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National Conference on Laser Technology and Optoelectronics (LTO)

Determination of the 87Rb atomic number density in thin rubidium vapors

He Xu

Published May 17, 2019• 0 citations
Publisher

Abstract

In recent years, the research in the fields of quantum computing, quantum communication and quantum precision measurement has set off the "second quantum revolution" and become an international hotspot. Background cold atomic vapor density is an important parameter in the vacuum systems. At present, the methods of measuring atomic vapor density include absorption line method and fluorescence method. However, for the negative exponential relation between the atomic vapor density and the measurement results of the existing methods, it is impossible to accurately measure the lowdensity atomic vapor. Therefore, based on the principle of magneto-optical Effect and coherence detection, we propose a method to measure the atomic vapor density of rarefied alkali metals. Taking 87Rb atom as an example, under different temperature and pressure conditions, the final measured atomic vapor density range is 1.47 x 1012~5.66 × 1013 m-3. The accuracy of this method is about 3~4 orders of magnitude higher than the existing method. The method effectively solves the measurement problem of low-density atomic vapor.

DOI
10.1117/12.2532048

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In recent years, the research in the fields of quantum computing, quantum communication and quantum precision measurement has set off the "second quantum revolution" and become an international hotspot. Background cold atomic vapor density is an important parameter in the vacuum systems. At present, the methods of measuring atomic vapor density include absorption line method and fluorescence method. However, for the negative exponential relation between the atomic vapor density and the measurement results of the existing methods, it is impossible to accurately measure the lowdensity atomic vapor. Therefore, based on the principle of magneto-optical Effect and coherence detection, we propose a method to measure the atomic vapor density of rarefied alkali metals. Taking 87Rb atom as an example, under different temperature and pressure conditions, the final measured atomic vapor density range is 1.47 x 10125.66 × 1013 m-3. The accuracy of this method is about 34 orders of magnitude higher than the existing method. The method effectively solves the measurement problem of low-density atomic vapor.
The temperature of the alkali metal cell, which affects the density of the alkali-metal vapor and the gas pressure, is usually difficult to measure directly. However, the temperature of the alkali-metal cell and the density of the alkali-metal vapor are important parameters that affect the performance of the atomic sensor. In this paper, a device that can directly measure the internal temperature of an alkali metal cell in real time is designed for the first time to explore the relationship between alkali-metal vapor density and temperature. Alkali-metal vapor density is measured using the absorption spectrum. The pressure broadening model, combined with the transition of four hyperfine levels, was used to fit the absorption line of 87Rb D1 under the action of 700 Torr N2, and a good fitting effect was obtained. The experimental results show that the density of 87Rb is less than the value calculated by the empirical formula. Based on the experimental results, we give the calculation formula of 87Rb density with an uncertainty of only 4% and obtain the temperature dependence index of the line width and linear displacement of 87Rb in N2 by analyzing the absorption spectrum.
Abstract An efficient method is proposed for quantifying rubidium vapor density by analyzing the spin‐exchange rate between alkali metals. This method effectively addresses two primary challenges in measuring alkali vapor density. First, it overcomes the limitation of absorption spectroscopy to measure vapor density under optically thick conditions, typically restricted to temperature higher than 420K, equivalent to alkali vapor density exceeding . Second, it mitigates the risk of disrupting shielding functionality due to the application of a strong magnetic field, often in the range of several tens of Gauss or higher, when employing the Faraday rotation for vapor‐density measurement. The spin‐exchange rate between atoms, inherently related to the vapor density, is evident in the electron‐paramagnetic‐resonance spectrum of spin‐polarized , thereby providing a possibility for measuring alkali vapor density. To eliminate overlap between two Lorentzian curves resulting from two decomposed components of an oscillating field, a small rotating magnetic field with identical amplitude and frequency but a 90‐degree phase shift along both the x and y axes is applied. This method is successfully employed to measure vapor density in the temperature range of 373–433 K, with the measured outcomes closely matching the saturation vapor curve.
