| Parametric down conversion in crystals with a quadratic nonlinearity has proved to be the most successful and widely used approach to generating nonclassical light such as squeezing light, entanglement beams, etc. The down-conversion process also can be used to generate so-called twin beams that have intensity correlations that are stronger than would be obtained with individual coherent states. Twin beams have been successfully applied to sub-shot-noise spectroscopy and quantum non-demolition measurements. The generation of entangled beams in parametric down-conversion was predicted by Reid and Drummond in the late 1980s, and is crucial for studies of quantum teleportation and networking. In general in order to experimentally generate nonclassical beams with carrier frequencyωone typically starts with a coherent soure atωwhich is frequency doubled to 2co. The light at 2co is then used to drive a down-conversion process to generate nonclassical light at frequencyω. These multiple steps add to the complexity of experimental arrangement and limit the possibility of generating nonclassical light at high frequencies.Especially frequency up-conversion second harmonic generation can directly generate nonclassical light. it may carry out two kinds of different wavelengths(short and long wavelengths) quadrature amplitude bright squeezing, continuous light beams that exhibit nonclassical statistics are of interest as a tool for studying quantum fields and for a number of applications including precision measurements, writing sub-wavelength spatial structures, and resources for quantum information and communication protocols. On the other hand, It is well known that second harmonic generation results in squeezing of the fundamental and harmonic beams. But the generation of multi-beam correlation in second harmonic generation is less well studied. Calculations have demonstrated the existence of correlations between the fundamental and harmonic fields including entanglement between the fundamental and harmonic fields and entanglement in type II second harmonic generation in the fundamental fields alone. The possibility of nonclassical spatial correlations in either the fundamental or harmonic fields alone and of entanglement in the fundamental field has also been in models that include diffraction. The generation of strong entanglement between optical fields at vastly different frequencies would immediately facilitate many interspecies quantum information protocols, for example, the frequency of states of light could be drastically altered using interspecies quantum teleportation protocols. Such a protocol would enhance the integrability of disparate nodes in a quantum information network. Applications could be envisaged in any situation where a nonclassical link is required between two experiments at differing optical frequencies. Such a situation might arise, for example, in experiments involving two different atomic species, or if a connection is required to an atomic frequency standard. In addition, the generation of harmonic entanglement will apply to other systems that share the same second-order nonlinear interaction term in the Hamiltonian.Our aim is to experimentally generate nonclassical states of light by a second harmonic generation frequency doubler with PPKTP crystal as nonlinear medium in which the fundamental wave is resonantly enhanced, and study their nonclassical quantum characteristics as noise fluctuation, intensity correlations, EPR entanglement, etc. we hope to have a profound understand and cognition about the quantum traits of light. the design of the frequency doubler owes to external-cavity singly resonant standing wave cavity. All the experiment setup is easily feasible.In this work we investigate firstly the infrared reflected fundamental field squeezing generating, then study the quantum intensity correlation properties of two-mode interacting each other inside the frequency doubler.The all experimental setup may depict as follows. A homemade all-solid-state single frequency Nd:YVO4 laser is used as pump source which delivers 700mW of infrared power at 1064nm. A narrow line-width F-P cavity (mode cleaner) is used to improve the transverse mode and reduce the excess intensity noise at 1064nm. After the mode-cleaner, the intensity noise of laser reaches the quantum noise limit at 5 MHz, and the ellipticity of the Gaussian beam profile is reduced to less than 1%. Frequency doubler is a singly resonant standing cavity. The SHG cavity is formed by two cavity mirrors with 20mm radii of curvature, which are separated by 40mm. the beam waist of the fundamental is about 46μm inside the cavity. The input coupler is coated for partial transmission of 2% at 1064nm and high reflectivity at 532nm. The output coupler is coated for high reflectivity at 1064nm and high transmission at 532nm. The PPKTP crystal is positioned at the center of the cavity, both end faces of it are antireflection coated at 1064nm and 532nm. The intracavity loss is determined to be 1% by measuring the finesse of the cavity. An electric