Detection of two-mode compression and degree of entanglement in continuous variables in parametric scattering of light

Generation of “twin beams” (of light with two-mode compression) in single-pass optical parametric amplifier (a crystal with a nonzero quadratic susceptibility) is considered. Radiation at the output of the nonlinear crystal is essentially multimode, which raises the question about the effect of the detection volume on the extent of suppression of noise from the difference photocurrent of the detectors. In addition, the longitudinal as well as transverse size of the region in which parametric transformation takes place is of fundamental importance. It is shown that maximal suppression of noise from difference photocurrent requires a high degree of entanglement of two-photon light at the outlet of the parametric amplifier, which is defined by Federov et al. [Phys. Rev. A 77, 032336 (2008)] as the ratio of the intensity distribution width to the correlation function width. The detection volume should be chosen taking into account both these quantities. Various modes of single-pass generation of twin beams (noncollinear frequency-degenerate and collinear frequency-nondegenerate synchronism of type I, as well as collinear frequency-degenerate synchronism of type II) are considered in connection with the degree of entanglement.     

Polarization tomography of a narrow-band biphoton field

Narrow-band orthogonally polarized collinear frequency-degenerate biphotons are generated in the process of the spontaneous parametric down-conversion of light in a nonlinear BBO crystal placed in an optical resonator. Quantum polarization tomography and polarization transformation of the generated biphoton states are performed. The results from our experiment agree well with theoretical calculations.     

Dispersion spreading of polarization-entangled states of light and two-photon interference

We study the interference structure of the second-order intensity correlation function for polarization-entangled two-photon light obtained from type II collinear frequency-degenerate spontaneous parametric down conversion (SPDC). The structure is visualized due to the spreading of the two-photon amplitude as two-photon light propagates through optical fiber with group-velocity dispersion (GVD). Because of the spreading, polarization-entangled Bell states can be obtained without any birefringence compensation at the output of the nonlinear crystal. Instead, proper time selection of the intensity correlation function is required. A birefringent material inserted at the output of the nonlinear crystal (either reducing the initial e-o delay between the oppositely polarized twin photons or increasing this delay) leads to a more complicated interference structure of the correlation function.     

Polarization effects in two-photon ionization of alkali atoms with two light beams

For spin-polarized as well as unpolarized alkali atoms, a compact expression is obtained for the rate of two-photon ionization by two light beams having different wave vectors, polarizations and orientations of their polarization ellipses. As an example, the expression is applied to one selected experimental geometry and 16 selected combinations of polarization states of the “exciting” and “ionizing” light beams.     

Experimental realization of deterministic entanglement transformations of bipartite pure states

We experimentally realize a kind of entanglement transformations of bipartite pure states with 100% efficiency. The protocol employs two-outcome positive operator-valued measures (POVMs) and can transform two-photon maximally entangled state to any two-photon entangled pure state with 100% efficiency. The average fidelity of all output states is 96%. Moreover, the scheme to implement arbitrary POVM on single-photon polarization state is also discussed. In principle, our setup can be applied to any kind of entanglement transformations of two-qubit entangled states to achieve optimal successful probability.     

Self-diffraction of light waves by a nonlocal photorefractive grating in a crystal with the \bar 43m symmetry

A frequency-degenerate steady-state two-wave interaction on a dynamic transmitting phase grating formed in a cubic crystal of the $\bar 43m$ symmetry group with a nonlocal photorefractive response is considered in the paraxial approximation. The conservation laws for the nonlinear system of equations of coupled waves, derived for an arbitrary orientation of interaction relative to the crystallographic axes and the polarization of incident light waves, indicate that the contribution to energy exchange between the interacting waves may come from fluxes in different directions. The possibility of nonunidirectional energy pumping from one wave to the other upon a change in their polarization state due to the interaction is demonstrated. For the transverse configuration of the interaction and linear polarization of incident waves, explicit analytic expressions for the scalar amplitudes of the orthogonal components of the light field are derived in the linear approximation in the coefficient of modulation of the interference pattern of light. The possibility of rotation of the polarization planes of light waves without a change in their intensity is demonstrated. For three particular configurations, the dependence of the efficiency of interaction of linearly polarized waves on the reduced length, orientation of the polarization vectors of the incident light waves, and the ratio of their intensities are analyzed.     

