At the view of the present situation of the oil-water interface measurement, all sorts of measurement method can not eliminate the interference of temperature and hang oil. In order to control water ratio of purification oil at the output, simple interface detector cannot have satisfy the actual needs. A key contribution of this paper is to propose a solution based on a new type of sensor head and a test of new solution to the problem of capacitive sensing at high water ratio. A LC oscillation circuits is used to detect the variation of capacitance, overcoming the influence of the hang oil and the temperature.
The squeezed state, as an important quantum resource, has great potential applications in quantum computing, quantum communication and precision measurement. In the noncritically squeezed light theory, the predicted noncritically squeezed light can be generated by breaking the spontaneous rotational symmetry occurring in a degenerate optical parametric oscillator (DOPO) pumped above threshold. The reliability of this kind of squeezing is crucially important, as its quantum performance is robust to the pump power in experiment. However, the detected squeezing degrades rapidly in detection, because the squeezed mode orientation diffuses slowly, resulting in a small mode mismatch during the homodyne detection. In this paper, we propose an experimentally feasible scheme to detect noncritically squeezing reliable by employing the spatial mode swapping technic. Theoretically, the dynamic fluctuation aroused by random mode rotation in the squeezing detection can be compensated for perfectly, and 3 dB squeezing can be achieved robustly even with additional vacuum noise. Our scheme makes an important step forward for the experimental generation of noncritically squeezed light.
<sec> The Hong-Ou-Mandel (HOM) interferometer using entangled photon source possesses important applications in quantum precision measurement and relevant areas. In this paper, a simultaneous measurement scheme of multiple independent delay parameters based on a cascaded HOM interferometer is proposed. The cascaded HOM interferometer is composed of <inline-formula><tex-math id="M3">\begin{document}$ n $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M3.png"/></alternatives></inline-formula> concatenated 50∶50 beam splitters and independent delay parameters <inline-formula><tex-math id="M4">\begin{document}$ {\tau }_{1} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M4.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M5">\begin{document}$ {\tau }_{2} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M5.png"/></alternatives></inline-formula>, ···, <inline-formula><tex-math id="M6">\begin{document}$ {\tau }_{n} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M6.png"/></alternatives></inline-formula>. The numbers <inline-formula><tex-math id="M7">\begin{document}$ n=1, 2\;\mathrm{a}\mathrm{n}\mathrm{d}\;3 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M7.png"/></alternatives></inline-formula> refer to the standard HOM interferometer, the second-cascaded HOM interferometer, and the third-cascaded HOM interferometer, respectively. Through the theoretical study of the cascaded HOM interference effect based on frequency entangled photon pairs, it can be concluded that there is a corresponding relationship between the dip position and the independent delay parameter in the second-order quantum interferogram. In the standard HOM interferometer, there is a dip in the second-order quantum interferogram, which can realize the measurement of delay parameter <inline-formula><tex-math id="M8">\begin{document}$ {\tau }_{1} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M8.png"/></alternatives></inline-formula>. In the second-cascaded HOM interferometer, there are two symmetrical dips in the second-order quantum interferogram, which can realize the simultaneous measurement of two independent delay parameters <inline-formula><tex-math id="M9">\begin{document}$ {\tau }_{1} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M9.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M10">\begin{document}$ {\tau }_{2} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M10.png"/></alternatives></inline-formula>. By analogy, in the third-cascaded HOM interferometer, there are six symmetrical dips in the second-order quantum interferogram, which can realize the simultaneous measurement of three independent delay parameters <inline-formula><tex-math id="M11">\begin{document}$ {\tau }_{1} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M11.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M11.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M12">\begin{document}$ {\tau }_{2} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M12.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M12.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M13">\begin{document}$ {\tau }_{3} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M13.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M13.png"/></alternatives></inline-formula>. Therefore, multiple independent delay parameters can be measured simultaneously based on a cascaded HOM interferometer. </sec><sec> In the experiment, the second-cascaded HOM interferometer based on frequency entangled photon source is built. The second-order quantum interferogram of the second-cascaded HOM interferometer is obtained by the coincidence measurement device. Two independent delay parameters <inline-formula><tex-math id="M14">\begin{document}$ {\tau }_{1} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M14.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M14.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M15">\begin{document}$ {\tau }_{2} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M15.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M15.png"/></alternatives></inline-formula> are measured simultaneously by recording the positions of two symmetrical dips, which are in good agreement with the theoretical results. At an averaging time of 3000 s, the measurement accuracy of two delay parameters <inline-formula><tex-math id="M16">\begin{document}$ {\tau }_{1} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M16.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M16.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M17">\begin{document}$ {\tau }_{2} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M17.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20210071_M17.png"/></alternatives></inline-formula> can reach 109 and 98 fs, respectively. These results lay a foundation for extending the applications of HOM interferometer in multi-parameter quantum systems. </sec>
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