Experimental Generation Of Squeezed Light On Cesium Transition Line Down To 10 Hertz

With the rapid development of science and technology,precision measurement is more and more widely used in scientific research and industrial fields.As a sensitive detection method,optical measurement has been widely applied in the field of weak signal detection.However,with the development of modern technology,the sensitivity of optical measurements has reached the quantum noise limit（QNL）in many situations.Squeezed light is one of the typical quantum light sources,and one of its quadrature noises is less than the QNL.Replacement of the coherent light of the original measurement system with the squeezed light can beyond the QNL,thereby improving the measurement sensitivity.At present,the analysis frequency of squeezed light generated in experiments is mostly in the radio frequency regime.However,in some applications,such as precision magnetic measurement,gravitational wave detection,seismic wave prediction,and the investigation of light-matter interactions,the analysis frequency is at audio frequencies and below.Therefore,it is of great significance to prepare squeezed light in low-frequency band.In the precision magnetic measurement,the sensitivity of the spin exchange relaxation-free magnetometer based on the atomic spin is in the order of 0.1f T/Hz,which is close to the QNL.For improving the sensitivity of the measurement,it is necessary to prepare the squeezed light resonant on the atom line.However,in the low-frequency band,especially in the Hertz range,there is a lot of technical noise.Secondly,the commonly used potassium titanyl phosphate（KTi OPO₄,KTP）crystal has strong absorption and heating effect on the short-wavelength light near the alkali transition,which makes it difficult to obtain the squeezed light in the audio frequencies and below.Based on the second-order nonlinear effect of periodically poled KTP（PPKTP）crystal,we theoretically and experimentally study the generation of the low-frequency squeezed light resonant on the cesium（Cs）D2 line in the Hz range.The main contents are as follows:Chapter 1 is a brief introduction of basic knowledge of quantum optics and nonlinear optics,as well as the theory of the generation and detection of squeezed light.And in Chapter 2,We have optimized and improved the squeezed light generation system:designing and building the optical parametric oscillator（OPO）cavity with high stability and small folding angle,building a mode cleaner to optimize the spatial mode of the laser,and optimizing the phase shifter to improve the stability of the phase scanning.Then,the locking schemes of the squeezed light generation system are introduced:frequency locking of Ti:sapphire laser,OPO cavity length locking,and quantum noise locking.Then in Chapter 3,based on the optical parametric amplifier（OPA）,we have generated a bright polarization squeezed light at 852 nm,which is obtained by combing the bright quadrature squeezed light generated by an OPA with coherent light in a polarization beam splitter.Three Stokes parameters squeezing are realized in the range of 2 MHz-10 MHz,and the maximum squeezing level is 4.3 d B.In Chapter 4,based on the photon statistics measurement,we have precisely determined the weak squeezing parameter of the squeezed vacuum state.Firstly,the relationship between the second-order degree of coherence g^（2）（0）and the weak squeezing parameter is determined theoretically.Then the influence of the nonlinear crystal temperature,the detection efficiency,and the pump power on the g^（2）（0）measurement are analyzed in detail.In the end,the squeezing parameter of the weak squeezed vacuum state is precisely determined by measuring the g^（2）（0）in the experiment.Finally,in Chapter 5,we have generated squeezed light on the atom line in the Hertz range.Firstly,we use optoelectronic feedback with the acousto-optic modulator to suppress the intensity noise of the laser light,and the intensity noise below 1k Hz is effectively suppressed more than 10 d B.Secondly,by reducing the parasitic interference,low-frequency vibration,beam jitter,and intensity noise,vacuum states from4 Hz to 200 k Hz with a flat spectrum are successfully detected.Based on the above work,a broadband squeezed vacuum light resonating with the Cs D2 line down to 4 Hz is experimentally generated,in which the maximum squeezing is 5.92 d B.Then,we reduce the probe light power injected into the OPA cavity to the single-photon level and control the squeezing angle by a single-photon modulation locking（SML）.Finally,a quadrature squeezed light resonating with Cs D2 line down to 10 Hz with the maximum squeezing of5.57 d B is obtained experimentally.The innovations of the above work are as follows:1)We have generated the bright polarized squeezed light resonating with Cs D2 line for the first time,and the stokes parameters（?）₁,（?）₂ and（?）₃ of the polarization light are squeezed from 2 MHz to 10 MHz,respectively,and the maximum squeezing is about 4.3 d B.2)We have presented a feasible method for determining the weak squeezing parameter of an OPO working far below the threshold based on the photon statistics measurement. The relationship between the second-order degree of coherence g^（2）（0）and the weak squeezing parameter is established.3)We have reported the generation of squeezed vacuum light resonant on Cs D2 line down to 4 Hz for the first time with a maximum squeezing of 5.92 d B.Secondly,we have presented a single-photon modulation locking to control the squeezing angle.With the help of SML,we have reported the generation of quadrature squeezed light resonating with Cs D2 line down to 10 Hz with a maximum squeezing of 5.57 d B for the first time.