Improving the spectral purity of single photons by a single-interferometer-coupled microring

We experimentally engineer a high-spectral-purity single-photon source using a single-interferometer-coupled silicon microring. By the reconfiguration of the interferometer, different coupling conditions can be obtained, corresponding to different quality factors for the pump and signal/idler. The ratio between the quality factor of the pump and signal/idler ranges from 0.29 to 2.57. By constructing the signal—idler joint spectral intensity, we intuitively demonstrate the spectral correlation of the signal and idler. As the ratio between the quality factor of the pump and signal/idler increases, the spectral correlation of the signal and idler decreases, i.e., the spectral purity of the signal/idler photons increases. Furthermore, time-integrated second-order correlation of the signal photons is measured, giving a value up to 94.95±3.46%. Such high-spectral-purity photons will improve the visibility of quantum interference and facilitate the development of on-chip quantum information processing.     

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.     

Lower Order and Higher Order Entanglement in Four‐Wave Mixing Process

Possibilities of generation of lower order and higher order intermodal entanglement in four‐wave mixing (FWM) process are rigorously investigated using Sen‐Mandal perturbative technique. The investigation has revealed that for a set of experimentally realizable parameters, one can observe lower order and higher order intermodal entanglement between pump and signal modes and signal and idler modes in a FWM process. In addition, trimodal entanglement involving pump, signal and idler modes is also reported.     

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     

Quantum bits generation using correlated photons: Fidelity and error corrections

We propose a new system of quantum bits generation using a soliton pulse within a micro ring resonator. A quantum gate can be formed using a polarization control unit incorporating into the system. The random signal and idler pairs can be formed within the photon correlation bandwidth, which can be generated and randomly formed the packet quantum bits, i.e. quantum packet codes. Each random code (logic) can be performed by combining the signal and idler of each entangled photon pair via the quantum gate. Results obtained have shown that the packet of quantum logic bits can be generated using the entangled photon pairs generated by the proposed system. The quantum bits transmission fidelity and error corrections are also described.     

Simultaneous chromatic dispersion monitoring,extinction ratio enhancement and wavelength conversion using four-wave mixing

This paper presents a multi-functional all-optical device performs simultaneous chromatic dispersion (CD) monitoring, extinction ratio (ER) improvement and tunable wavelength conversion using four-wave mixing (FWM) in a single piece of highly nonlinear optical fiber (HNLF). The multi-functional ability arises from the nonlinear power transfer function (PTF) between the input signal and the output idler wave, which not only maps the CD information of the signal onto the average power of idler wave, but also transfers the data information to the idler wave with ER enhancement, making the device compact and cost effective. Furthermore within the wavelength tuning range the monitoring sensitivity and output ER can be much higher compared to all optical monitors and regenerators proposed before. Numerical simulations are then used to demonstrate the efficiency of this method.     

Photon statistics of pulse-pumped four-wave mixing in fiber with weak signal injection

We study the photon statistics of pulse-pumped four-wave mixing in fibers with weak coherent signal injection by measuring the intensity correlation functions of individual signal and idler fields. The experimental results show that the intensity correlation function of individual signal(idler) field g_(s(i))~(2) decreases with the intensity of signal injection. After applying narrow band filter in signal(idler) band, the value of g_(s(i))~(2) decreases from 1.9 ± 0.02(1.9 ± 0.02) to 1.03 ± 0.02(1.05 ± 0.02) when the intensity of signal injection varies from 0 to 120 photons/pulse. The results indicate that the photon statistics changes from Bose–Einstein distribution to Poisson distribution. We calculate the intensity correlation functions by using the multi-mode theory of four-wave mixing in fibers. The theoretical curves well fit the experimental results.Our investigation will be useful for mitigating the crosstalk between quantum and classical channels in a dense wavelength division multiplexing network.     

Quantum packet switching generation using correlated photons in a microring resonator system

We propose a new system of a packet of quantum bits generation using a soliton pulse within a microring resonator. A quantum gate can be formed by using a polarization control unit incorporating into the system. The random signal and idler pairs can be formed within the photon correlation bandwidth, which can be generated, and randomly form the packet quantum bits, i.e. quantum packet switching. Each random code (logic) can be performed by combining the signal and idler of each entangled photon pair via the quantum gate. Results obtained have shown that the packet of quantum logic bits can be generated using the entangled photon pairs generated by the proposed system.     