Atomic density is a basic and important parameter in quantum optics, nonlinear optics, and precision measurement. In the past few decades, several methods have been used to measure atomic density, such as thermionic effect, optical absorption, and resonance fluorescence. The main error of these experiments stemmed from depopulation of the energy level, self-absorption, and the broad bandwidth of the laser. Here we demonstrate the atomic density of 87 Rb vapor in paraffin coated cell between 297 K and 334 K mainly using fluorescence measurement. Optical pumping, anti-relaxation coating, and absorption compensation approaches are used to decrease measurement error. These measurement methods are suitable for vapor temperature at dozens of degrees. The fitting function for the experimental data of 87 Rb atomic density is given.
Micro-electro-mechanical systems (MEMS)-based (chip-scale) alkali vapor cells are key components in emerging quantum technologies, where device performance critically depends on the atomic number density. Thus, it is important to have an accurate estimate of the atomic number density in MEMS-based alkali vapor cells to optimize light-matter interactions and design efficient quantum sensing systems. Here, a quantitatively validated method is presented for determining the rubidium (Rb) atomic number density in warm vapor using Single-Pass Absorption Spectroscopy (SPAS). The absolute transmission spectra are measured and modeled using the 780.24~nm and 420.29~nm transitions in Rb-filled MEMS vapor. The theoretical model employs a density-matrix formalism within the Lindblad framework and incorporates directly measurable experimental parameters, such as laser beam power, diameter, and cell temperature. The model explicitly accounts for optical pumping, Doppler broadening, and transit-time broadening effects and exhibits quantitative agreement (>99%> 99\%>99%) with experimental spectra over a broad range of temperatures (293-353~K), laser probe powers of approximately 10~μμμW-100~μμμW at the 780.24~nm transition and 8~μμμW-80~μμμW at the 420.29~nm transition, and cell lengths (2--100~mm). This method demonstrates a practical and reliable approach for determining the density of alkali vapor cells for quantum sensing, metrology, and quantum communication applications.
High sensitivity measurements of inertia and magnetic field could be achieved by utilizing a category of devices, which manipulate the atomic spins in the spin-exchange-relaxation-free regime. The alkali cell which contains the alkali metal vapor is used to sense magnetic field and inertia. The atomic number density of alkali vapor and the polarization of alkali metal vapor are two of the most important parameters of the cell. They play an important role in the research on atomic spins in the spin-exchange-relaxation-free regime. Besides, optical polarization plays an important role in quantum computing and atomic physics. We propose a measurement of alkali vapor polarization and alkali number density by detecting the optical rotation in one system. This method simplifies existing experimental equipment and processes. A constant bias magnetic field is applied and the Faraday rotation angle is detected by a bunch of the probe beam to deduce alkali-metal density. Then the magnetic field is closed and a bunch of the pump laser is utilized to polarize alkali-metal. Again, the probe beam is utilized to obtain the polarization of alkali metal. The alkali density obtained at first is used to deduce the polarization. This paper applies a numerical method to analyze the Faraday rotation and the polarization rotation. According to the numerical method, the optimal wavelength for the experiment is given. Finally, the fluctuation of magnetic field and wavelength on signal analysis are analyzed.
Abstract The actual vapor density characterizing the alkali metal spin-exchange rate remains a compelling issue for spin-exchange optical pumping. Based on the deduced relationship between the Faraday rotation angle and the rubidium vapor number density using the electrodynamics theory, we report a measurement of the number density for rubidium vapor sealed inside a cell based on a Faraday modulator. The measurement relies on the optical rotation angle due to rubidium vapor under a bias magnetic field (∼0.08 T) produced by a samarium–cobalt magnet. A Faraday modulator with a lock-in amplifier is used to accurately measure the tiny optical rotation angle in a temperature range of 387–468 K. In addition, a synchronization verification is performed by the photoelastic modulator (PEM). The recurring data showed that the two methods are consistent with each other. Compared with the PEM method, the Faraday modulator detection system does not need to adjust the optical axis difference of 45° in the PEM detection system, thereby reducing the complexity of the experiment and the error caused by the alignment of the optical axis, which showed that the Faraday modulator detection method more advantageous in measuring the alkali metal density.