通信波长频率一致纠缠光源的频谱测量

本文利用光栅单色仪实现了对超短脉冲抽运周期极化磷酸氧钛钾晶体产生的通信波长频率一致纠缠光子源的频谱特性分析.测量到双光子的联合频谱呈正关联分布,为频率一致纠缠光源.信号光、闲置光中心波长分别为1574.4 nm和1574.9 nm,频谱宽度分别为35.3 nm和37.6 nm,双光子符合包络宽度约为3 nm.根据单光子频谱宽度与双光子符合包络宽度的比值可以得到双光子的频率纠缠参量R约为12,表征了信号光子与闲置光子之间具有较高的频率纠缠度.     

Generating entangled two-photon states with coincident frequencies

It is shown that parametric down-conversion, with a short-duration pump pulse and a long nonlinear crystal that is appropriately phase matched, can produce a frequency-entangled biphoton state whose individual photons are coincident in frequency. Quantum interference experiments which distinguish this state from the familiar time-coincident biphoton state are described.     

Qutrits and ququarts in spontaneous parametric down-conversion, correlations and entanglement

We investigate features of biphoton qutrits and ququarts with the symmetry of biphoton wavefunctions fully taken into account. Entanglement quantifiers of such states are found as functions of qutrit and ququart parameters. For biphoton qutrits, a general relation between degrees of entanglement and polarization is established. In the case of biphoton ququarts, photon frequencies are considered as variables independent of polarization variables, with both polarization and frequency degrees of freedom equally contributing to the total ququart entanglement. Averaging over one of these two degrees of freedom is shown to reduce pure ququart states to mixed two-qubit states. The degree of polarization and degrees of correlations in mixed polarization states are investigated.     

Purity and entanglement of two-photon polarization states generated by spontaneous parametric down-conversion

We elucidate the dependence of purity and entanglement of two-photon states generated by spontaneous parametric down-conversion on the parameters of the source, such as crystal length, pump beam divergence, frequency bandwidth, and detectors angular aperture. The effect of crystal anisotropy is taken into account. Numerical simulations are presented for two types of commonly used source configurations.     

超短脉冲抽运下频率一致纠缠光源量子特性实验研究

利用自发参量下转换过程产生的频率纠缠光源在量子信息处理及相关领域中 具有十分重要的应用. 本文利用中心波长为792 nm, 脉冲宽度小于20 fs的脉冲激光源抽运满足II类准相位 匹配条件的周期极化磷酸氧钛钾晶体, 实验产生了偏振相互正交的频率一致纠缠光子对. 基于Hong-Ou-Mandel干涉仪的二阶量子符合干涉装置, 测量到该纠缠双光子对的干涉可见度约为42%, 表明其频率不可分特性较理想频率一致纠缠光源大大降低. 通过理论分析给出, 由于超短脉冲光源对应的宽频谱带宽影响, 相位匹配函数中的高阶色散项不再忽略, 从而导致纠缠光子对的频率不可分性减弱. 进一步利用实验参数给出的数值模拟结果与实验结果符合, 证实了脉冲抽运源带宽对频率一致纠缠光源的量子不可分特性的影响. 关键词： 频率一致纠缠 Hong-Ou-Mandel干涉测量 符合计数     

Quadripartite entanglement from cascaded χ nonlinear interactions

We theoretically analyze the generation of quadripartite continuous-variable entanglement, with different frequency, by cascaded nonlinear process in a quadruply resonant optical cavity. This process is realized by one optical parametric down-conversion operated below threshold cascaded by two sum-frequency generations in a quasiperiodic optical supperlattice. It shows that the signal, idler and two sum frequency modes are entangled according to the sufficient inseparability criterion proposed by van Loock and Furusawa. The quantum correlation spectra among these four modes versus normalized analysis frequency and nonlinear coupling coefficients are discussed as well in detail.     

Generation of broadband biphotons and their compression in an optical fiber

The generation of biphoton wave packets with a broad spectrum owing to quasi-phase-matched spontaneous parametric down-conversion in a crystal with an aperiodic poling has been considered. Biphotons at the exit of such a sample are not Fourier limited; i.e., their correlation time is much larger than the inverse width of the spectrum. It has been shown that if the inverse grating vector varies linearly along the sample length, the generated biphoton wave packets can be compressed in time by transmitting two-photon light through an optical fiber.     

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.     

Reconstruction of the polarization states of a biphoton field

A method for reconstructing an arbitrary quantum state of an optical system in Hilbert space with dimension d = 4 is discussed. Such states can be realized using a collinear frequency-nondegenerate regime of generating spontaneous parametric down-conversion. The method has been tested for a number of polarization states of a biphoton field. The high accuracy of the reconstruction of the states (above 99%) indicates that the procedures proposed for reconstructing the quantum state of the system are adequate.     

