铷原子气体自旋噪声谱的测量与改进

利用自主设计并制作的基于现场可编程门阵列的实时傅里叶变换采集卡(FFTsDAC),采用线偏振光检测碱金属铷原子气样品中的自旋随机涨落(即自旋噪声谱).详细讨论了背景噪声以及自旋噪声随探测光光强的变化关系,证实了自旋噪声来自于系统中自旋的随机涨落.对比了两种FFTsDAC(8 bit采样的FFTsDACl和12 bit采样的FFTsDAC2)的测量性能,分析了影响实验信噪比的因素.FFTsDAC2具有更高的测量效率和采样深度以及更长的单次采样时间,因而具有更高的信噪比和更好的频率分辨率,与数值模拟的结果一致.

Measurement and improvement of rubidium spin noise spectroscopy

Spin noise spectroscopy (SNS) is a new kind of Faraday rotation technique, which does not need spin injection to generate polarized spin. This method uses a linearly polarized laser to detect the spontaneous spin fluctuation in a thermal equilibrium state. However, the signal of spontaneous spin fluctuation is so weak (~μV) in the thermal equilibrium system that a big signal-noise ratio (SNR) is often demanded. Here, we report on the build-up and improvement of a spin noise spectrum measurement system. A home-made field-programmable gate array (FPGA) based data-acquisition card with real-time fast Fourier transform (DAC-FFT) is used to improve the SNR of the SNS measurement system. The reduction of intrinsic noise in the experimental system is discussed in detail. Both the dependence of background noise and the dependence of spin noise on the intensity of probe laser are analyzed. We find that the background noise is proportional to the intensity of the probe laser, while the spin noise signal shows square dependence on probe laser intensity. The spin noise indeed comes from the spontaneous spin fluctuation as experimentally confirmed via an acousto-optic modulator (AOM) inserted in the measurement system. The measurement performances of two FPGA based DAC-FFTs (the 8-bit FFTsDAC1 and the 12-bit FFTsDAC2, respectively) are compared. Several factors are found to affect the SNR of the system, including the measurement efficiency and the acquisition resolution. The FFTsDAC2 has longer single acquisition time and faster data transmission speed (with USB 3.0) than the FFTsDAC1, when the total measurement time is set to be the same, the effective measurement time realized in FFTsDAC2 is longer than in FFTsDAC1. With better measurement efficiency and sampling depth and longer single acquisition time, the FFTsDAC2 has a better SNR and finer frequency resolution with a much narrower full width at half maximum (FWHM) value. Moreover, the simulations of the measurement process show the effect of the single acquisition time on the FWHM of spin noise peak, further clarifying the reason why the spin noise spectrum measured by FFTsDAC2 is more accurate.