Experimental Preparation And Detection Of 795 Nm Polarization Squeezing At Rubidium D1 Line

As an important nonclassical light field,the squeezed light has been widely used in the fields of quantum teleportation,quantum dense coding,quantum storage,gravitational wave detection and etc,since it is experimentaly generated in 1985.In the field of squeezed light,in addition to quadrature squeezed light,photon number squeezed light and intensity difference squeezed light,the recently developed polarization squeezed light has also been widely concerned.The polarization squeezed light can be understood as a linearly polarized coherent light,whose vertical vacuum channel is filled by a squeezed vacuum field.In the field of quantum storage,the polarization squeezed light can directly interact with the spin wave of atomic ensemble.In addition,the polarization squeezed light can also improve the sensitivity of the magnetic field measurement.At present,the sensitivity of atomic magnetometer based on atomic spin effect reaches 0.16 f T/Hz1/2,which is close to the shot noise limit.To further improve the sensitivity,two fundamental quantum noises must be considered,including atomic spin projection noise and polarization noise of the light field.The polarization squeezed light can realize the sensitivity of the magnetic field measurement beyond the shot noise limit.In the experiment,the polarization squeezed light can be generated by combining the squeezed vacuum field and coherent light with a certain phase.For this scheme,the generation of squeezed vacuum field is the first step.In addition,for many applications in the field of precision measurement,such as gravitational wave detection and magnetic field measurement,the signal is mainly in the low frequency range,which requires that the squeezed light can be extended to low Fourier frequency.Based on the second-order nonlinear effect,we carry out the theoretical and experimental study on the 795 nm squeezed vacuum field,polarization squeezed light and low-frequency squeezed light,the main contents are as follow:1)We gave a brief review on the history of the development of squeezed light,and introduced some basic knowledge of quantum optics and nonlinear optics;2)Using PPKTP crystals as the nonlinear media,two configurations including the four-mirror bow-tie ring cavity with PPKTP crystal inside and semi-monolithic PPKTP cavity were used to realize the frequency doubling of the 795 nm laser,and the high-efficiency 397.5nm UV laser was obtained.In addition,the thermal effect in the frequency doubling process was analyzed,and the power stability and the beam quality were also mearsured;3)Using the 397.5nm UV laser to pump an optical parametric osciallator(OPO),we realized-5.6d B of single-mode squeezed vacuum field at 795nm;4)Running the OPO as an optical parametric amplifier(OPA),we obtained the quadrature squeezed light.Combining the the quadrature squeezed light with the coherent light,we realized the polarization squeezed light;5)We analyzed the noise coupling mechanism of the balanced homodyne system,and discussed the possible noise sources of the system in the low frequency band in detail.When the cavity was operating in the OPO mode,we can still detect the polarization squeezing of the-2.8d B at frequencies as low as 2.5k Hz through some experimental improvements.The innovation points are as follows:1)We compared characteristic of the four-mirror bow-tie ring cavity and semi-monolithic cavity for frequency doubling.Compared with bow-tie ring cavity,the semi-monolithic cavity showed strong thermal effect,which is not suitable for frequency doubling at this wavelength.This work can provide reference when other research groups carry out the related work;2)We achieved the highest squeezing degree of-5.6d B at 795 nm up to now,and analyzed the limiting factor of squeezing degree;3)At present,the research of low frequency squeezing is mainly focused on 1064 nm,this paper studied the preparation of 795 nm low-frequency squeezed light.After a preliminary exploration,-2.8d B of squeezing can be detected in the frequency band of 2.5k Hz-100 k Hz.To obtain a lower frequency band of squeezing,we also propose some improving schemes for the following experiments.