servo was utilized to lock the cavity to ensure that the cavity frequency coincide with the pump laser frequency.In condition of harmonic output by frequency doubling in one port resonator, we firstly study the reflected fundamental light noise fluctuation on different parameters. we investigated the noise fluctuation characteristics of the fundamental reflected from the doubler when the input coupler transmission is 5%, 2% and 1%. When the input cavity coupler power transmissivity is 5% and pump power is 9mW, amplitude noise reduction of 0.67dB(considering the detection efficiency of 79%, the inferred squeezing is 0.85dB) below the shot noise limit is experimentally observed. The quadrature amplitude noise of the reflected light is measured by a self-homodyne system. At the same time, we investigated the amplitude squeezing of the harmonic as a function of fundamental pump power under three different input coupling transmissions.Based on the observation of noise reduction of the reflected infrared light and the second harmonic light, we study experimentally the generation of muti-beam correlation by second harmonic generation, which is less well studied than in the case of parametric down conversion. There has demonstrated the existence of correlation between the fundamental and harmonic fields.To measure the quantum correlation between the fundamental and the SH fields, the photo-detectors of the SH and fundamental fields were well balanced with a thermal white-light source that provided a dc photocurrent equal to that given by the corresponding squeezed light.Before measuring the quantum correlation between fundamental and SH fields, we first investigated the amplitude noise of the fundamental and SH fields individually. The green light was separated from the infrared by using a dichroic beam splitter, and its amplitude noise was directly measured by a photo-detector(FND-lOOQ) directly detect the amplitude noise of the fundamental field, the corresponding QNL calibrated by a thermal white-light source. The reflected pump Beam, after the interaction with the SHG cavity, is reflected by a polarizing beam splitter after double passage through the Faraday rotator. A detector sysytem based on an ETX-300 photodiode was used to directly detect the amplitude noise of the fundamental field (the QNL was also calibrated by a thermal white-light source).In conclusion, amplitude quadrature squeezing of both the fundamental and SH fields was generated with degrees of squeezing of 0.5 and 0.3 dB, respectively, in an external cavity-enhanced singly resonant PPKTP frequency doubler. At the same time, the quantum anti-correlation of 0.6dB (considering the detection efficiency of 79% and 60%, the inferred squeezing is 1.1dB) between these squeezed fundamental and SH fields was also observed experimentally. When the input cavity coupler power transmissivity is 2%, and the pump power is 6mW.Anyway, we have demonstrated a new approach to generating bright correlation beams using SHG, we present the first measurement (to our knowledge) of quantum correlation between fundamental and SH fields with a vast wavelength difference of 532nm, Experimental results clearly verify the theoretical predictions and take a key step toward the generation of a two-color nonclassical state at vastly different frequencies. The quantum correlation indicates the squeezing of either mode not give a complete measure of the nonclassicality of the source, and it is shown that these correlations can be used to enhance the squeezing. This two color lights with quantum correlation characteristic are especially useful in quantum communication, For example, it can be used in any situation where a nonclassical link is required between two systems at different optical frequencies. Such a two-color source is also an ideal candidate for long distance quantum communication, because the wavelength of fundamental and second harmonic fields can be readily chosen to be around 1560nm which lies in the low-loss transmission window of optical fibers and 780nm which is compatible with the absorption lines of alkaline atom and can be used for storage and processing of quantum information.In theory we analyze nonclassical characters of ouput fields in singly resonant second harmonic generation. firstly, using the boundary condition and Fourier transform we derived the noise characteristics of the second harmonic field exiting the singly resonant doubler and the infrared light reflected from the cavity, then induce the quantum correlation property between the two kinds of fields based on the noise fluctuation squeezing spectrum. We also predict the quantum entanglement effect between them by Criteria of Quantum teleportation for continuous variables, at last we give the analysis of quantum correlation detection principle, This offers the theoretical criterion of experimental detection. Finally, we theoretically simulate the above part experimental results, and the results are in basic agreement with theoretical predictions.
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