We report the experimental generation of a squeezed vacuum at frequencies ranging from 2.5 kHz to 200 kHz that is resonant on the cesium D2 line by using a below-threshold optical parametric oscillator (OPO). The OPO is based on a periodically-poled KTiOPO 4 (PPKTP) crystal that is pumped using a bow-tie four-mirror ring frequency doubler. The phase of the squeezed light is controlled using a quantum noise locking technique. At a pump power of 115 mW, maximum quadrature phase squeezing of 3.5 dB and anti-squeezing of 7.5 dB are detected using a home-made balanced homodyne detector. This squeezed vacuum at an atomic transition in the kilohertz range is an ideal quantum source for quantum metrology of enhancing measurement precision, especially for ultra-sensitive measurement of weak magnetic fields when using a Cs atomic magnetometer in the audio frequency range.
Atomic spin relaxation in a vapor cell, which can be characterized by the magnetic resonance linewidth (MRL), is an important parameter that eventually determines the sensitivity of an atomic magnetometer. In this paper, we have extensively studied how the pump intensity affects the spin relaxation. The experiment is performed with a cesium vapor cell, and the influence of the pump intensity on MRL is measured at room temperature at zero-field resonance. A simple model with five atomic levels of a Λ-like configuration is discussed theoretically, which can be used to represent the experimental process approximately, and the experimental results can be explained to some extent. Both the experimental and the theoretical results show a nonlinear broadening of the MRL when the pump intensity is increasing. The work helps to understand the mechanism of pump induced atomic spin relaxation in the atomic magnetometers.
Magnetometry has already been widely used in mineral exploration, medical exploration and precision measurement physics. One is trying to improve the sensitivity of the magnetometer. One of the most widely used magnetometers is based on the Bell-Bloom structure, which can be realized by modulating the pump light. The sensitivity of the Bell-Bloom magnetometer is determined by the magnetic resonance linewidth (MRL) and the signal-to-noise under the condition of magnetic resonance (SNR). Both are affected by the pump intensity and the relaxation rate of the atoms. In order to achieve a higher sensitivity, how these factors affect the magnetic field measurement should be analyzed. In this paper, the influence of the pump light on the sensitivity of the linearly polarized Bell-Bloom magnetometer is investigated based on the Bloch equation with amplitude modulated pump beam and the rate equations with spin relaxation. The rate equations are obtained from the Liouville equation, and the theoretical analysis is based on the cesium. The pump beam is linearly polarized and is resonant to D<sub>1</sub> transition of cesium. Both the direct pump (pump frequency is resonant to <inline-formula><tex-math id="M500">\begin{document}${6^2}{{\rm{S}}_{1/2}}\;F = 4$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20190030_M500.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20190030_M500.png"/></alternatives></inline-formula>−<inline-formula><tex-math id="Z-20190422020150-2">\begin{document}${6^2}{{\rm{P}}_{1/2}}\;F' = 3$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20190030_Z-20190422020150-2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20190030_Z-20190422020150-2.png"/></alternatives></inline-formula> transition) and the indirect pump (pump frequency is resonant to <inline-formula><tex-math id="M501">\begin{document}${6^2}{{\rm{S}}_{1/2}}\;F = 3 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20190030_M501.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20190030_M501.png"/></alternatives></inline-formula>−<inline-formula><tex-math id="Z-20190422020310-3">\begin{document}${6^2}{{\rm{P}}_{1/2}}\;F' = 4$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20190030_Z-20190422020310-3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20190030_Z-20190422020310-3.png"/></alternatives></inline-formula> transition) are analyzed. The experiment is performed based on a 20-mm cube cesium vapour cell with 20-Torr helium as buffer gas. The linearly polarized probe beam is tuned to resonance to cesium D<sub>2</sub> transition <inline-formula><tex-math id="M502">\begin{document}${6^2}{{\rm{S}}_{1/2}}\;F = 4$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20190030_M502.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20190030_M502.png"/></alternatives></inline-formula>−<inline-formula><tex-math id="Z-20190422020405-4">\begin{document}$ {6^2}{{\rm{P}}_{3/2}}\;F' = 5$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20190030_Z-20190422020405-4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20190030_Z-20190422020405-4.png"/></alternatives></inline-formula>, and the intensity of the probe is 0.2 W/m<sup>2</sup>. The spectra of magnetic resonance are measured by using the lock-in detection with a scanning of the modulation frequency. Then the sensitivity can be obtained by measuring MRL and SNR. The experimental results show that the sensitivity and the pump intensity are related nonlinearly, which is coincident with theoretical result. Higher sensitivity can be obtained under the condition of indirect pump. In addition, the effect of atomic spin relaxation on sensitivity is also analyzed with the indirect pump beam. This work clarifies the dynamics of the Bell-Bloom magnetometer to some extent. The highest sensitivity obtained is <inline-formula><tex-math id="M503">\begin{document}$31.7\;{\rm{pT}}/\sqrt {{\rm{Hz}}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20190030_M503.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20190030_M503.png"/></alternatives></inline-formula> in our experiment, which can be optimized by using other kinds of vapour cells and different measuring methods.
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