Sensitive determination of the spin polarization of optically pumped alkali-metal atoms using near-resonant light

Abstract

A new method to measure the spin polarization of optically pumped alkali-metal atoms is demonstrated. Unlike the conventional method using far-detuned probe light, the near-resonant light with two specific frequencies was chosen. Because the Faraday rotation angle of this approach can be two orders of magnitude greater than that with the conventional method, this approach is more sensitive to the spin polarization. Based on the results of the experimental scheme, the spin polarization measurements are found to be in good agreement with the theoretical predictions, thereby demonstrating the feasibility of this approach.Since the ingenious idea of optical pumping was proposed by Kastler in 1950 1 , it has played an important role in atomic physics 2-4 . Optical pumping is typically used to polarize alkali-metal atoms. Once an ensemble of alkali metal atoms is polarized, many perfect physical properties can be observed 2 . As a result, optically pumped alkali-metal vapor has been widely used in a variety of significant areas, such as atomic magnetometers 5-9 , Faraday filters 10-12 , atomic clocks 13 , quantum memory and teleportation 14,15 , and nuclear magnetic resonance 16,17 . The spin polarization, which reflects the spin coherence of an atomic ensemble, is a vital parameter of optically pumped alkali-metal atoms. For example, the spin polarization directly determines the performance of atomic magnetometers and has an optimal value for an atomic magnetometer 6,8 , and it is also helpful for people to research and design Faraday filters by obtaining accurate knowledge of the spin polarization 12 . Therefore, it is essential to measure the spin polarization of alkali-metal atoms accurately.The spin polarization is usually determined by Faraday rotation using far-detuned light 18,19 . In such a measurement, for the D 1 line transition of alkali-metal atoms, the Faraday rotation angle θ is given by 19where l is the length over which the probe light interacts with the alkali-metal vapor, e is the electron charge, N is the number density of the alkali-metal vapor, m e is the electron mass, c is the speed of light, δ is the probe detuning from the D 1 line transition, and P is the spin polarization of the alkali-metal atoms. A necessary condition of equation (1) is that δ is much greater than the hyperfine splitting 19 . In this case, θ is scarcely sensitive to P, unless N is large enough. For the vapor number density of 10 11 to 10 12 cm −3 , as θ is usually only several milliradians 18 , it is difficult to obtain accurate knowledge of the spin polarization using this method. However, in many practical applications, alkali-metal vapor operating at near room temperature, which corresponds to the number density close to 10 11 cm −3 , has great advantages and has been widely utilized. For example, room temperature operation can simplify the structure and can reduce the energy consumption for atomic magnetometers, as indicated by the many reported room temperature atomic magnetometers 8,20 . Therefore, a sensitive metho...

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