Signal-to-noise ratio of spin noise spectroscopy in rubidium vapor

Spin noise spectroscopy is a non-demolition technique to detect the spin dynamics, and it is a good way to realize spin property under thermal equilibrium. Since spin noise arises from spin fluctuation at thermal equilibrium, it is a weak signal, therefore, various methods are used to enhance the signal-to-noise ratio (SNR) of the measurement system. To study the influence from different factors on the quality of spin noise spectroscopy, we report spin noise spectroscopy measurements in Rubidium vapor with three methods: a commercial frequency analyzer, a data acquisition card (DAC) with fast Fourier transform (FFT) done by a computer, and a DAC with real-time FFT based on FPGA (field-programmable gate array), respectively. According to the experimental results, we discuss several parameters and their influences on the SNR of the spectrum, including spectrum accumulation time, measurement efficiency and acquisition resolution. We find that the accumulation time is the most important factor for achieving high-quality spectrum. Measurement efficiency indicates how a good quality of the spin noise spectroscopy can be achieved in a finite time period, and we make a comparison of measurement efficiency among three methods. However, improvement of acquisition resolution does not make much more contribution to the quality of spin noise spectroscopy. Taken all into account, the DAC with real-time FFT performs best due to its bigger data utilization ratio, higher measurement efficiency and the multiplex advantage, thus it is more helpful for spin noise spectroscopy measurement in the study of spin dynamics.