The influence of laser fluctuations on entanglement generation in a non-degenerate parametric amplifier

We consider the effects of the phase and the amplitude fluctuations of the pump field upon the entanglement generation in a non-degenerate optical parametric amplifier. We show that the entanglement between the signal and idler modes in a NOPA system are suppressed by these fluctuations. Our results also show that entanglement is more sensitive to phase fluctuations than to amplitude fluctuations.     

Characterization of the pairwise correlations in different quantum networks consisting of four-wave mixers and beamsplitters

We investigate the performances of the pairwise correlations(PCs) in different quantum networks consisting of fourwave mixers(FWMs) and beamsplitters(BSs). PCs with quantum correlation in different quantum networks can be verified by calculating the degree of relative intensity squeezing for any pair of all the output fields. More interestingly, the quantum correlation recovery and enhancement are present in the FWM+BS network and the repulsion effect phenomena(signal(idler)-frequency mode cannot be quantum correlated with the other two idler(signal)-frequency modes simultaneously)between the PCs with quantum correlation are predicted in the FWM + FWM and FWM + FWM + FWM networks. Our results presented here pave the way for the manipulation of the quantum correlation in quantum networks.     

Analysis of directly produce pump,signal, and idler three-color continuous-variable entanglement

Analysis is presented in this paper to directly produce three-color continuous-variable (CV) entanglement by nondegenerate optical parametric oscillator (NOPO) with an injected signal. The quantum entanglement among the pump, signal, and idler beams are calculated by applying sufficient inseparability criteria for CV entanglement which proposed by van Loock and Furusawa [Phys. Rev. A 67, 052315 (2003)]. The results indicates that strong three-color CV entangled beams, i.e. pump, signal, and idler beams, can be generated when the NOPO operating above the threshold. The degree of entanglement dependence on damping rates is also investigated.     

长脉冲抽运光子晶体光纤四波混频和超连续谱的理论研究

本文数值研究了长脉冲抽运光子晶体光纤四波混频和超连续谱的产生.依据准连续波近似下的相位匹配条件和能量守恒定律,能够理论上确定在光子晶体光纤正常色散区抽运时,四波混频效应产生的信号光和空闲光波长;以及在光纤反常色散区抽运时,调制不稳现象产生的两个对称旁瓣波长.利用自适应分步傅里叶法,定量地模拟了利用波长为1064 nm的亚纳秒激光器抽运两种不同色散特性的光子晶体光纤四波混频效应和超连续谱的产生,模拟结果与实验结果符合非常好. 关键词： 光子晶体光纤 超连续谱 四波混频 自适应分步傅里叶法     

Entanglement and Photon Anti-Bunching in Coupled Non-Degenerate Parametric Oscillators

We analytically and numerically show that the Hillery-Zubairy’s entanglement criterion is satisfied both below and above the threshold of coupled non-degenerate optical parametric oscillators (NOPOs) with strong nonlinear gain saturation and dissipative linear coupling. We investigated two cases: for large pump mode dissipation, below-threshold entanglement is possible only when the parametric interaction has an enough detuning among the signal, idler, and pump photon modes. On the other hand, for a large dissipative coupling, below-threshold entanglement is possible even when there is no detuning in the parametric interaction. In both cases, a non-Gaussian state entanglement criterion is satisfied even at the threshold. Recent progress in nano-photonic devices might make it possible to experimentally demonstrate this phase transition in a coherent XY machine with quantum correlations.     

Bright source of spectrally uncorrelated polarization-entangled photons with nearly single-mode emission

We present results of a bright polarization-entangled photon source operating at 1552 nm via type-II collinear degenerate spontaneous parametric down-conversion in a periodically poled potassium titanyl phosphate crystal. We report a conservative inferred pair generation rate of 123,000 pairs/s/mW into collection modes. Minimization of spectral and spatial entanglement was achieved by group velocity matching the pump, signal, and idler modes and through properly focusing the pump beam. By utilizing a pair of calcite beam displacers, we are able to overlap photons from adjacent down-conversion processes to obtain polarization-entanglement visibility of 94.7+/-1.1% with accidentals subtracted.     

Experimental observation of three-color optical quantum correlations

Quantum correlations among bright pump, signal, and idler beams produced by an optical parametric oscillator, all with different frequencies, are experimentally demonstrated. We show that the degree of entanglement between signal and idler fields is improved by using information on pump fluctuations. To our knowledge this is the first observation of three-color optical quantum correlations.     