Observation of two-photon coherence in plasmon-assisted transmission

We experimentally study two-photon coherence in plasmon-assisted transmission with a two-photon Mach–Zehnder (MZ) interferometer. Two collinear photons of identical or orthogonal polarization are simultaneously incident on one optically thick metal film, perforated with a periodic array of subwavelength holes. Interference fringes with two-photon de Broglie wavelength are observed, which indicates that the quantum coherence of biphoton is preserved in surface plasmon assisted transmission.     

Optical polarization echo: Manifestation and study by methods of quantum tomography of states and processes

The polarization echo effect is demonstrated, which is manifested in the identity of a composite polarization transformation irrespective of the spectrum of the input polarization state. The pure input state transfers during its evolution to a completely mixed state and then returns to the initial state. The stages of this evolution are completely analogous to the corresponding stages of the “classical” spin echo effect. The adequacy of transformations performed for conditionally prepared one-photon polarization states is verified by the tomography of the quantum process.     

High-efficiency Zn isotope separation in a photochemical reaction induced by two-photon excitation

High-efficiency laser isotope separation was accomplished for zinc atoms in the situation when isotope shifts are much smaller than the Doppler width. The atoms were excited via two-photon absorption of two counter-propagating waves slightly detuned from the intermediate state, so that all “0requisite” atoms were excited. The isotope separation results from a photochemical reaction between the selectively excited zinc isotope and the CO₂ molecules. The reaction rate is 3–5 orders of magnitude higher than for the unexcited atoms. The mechanisms of two-photon resonance line broadening are considered both theoretically and experimentally, and the possibilities of the method for obtaining isotopes in quantitative yields are estimated.     

Biphoton light generation in polarization-frequency bell states

Four polarization-frequency Bell states are obtained experimentally for photon pairs (biphotons) emitted during spontaneous parametric scattering from continuous pumping in the collinear frequency-nondegenerate regime. The polarization properties of such states are investigated. It is shown that biphoton light in the singlet Bell state is not polarized in the second or fourth order in the field.     

基于焦斑空间频率全域优化的偏振匀滑设计

偏振匀滑是利用光束偏振特性降低焦斑对比度的一种技术,它最大可降低焦斑对比度为原来的1/√2. 焦斑的频谱分析显示,传统楔形晶体的偏振匀滑对焦斑对比度的改善只集中在某些特定的空间频率,本文因此提出了一种可全域降低焦斑空间频率的偏振匀滑方法,它采用单轴晶体对入射光的角度不同而产生的相移不同的方法,实现激光两种正交偏振态在靶点的分离. 理论分析和数值模拟表明,新方法可以实现焦斑空间频率中高频段的全频域降低,焦斑对比度也可同时达到1/√2的最大程度的改善. 分析了连续相位板作为新方法引入激光入射角分布不同的条件,确定了刻蚀连续相位板面形的晶体同时实现焦斑整形和偏振匀滑的边界条件. 关键词： 偏振匀滑 焦斑功率谱 响应函数 焦斑整形     

The classical Ising model: A quantum renormalization group approach

Proceeding from the equivalence between the d-dimentional classical Ising model and the (d?1)-dimentional quantum mechanical Ising model in a transverse magnetic field, we study the critical properties of the classical model via the quantum mechanical model. Quantum renormalization group transformations based on the truncation method and the ground state projection operator method are used to calculate the critical exponents. They are found to agree well with the “exact” values.     

基于非线性光纤环镜的40Gb/s可重构光逻辑门

提出了一种新型的基于非线性光纤环镜(NOLM)的可重构全光逻辑门实现方案。传统的基于NOLM的全光逻辑利用自相位调制效应或交叉相位调制效应,透射传输函数重构的自由度低,可实现的逻辑门种类较少。该方案在传统的结构基础上,分析了NOLM中探测光的偏振态的演化,以及输入光偏振态和环内偏振控制器对NOLM的传输特性的影响。理论分析和数值仿真结果表明在考虑NOLM中的非线性偏振旋转效应的情况下,可以更加自由地构建不同透射传输函数,从而利用单一NOLM结构,仅通过调节偏振控制器,即能够可重构地实现绝大部分基础组合逻辑。实验中,完成了两路40Gb/s的数据信号之间的"非"、"与"、"或"、"或非"、"同或"、"异或"等各种基础组合逻辑,验证了方案的可行性。     

Two-photon excitation of confined biexcitons in CuCl quantum dots

We have studied direct creation processes of confined biexcitons in CuCl quantum dots by polarization-dependent resonant two-photon excitation spectroscopy. The two-photon absorption band for the lowest state of the biexciton (total angular momentum J=0) which appears on the lower energy side of confined exciton band was identified from the analysis of the polarization dependence of the photoluminescence excitation spectrum of the biexciton. Furthermore, the two-photon excitation process for the excited state of the biexciton (J=2) was also found with polarization dependence different from the J=0 biexciton state.     