脉冲激光作用下的量子定位实验方案的设计及分析

基于量子相干和量子空间定位的原理,设计了量子空间定位的实验方案.讨论了在脉冲激光作用下,由第二类自发参量下转换所产生的纠缠光子对的光谱特征、对应的相干函数特点以及对量子定位的影响.结果表明,随着激光脉冲宽度的增加,纠缠光子对的相干性减小,量子定位的测量准确度降低.     

The influence of non-Kolmogorov turbulence on the entanglement of spatial two-qubit states in a slant channel

The effects of atmospheric turbulence on the entanglement of spatial two-qubit states that are prepared using the signal and idler photons produced by parametric down-conversion are studied. Utilizing the non-Kolmogorov model for atmospheric turbulence and Rytov approximation method, we quantify the effects of atmospheric turbulence on the entanglement of the two-qubit state in terms of Wootters's concurrence. Our results show that the effects of the zenith angle of communication channel and the outer scale of turbulence on the concurrence of a spatial two-qubit state can be ignored and the smaller inner scale of turbulence, the smaller refractive-index power α, the shorter wavelength of beams and the longer propagation distance will lead to the larger fluctuations of the concurrence of a spatial two-qubit state.     

Experimental realization of a three-qubit entangled W state

We report on the experimental observation of the three-photon polarization-entangled W state using spontaneous parametric down-conversion. This state is inequivalent to the Greenberger-Horne-Zeilinger state under stochastic local operations and classical communications and thus is the representative of the second class of genuine tripartite entanglement. We study the characteristic features of entanglement and demonstrate the high degree of two-photon entanglement in the W state.     

Anisotropically and high entanglement of biphoton states generated in spontaneous parametric down-conversion

We show both theoretically and experimentally that biphoton wave packets generated via spontaneous parametric down-conversion can be strongly anisotropic and highly entangled. The conditions under which these effects exist are found and discussed.     

Correlation Properties of an Optical–Terahertz Biphoton Field

JETP Letters - The effect of the structure of angular modes of an optical–terahertz biphoton field appearing in strongly nondegenerate parametric down-conversion on the possibility of...     

Schmidt Modes and the Time-Dependent Wavefunction of a Broadband Biphoton Field

JETP Letters - The wavefunction of a broadband biphoton field arising in parametric down-conversion in an aperiodically polarized nonlinear crystal with a linear chirp under the action of a short...     

Experimental Proposal of Probabilistic Superdense Coding with Linear Optical Elements

提出一种多人控制的N粒子GHZ纠缠态的概率传送方案，m粒子GHZ态就可以胜任m个监控者参与的控制工作。接受者Bob在所有参与者的本地操作和有关测量结果的经典信息的协助下进行适当的幺正操作成功实现量子态的异地重建，传送过程中任意一个参与者的缺席都将导致传送的失败。     

Quantification of continuous variable entanglement with only two types of simple measurements

Here we propose an experimental set-up in which it is possible to obtain the entanglement of a two-mode Gaussian state, be it pure or mixed, using only simple linear optical measurement devices. After a proper unitary manipulation of the two-mode Gaussian state only number and purity measurements of just one of the modes suffice to give us a complete and exact knowledge of the state’s entanglement.     

Entangled optical vortex links

Optical vortices are lines of phase singularity which percolate through all optical fields. We report the entanglement of linked optical vortex loops in the light produced by spontaneous parametric down-conversion. As measured by using a Bell inequality, this entanglement between topological features extends over macroscopic and finite volumes. The entanglement of photons in complex three-dimensional topological states suggests the possibility of entanglement of similar features in other quantum systems describable by complex scalar functions, such as superconductors, superfluids, and Bose-Einstein condensates.     

Transverse compression of two-photon wave packets

A method for generating two-photon wave packets (biphotons) with a small width of the spatial intensity correlation function (correlation radius) has been considered. We propose to prepare a two-photon wave packet inhomogeneously broadened in the angle by means of spontaneous parametric down-conversion in an inhomogeneous crystal. Such biphotons are not diffraction limited; i.e., their correlation radius is much larger than the inverse width of the spatial spectrum. However, the correlation radius of such a biphoton decreases as the biphoton propagates in free space, and transverse compression occurs at a certain distance from the crystal; i.e., the biphoton becomes diffraction limited.     

Control of the frequency spectrum of a biphoton field due to the electro-optical effect

A method for controlling the spectrum of spontaneous parametric down-conversion has been implemented. This method is based on the application of the gradient of an electrostatic field to a nonlinear crystal where down-conversion occurs. Due to the electro-optical effect, i.e., the dependence of the refractive indices on the applied field, the phase-matching conditions are different along the length of the crystal and, as a result, the spectrum of the emitted biphoton field is modified.