Nonclassical properties of a contradirectional nonlinear optical coupler

We investigate the nonclassical properties of output fields propagated through a contradirectional asymmetric nonlinear optical coupler consisting of a linear waveguide and a nonlinear (quadratic) waveguide operated by second harmonic generation. In contrast to the earlier results, all the initial fields are considered weak and a completely quantum-mechanical model is used here to describe the system. Perturbative solutions of Heisenberg's equations of motion for various field modes are obtained using Sen–Mandal technique. Obtained solutions are subsequently used to show the existence of single-mode and intermodal squeezing, single-mode and intermodal antibunching, two-mode and multi-mode entanglement in the output of contradirectional asymmetric nonlinear optical coupler. Further, existence of higher order nonclassicality is also established by showing the existence of higher order antibunching, higher order squeezing and higher order entanglement. Variation of observed nonclassical characters with different coupling constants and phase mismatch is discussed.     

Broadband frequency mode entanglement in waveguided parametric downconversion

We report the observation of beatings of the coincidence event rate in a Hong-Ou-Mandel interference (HOMI) between signal and idler photons from a parametric downconversion (PDC) process inside a multimode KTP waveguide. As an explanation we introduce biphotonic states entangled in their broadband frequency modes generated by waveguide mode triples and propose a suitable entanglement detection scheme.     

Three-colour entanglement generated by an injection-seeded nondegenerate optical parametric oscillator

Three-colour continuous-variable (CV) entanglement can be directly generated by an injection-seeded nondegenerate optical parametric oscillator (INOPO). The quantum correlations among the pump, signal, and idler beams are calculated and discussed by applying sufficient inseparability criteria for CV entanglement. The results clearly show that strong three-colour CV entanglement can be produced by operating the pump above the oscillation threshold. The INOPO is easier to realize experimentally and more steady in comparison to that without an injected signal since the injected signal can increase the nonlinear conversion efficiency and the stability, as well as allow a large degree of tunability. This scheme can be very useful for the applications in quantum communication and computation networks.     

Stimulated Brillouin scattering limited intensity-dependent four-wave mixing between pump and signal in a non-zero dispersion shifted fiber

The intensity-dependent four-wave mixing (FWM) efficiency is derived by the nonlinear Schrodinger equations including the self phase modulation and cross phase modulation between the two Fabry–Perot modes of a pump and a single-wavelength signal. The maximum FWM for mixing the pump with signal in non-zero dispersion shifted fiber is explained as the minimum phase mismatch for FWM due to equal group delay for the pump and signal. It was also found that the experimental measurements for the generated FWM power are limited by the stimulated Brillouin scattering (SBS) threshold. Under the SBS threshold, the experimental results are still coincided well with the theoretical calculations of the FWM generated power ratio to have the difference of 0.12 dB for the input signal power of 12 mW.     

Nontransparency and optical phase erasure characteristic of four-wave mixing

Four-wave mixing (FWM) is a well-known nonlinear optical phenomenon and has attracted great interest in nonlinear all-optical signal processing owing to its ultrafast and transparent properties. Here we report the experimental observation via spectra on the nontransparency and optical phase erasure characteristic of FWM. The distinctive feature of “optical phase erasure” is a consequence of square relationship between the converted idler and input signal. It is interesting to find that the optical phase information carried by the input signal is erased in the converted idler by exploiting FWM. Such new attribute of FWM enables optical phase information removal and all-optical format conversion from carrier-suppressed return-to-zero (CSRZ) to return-to-zero (RZ). Moreover, similar phenomena are observed in the higher-order FWM. In addition, the nontransparency phenomenon and optical phase erasure characteristic are also demonstrated by theoretical analyses with optical spectra and phase diagrams.     

Direct production of tripartite pump-signal-idler entanglement in the above-threshold optical parametric oscillator

We calculate the quantum correlations existing among the three output fields (pump, signal, and idler) of a triply resonant nondegenerate optical parametric oscillator operating above threshold. By applying the standard criteria [P. van Loock and A. Furusawa, Phys. Rev. A 67, 052315 (2003)], we show that strong tripartite continuous-variable entanglement is present in this well-known and simple system. Furthermore, since the entanglement is generated directly from a nonlinear process, the three entangled fields can have very different frequencies, opening the way for multicolored quantum information networks.     