Search for light penetrating bosons with the use of a rotatable detector arrangement at a nuclear reactor

The search for the two-photon decay of light penetrating bosons (axions or others), produced in a nuclear reactor, was supplemented by measurements with a rotatable set-up at the Jülich 10 MW research reactor FRJ-1. Measurements were performed in “effect” and “background” positions, the latter not allowing the simultaneous detection of both photons due to decay kinematics. Preliminary measurements, carried out at reactor ON condition only, show no positive effect.     

光梯度力驱动的纳米硅基光开关

通过一道光改变另一道光的传输路线是光子集成网络中重要而长远的目标, 然而, 由于硅材料的光学非线性较弱, 在硅材料上实现开关的全光控制难以实现. 因此本文提出了一种由光梯度力驱动的纳米硅基光开关, 实现了硅基光开关的全光控制. 该光开关由一个部分悬空的微环谐振器和一个交叉波导结构构成, 当通入一道控制光时, 悬空的微环谐振器在光梯度力的作用下发生弯曲, 微环谐振器的谐振波长随之发生变化, 从而实现光信号的传输路线发生改变. 该光开关利用纳米光子制造技术在标准绝缘体上硅晶圆上制造, 实验数据得出其最小消光比为10.67 dB, 最大串扰为 -11.01 dB, 开关时间分别为180 ns和170 ns. 该光开关具有尺寸小, 响应速度快, 低损耗和可拓展等优点, 在片上集成光路、高速信号处理以及下一代光纤通信网络中具有潜在应用.     

Quantum control of the angular momentum distribution in multiphoton absorption processes

The newly developed technique of polarization pulse shaping is applied to the control of two-photon absorption in atomic rubidium. This technique enables the manipulation of the transient vector properties of a light matter interaction. We establish that control can be exerted on the angular distribution of the final state, and demonstrate the ability to control the final state population of a nearly degenerate system, as well as to perform M-state resolved spectroscopy.     

Macroscopic pure state of light free of polarization noise

The preparation of completely nonpolarized light is seemingly easy; an everyday example is sunlight. The task is much more difficult if light has to be in a pure quantum state, as required by most quantum-technology applications. The pure quantum states of light obtained so far are either polarized or, in rare cases, manifest hidden polarization; even if their intensities are invariant to polarization transformations, higher-order moments are not. We experimentally demonstrate the preparation of the macroscopic singlet Bell state, which is pure, is completely nonpolarized, and has no polarization noise. Simultaneous fluctuation suppression in three Stokes observables below the shot-noise limit is demonstrated, opening perspectives for noiseless polarization measurements. The state is shown to be invariant to polarization transformations. This robust highly entangled isotropic state promises to fuel important applications in photonic quantum technologies.     

Investigations of aluminum films with synchrotron radiation of wavelengths 500 to 1000 Å

Measurements of the photocurrent from thin Al cathodes in a windowless electron multiplier as a function of wavelength, polarization, angle of incidence, and film thickness have been carried out in the extreme vacuum ultraviolet. A normal incidence monochromator utilizing synchrotron radiation provided highly polarized light. A marked difference is found between the photocurrent measured during irradiation of thin films with “s” and “p” polarized light at wavelengths near the Al plasma wavelength (835 Å). For films thicker than about 500 Å pronounced interference effects are found in both “p” and “s” light at wavelengths less than the plasma wavelength. The observations can be explained by assuming the photocurrent is related to the photon density (electromagnetic energy density) in the photocathode. Calculations of the energy density in films irradiated with light give the structure found in the measured photocurrent. The measurements indicate our monochromator yields light with a degree of polarization consistent with the calculated polarization of the synchrotron radiation incident upon our grating (about 85%).     

Spatial frequency response and resolution in holography

Measurements were taken in order to compare “spatial frequency response” (SFR) and resolution properties of several types of holographic methods. The best SFR was obtained for the “Fourier holographic method”; good resolution and SFR were also found for the “image plane hologram” (IPH) reconstructed with a laser light.