically generated under extended phase-matching conditions.