Improving the purity of heralded single-photon sources through spontaneous parametric down-conversion process

The high-purity single-photon source plays an important role in the field of quantum information. Usually, it is generated through spontaneous parametric down-conversion process. In this paper, we investigate and summarize a few approaches on obtaining single-photon sources with a high purity using either PPKTP or PPLN nonlinear crystals. Moreover, we present improved schemes to increase the purity based on existing work, corresponding applicable conditions and procedures are discussed and analyzed. Besides, we carry out numerical simulations and show that nearly perfect purity can be reached even without using any filters. Therefore, this work might provide valuable references for the generation and application of high purity single-photon sources.     

Spontaneous parametric down-conversion

Spontaneous parametric down-conversion (SPDC) in media with no inversion center and the use of this phenomenon in the spectroscopy of natural oscillation states of a crystal lattice (i.e., optical phonons) are retrospectively described. We think that the SPDC spectroscopy method is estimated inappropriately and hope to again attract the attention of readers to one of the most interesting quantum phenomena of nonlinear optics that has no classical analog. The capabilities of SPDC spectroscopy will certainly be used in both fundamental science and technology of new materials.     

Revisiting non-degenerate parametric down-conversion

The quantum dynamics of a two-mode non-resonant parametric down-conversion process is studied by recasting the time evolution equations for the basic operators in an equivalent spin equation form with simpler exact solutions for a pump field with harmonic time dependence. Expectation values of suitable operators for studying important features such as squeezing and quantum revivals are presented in simple forms.      

Quantum structures in traveling-wave spontaneous parametric down-conversion

We analyze theoretically spatial structures appearing in the far diffraction zone of the electromagnetic field emitted in the cavityless parametric down-conversion. We investigate in detail the spatial correlation functions of intensity and demonstrate the existence of strong quantum correlations between the regions of the far field symmetrical with respect to the optical axis. Our simplified model allows us to obtain analytical results for some limiting cases. We demonstrate that in the limit of small diffraction and ideal quantum efficiency of photodetection the noise reduction in the photocurrent difference between symmetrical regions in the far diffraction field becomes complete at zero frequency of photocurrent fluctuations. Received 5 February 2001 and Received in final form 11 April 2001     

Entanglement in optical parametric down-conversion with losses and noise

We show that radiation generated in optical parametric down-conversion with losses and noise is entangled for all times if the coupling coefficient is higher than half of damping constant and the product of damping constant and mean number of noise photons. For the process stimulated by means of chaotic light there is a saturable bound of its intensity for the generation of nonclassical light. Otherwise the quantum behaviour and entanglement are fully reduced. Under some restrictions for noise nonclassical light can also be generated at and below the threshold.     

Bistability,self-pulsing and chaos in optical parametric oscillators

Summary We consider a degenerate parametric oscillator and show that, when the cavity is detuned both with respect to the pump and with respect to its subharmonic, the system can display bistable or self-pulsing behaviour. Bistability (self-pulsing) requires that the product of the two detuning parameters is positive (negative). In the self-pulsing regime, the system exhibits period doubling and chaos. We generalize our steady-state analysis to the case in which the cavity has a poor finesse for the pump mode.

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Riassunto Si considera un oscillatore parametrico degenere e si mostra che, quando la cavità è fuori risonanza sia rispetto alla pompa che rispetto alla sua subarmonica, il sistema può esibire comportamento bistabile o autoimpulsato. Il comportamento bistabile (autoimpulsato) richiede che il prodotto dei due parametri di fuori risonanza sia positivo (negativo). Nel regime autoimpulsato, il sistema mostra raddoppiamento di periodo e caos. Si generalizza l'analisi stazionaria al caso in cui la cavità ha qualità cattiva per il modo di pompa.

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Резюме Мы рассматриваем вырозденный параметрический осциллятор и показываем, что, когда резонатор находится вне резонанса относительно накачки и относительно субгармоник, тогда система может обнаруживать бистабильное или самопульсирующее поведение. Бистабильность (самопульсирование) требует, чтобы произведение двух параметров расстройки являлось положительным (отрицательным). В режиме самопульсирования система обнаруживает удвоение и хаос. Мы обобщаем наш стационарный анализ на случай, в котором резонатор имеет плохое качество для моды накачки.

     

A new bipartite entangled state describing the parametric down-conversion process and its applications in quantum optics

We construct a new bipartite entangled state(NBES),which describes both the squeezing and the entanglement involved in the parametric down-conversion process and can be produced using a symmetric beam splitter.Constructing asymmetric ket-bra integrations based on the NBES leads to some new squeezing operators,which clearly exhibit the relationships between squeezing and entangled state transformations.Moreover,an entangled Wigner operator with a definite physical meaning is also presented.     

Spontaneous coherence of atomic states within a metallic slit

Summary It was found that, when metastable hydrogen 2s_1/2-atoms pass through a two-electrode atomic interferometer, the yield of 2p-atoms essentially depends on the field direction. This large-scale effect can hardly be understood on the basis of quantum-mechanical concepts on the behaviour of an H-atom in electric field. In further experiments the said effect was shown to be a secondary one, namely, it arises due to transformation of the pure 2s _1/2-state into the (2s−2p)-superposition as H-atoms fly through the interferometer entrance slit. Let us take a big metallic screen and make in it a small metallic hole M. A. Leontovitch-Talk in a seminar The authors of this paper have agreed to not receive the proofs for correction.     