Evolution of first-order sidebands from multiple FWM processes in HiBi optical fibers

The generation of two idler waves inside a high birefringent (HiBi) optical fiber through three four-wave mixing (FWM) processes is studied theoretically. The coupled-equations for the field amplitudes are derived and analytically solved, in the co-polarized and orthogonal polarization schemes. The obtained solutions take into account the delayed Raman response of the medium. The polarization sensitivity of the generation of the idler waves is analyzed. Results show that the stimulated Raman scattering does not change the efficiency of the idler wave generation in the co-polarized scheme, whereas in the orthogonal polarization scheme that nonlinear process decreases the efficiency of the four-wave mixing processes. Results also show that this set of multiple four-wave mixing processes is physically quite different from the typical single or dual pump four-wave mixing configurations. Findings show that the power transfer from the pumps to the idler fields can lead to a monotonous growth, or a periodic evolution of the sidebands along the fiber. Results show that the process efficiency varies greatly with the angle between the two pump polarizations.     

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.     

Optical-fiber source of polarization-entangled photons in the 1550 nm telecom band

We present a fiber-based source of polarization-entangled photons that is well suited for quantum communication applications in the 1550 nm band of standard fiber-optic telecommunications. Polarization entanglement is created by pumping a nonlinear-fiber Sagnac interferometer with two time-delayed orthogonally polarized pump pulses and subsequently removing the time distinguishability by passing the parametrically scattered signal and idler photon pairs through a piece of birefringent fiber. Coincidence detection of the signal and idler photons yields biphoton interference with visibility greater than 90%, while no interference is observed in direct detection of either signal or idler photons. All four Bell states can be prepared with our setup and we demonstrate violations of the Clauser-Horne-Shimony-Holt form of Bell's inequality by up to 10 standard deviations of measurement uncertainty.     

Dynamics of a Dipolariton Optical Parametric Oscillator in a Semiconductor Microcavity

The dynamics of dipolariton states in a planar microcavity has been studied upon pumping of a state corresponding to the middle dipolariton branch. It has been shown that, under exact resonance conditions, both periodic and aperiodic regimes of conversion of pump dipolaritons to dipolaritons of idler and signal modes occur. The periodic regime of conversion of signal and idler dipolaritons without involving pump dipolaritons on the middle branch has also been analyzed.     

Enhanced four-wave mixing in a hollow-core photonic-crystal fiber

Hollow-core photonic-crystal fibers are shown to substantially enhance four-wave mixing (FWM) of laser pulses in a gas filling the fiber core. Picosecond pulses of Nd:YAG fundamental radiation and its second harmonic are used to generate a signal at the frequency of the third harmonic by the FWM process 3omega = 2omega + 2omega - omega. The efficiency achieved for this process in a 9-cm-long, 13-microm-hollow-core-diameter photonic-crystal fiber, designed to simultaneously transmit a two-color pump and the FWM signal, is shown to be approximately 800 times higher than the maximum FWM efficiency attainable with the same laser pulses in the tight-focusing regime.     

Two-spatial-mode parametric amplifier in lithium niobate waveguides with asymmetric Y junctions

For optical parametric amplifiers in proton-exchange waveguide devices reported to date, both the signal and the idler photons are in the TM00 mode, causing difficulty in distinguishing signal from idler. We report a two-mode optical parametric amplifier. The key components, asymmetric Y junctions, successfully launch the pump wave in a pure TM10 mode and separate the signal and the idler in the TM10 and TM00 modes with a power ratio of as much as 27.5 dB. With high parametric gain, optical parametric generation with a threshold of 300 pJ/pulse was demonstrated for 1.8 ps long pump pulses near 780 nm. These periodically poled lithium niobate waveguides are efficient integrable photon sources.     

Quantum cascade structure for mid-IR four-wave mixing

The design and modeling of a quantum cascade optical amplifier (QCOA) using intra-cavity non-linear interactions to achieve wavelength conversion is proposed. The model is based on the nonlinear equation coupled with Maxwell wave equations for different emission modes. In the proposed structure, four wave mixing (FWM) output exhibits a peak as a function of pump and probe frequency if they are tuned to the energy levels of the QC structure subbands. Results demonstrate that the FWM output signal power significantly depends on how subbands are engineered and interact with optical pulses which propagate in multi layer medium. In addition, we show that by adjusting pump and probe signal frequencies, FWM output power can be tuned.