Generation and Manipulation of Spin-Orbit Coupling mode via Four-Wave Mixing with Quantum Interference

Recently, the vectorial nonlinear optical processes driven by spin-orbit coupling (SOC) light have come to the fore, leading to striking optical phenomena and important applications both in classical and quantum optics. However, research on the SOC-mediated light-atom interactions is still in its infancy. Here, the generation and manipulation of SOC mode through a vectorial four-wave mixing (FWM) process in ⁸⁵Rb vapor is demonstrated. Under the excitation of two SOC pump beams, multiple FWM paths can be simultaneously established due to the rich atomic Zeeman sublevels coupling with different circularly polarized components of SOC fields. A higher-order cylindrical or more general SOC FWM signal is observed in the vectorial FWM process, which obeys angular momentum conservation and Gouy phase matching. In particular, quantum interference between different FWM paths plays a crucial role in manipulating the SOC mode of the generated FWM signal, revealing that the conversion of SOC mode is intrinsically quantum. This work provides a pathway toward deeper insight into the vectorial nonlinear optical processes and may advance technology for shaping spatially structured light, which is essential in applications such as nonlinear polarization imaging and the optical communication realm.     

Theory for quantum state of photon pairs generated from spontaneous parametric down-conversion nonlinear process

A theory is presented for the quantum state of photon pairs generated from spontaneous parametric down-conversion nonlinear process. In this theory, the influence of the final sizes of nonlinear optical crystals on optical eigenmodes is explicitly taken into consideration. It was found that these photon pairs are not in entangled quantum states. Polarization correlations between the signal beam and the idler beam are explained. It is also shown that the two photons generated from SPDC are not spatially separated, therefore the polarization correlation between the signal and idler beams is not an evidence for quantum non-locality. The text was submitted by the authors in English.     

All-optical wavelength conversion and multicasting for polarization-multiplexed signal using angled pumps in a silicon waveguide

We report all-optical wavelength conversion and multicasting for a 20 Gb/s (2×10 Gb/s) polarization-multiplexed (Pol-MUX) nonreturn-to-zero amplitude-shift keying signal using four-wave mixing (FWM) in a silicon waveguide. The temporal waveforms and eye diagrams demodulated from both polarization channels of the converted idler are clearly observed. Moreover, the wavelength multicasting of the Pol-MUX signal is also demonstrated by using two incident pumps.     

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.     

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.     

Controllable four-wave mixing based on quantum dot-cavity coupling system

We theoretically study the four-wave mixing(FWM) response in a quantum dot-cavity coupling system, where a two-level quantum dot(QD) is placed in an optical cavity while the cavity mode is coupled to the nanomechanical resonator via radiation pressure. The influences of the QD-cavity coupling strength, the Rabi coupling strength of the QD, and the power of the pump light on the FWM intensity are mainly considered. The numerical results show that the FWM intensity in this hybrid system can be significantly enhanced by increasing the QD-cavity coupling strength. In addition, the FWM intensity can be effectively modulated by the Rabi coupling strength and the pump power. Furthermore, the effects of the cavity decay rate and the cavitypump detuning on the FWM signal are also explored. The obtained results may have potential applications in the fields of quantum optics and quantum information science.     

Two-color continuous-variable entanglement generated in nondegenerate optical parametric oscillator

A scheme is proposed to produce two-color continuous-variable (CV) entanglement in nondegenerate optical parametric oscillator with an injected signal (INOPO). The quantum correlations between signal and idler beams are calculated by applying sufficient inseparability criteria for bipartite CV entanglement. The results indicate that two-color CV entangled beams can be produced when the INOPO is operating both below and above the threshold. The injected signal does not markedly diminish the quantum correlations between the signal and idler beams. The degree of entanglement dependence on damping rates is also investigated.     

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.     

Tomographic measurement of joint photon statistics of the twin-beam quantum state

We report the first measurement of the joint photon-number probability distribution for a two-mode quantum state created by a nondegenerate optical parametric amplifier. The measured distributions exhibit up to 1.9 dB of quantum correlation between the signal and idler photon numbers, whereas the marginal distributions are thermal as expected for parametric fluorescence.     

Generation of photon pairs in dispersion shift fibers through spontaneous four wave mixing: Influence of self-phase modulation

Correlated signal and idler photon pairs with small detuning in the telecom band can be generated through spontaneous four-wave mixing in dispersion shift fibers. However, photons originated from other nonlinear processes in optical fibers, such as Raman scattering and self-phase modulation, may contaminate the photon pairs. It has been proved that photons produced by Raman scattering are the background noise of photon pairs. Here we show that photons induced by self-phase modulation of pump pulses are another origin of background noise. After studying the dependence of self-phase modulation induced photons in signal and idler bands, we demonstrate that the quantum correlation of photon pairs can be degraded by the self-phase modulation effect. The investigations are useful for characterizing and optimizing an all fiber source of photon pairs.     

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.     

Three-field noise correlation via third-order nonlinear optical processes

We report experimental observation of three-field noise correlation between two coexisting four-wave mixing (FWM) signals and the probe signal in a coherently-prepared rubidium vapor system. The two FWM signals in a double-Λ type quasi-four-level atomic system are obtained in different directions via their respective phase-matching conditions and can be detected individually. These signals and the probe beam are found to be strongly correlated or anticorrelated with each other.