基于线性光学的多通道混合纠缠态

本文提出了一种基于线性光学的多通道混合纠缠态方案。最初的混合纠缠态是在连续变量猫态和分离变量单光子量子比特之间产生。该方案基于线性光学元件,利用分束器在初始混合纠缠态基础上产生多通道混合纠缠态(M组)。在此基础上,对每对混合纠缠态相对初始混合纠缠态的保真度进行了计算,并分析了保真度随所分份数多少及猫态大小的变化关系。这种多通道混合纠缠态将会为涉及分离变量与连续变量的多通道量子信息传输及存储、量子网络节点信息分布等提供重要的资源。     

Spontaneous parametric down-conversion and quantum walks in arrays of quadratic nonlinear waveguides

We analyze the process of photon-pair generation with simultaneous quantum walks in a quadratic nonlinear waveguide array. We demonstrate that the spontaneous parametric down-conversion in the array allows for creating quantum states with strongly pronounced spatial correlations, which are qualitatively different from those possible in bulk crystals or through quantum walks in linear waveguide arrays. Most importantly, the photon correlations can be controlled entirely classically by varying the spatial profile of the pump beam or the phase-matching conditions.     

Entanglement in cascaded-crystal parametric down-conversion

We use spontaneous parametric down-conversion in a cascade of crystals, driven by a single monochromatic cw pump laser, to study the interference of entangled photon pairs. By changing the distance between the crystals, the observed quantum interference pattern varies continuously from that associated with a longer single crystal to that associated with independent emissions from two distinct crystals. Postselection via spectral filtering suppresses this phenomenon. These findings are expected to advance the field of quantum-state engineering.     

Four-photon correlations in parametric down-conversion

Four-photon correlations of the output radiation of a parametric amplifier with a vacuum at the input are considered for an arbitrary parametric gain coefficient. Such states are interpreted in the literature as four-photon states. It is shown that the fourth-order correlation function for such states in the limit of a small number of photons has an asymptotics typical of two-photon states. Nevertheless, even in the “classical” limit of high intensities, the level of four-photon correlations, i.e., the value of the normalized fourth-order correlation function, is substantially greater than that for coherent and even thermal fields.     

Ultra‐fast heralded single photon source based on telecom technology

The realization of an ultra‐fast source of heralded single photons emitted at the wavelength of 1540 nm is reported. The presented strategy is based on state‐of‐the‐art telecom technology, combined with off‐the‐shelf fiber components and waveguide non‐linear stages pumped by a 10 GHz repetition rate laser. The single photons are heralded at a rate as high as 2.1 MHz with a heralding efficiency of 42%. Single‐photon character of the source is inferred by measuring the second‐order autocorrelation function. For the highest heralding rate, a value as low as 0.023 is found. This not only proves negligible multi‐photon contributions but also represents one of the best measured values reported to date for heralding rates in the MHz regime. These performances, associated with a device‐like configuration, are key ingredients for both fast and secure quantum communication protocols.

     

光纤耦合双光学腔系统的相干动力学

为了研究量子相干性在腔量子电动力学系统中的动力学和分布特性,基于两个各自捕获原子系综的光学腔建立了双光学腔系统,腔与腔之间由光纤耦合.利用相对熵度量的量子相干性,引入量子相干非平衡性的概念,分析了系统中相干动力学和光纤-腔耦合强度对相干性分布的影响.结果表明:在强耦合极限下,光纤-腔耦合强度的增加有利于保持两腔中的原子的整体相干性;光纤-腔耦合强度、原子-腔耦合强度以及原子数三个参数之间满足特定条件时,腔内的原子相干性可以传输至另一个腔.考虑腔、光纤及原子都存在耗散的情形,对比了不同耗散速率和非耗散情形下的相干性演化,发现耗散使得耦合双腔系统的相干性以及各个腔中的原子相干性发生衰减.     

Quantum interference and diffraction of parametric down-converted biphotons

We present two-photon diffraction and interference experiments utilizing parametric down-converted photon pairs (biphotons) and a transmission grating. The biphoton exhibits a diffraction-interference pattern equivalent to an effective single particle with half wavelength of the constituent photons.     

Numerical simulation of spatial fluctuations in parametric image amplification

We study the properties of the spatial fluctuations in the far-field parametric fluorescence output of a type 1 degenerate traveling-wave parametric amplifier. Results of a semi-classical simulation are compared with experiments in a LBO crystal. This simulation is then used to predict amplified images of a continuous background, in a phase-sensitive as well as in a phase-insensitive configuration. Received 12 March 2001     

von Neumann Mutual Information for Anisotropic Coupled Oscillators Interacting with a Single Two-Level Atom

We consider the interaction between a two-level atom and two electromagnetic fields injected simultaneously within a cavity, with the interaction between the fields in parametric frequency-converter form. The wave function in Schr ödinger picture is obtained under certain conditions and consequently the density matrix. By employing a generalization of the von Neumann mutual information (in the context of Tsallis' nonextensive statistics) we measure the degree of entanglement for the present system. An important change is observed in the generalized mutual information depending on the entropic index. We also measure the minimum degree of entanglement during the transition from collapse to revival and vice-versa. Successive revival peaks show a lowering of the local maximum point indicating a dissipative irreversible change in the atomic state.     

Multimode description of stimulated parametric down-conversion

We derive multimode generating functions, photon-number distributions, integrated-intensity distributions, moments and quadrature fluctuations for stimulated parametric down-conversion in relation to experiments measuring joint distributions. The border between classical and quantum behaviour is given.     

Spontaneous nonparametric down-conversion of light

The spontaneous down-conversion of laser light in a dielectric plate and/or metal-dielectric interface is considered. It is found that strong enhancement of the process takes place near the critical intensity of excitation. The enhancement of the efficiency takes place if the amplitude of oscillations of the eikonal coincides with the wavelength of emission. The down-conversion in metal-coated SNOM tip may be used for the generation of completely polarization- entangled photon pairs.