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Chinese Physics B

Spin-exchange relaxation of naturally abundant Rb in a K–Rb–21Ne self-compensated atomic comagnetometer*

Yan Wu Lu, Yueyang Zhai, Yong Zhang, Wenfeng Fan, Xing Li, Wei Quan

Published Feb 13, 2020• 12 citations
Publisher

Abstract

The total effective spin-exchange relaxation of naturally abundant Rb in a K–Rb–21Ne comagnetometer is analyzed, and the results show that the coexistence of 87Rb and 85Rb isotopes in the same volume can lead to a large extra spin-exchange broadening compared to pure 87Rb. This broadening mainly comes from the contribution of the equivalent reduction in the Rb spin-exchange rate. On this basis, an approximate relaxation model is proposed and experimentally demonstrated to be more accurate than that from a previous work. This study also provides a method for determining the properties of alkali-metal vapor cells.

DOI
10.1088/1674-1056/ab75d3

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The total effective spin-exchange relaxation of naturally abundant Rb in a K–Rb–21Ne comagnetometer is analyzed, and the results show that the coexistence of 87Rb and 85Rb isotopes in the same volume can lead to a large extra spin-exchange broadening compared to pure 87Rb. This broadening mainly comes from the contribution of the equivalent reduction in the Rb spin-exchange rate. On this basis, an approximate relaxation model is proposed and experimentally demonstrated to be more accurate than that from a previous work. This study also provides a method for determining the properties of alkali-metal vapor cells.
The elimination of relaxation resulting from spin-exchange (SE) interaction is crucial for ultrasensitive atomic comagnetometers. In this study, we demonstrate the SE relaxation is only partially suppressed and significantly broadens the magnetic linewidth in the K-Rb-21Ne comagnetometer. The SE relaxation arises from the compensation magnetic field when operating in the self-compensation regime. We propose a new method to measure the SE relaxation in the self-compensation regime where the alkali-metal and noble-gas spin ensembles are coupled. In the presence of SE relaxation, we find the optimal alkali-metal polarization for maximizing the sensitivity is shifted from the typical value. Under various conditions, we present a detailed study of the SE relaxation and the scale factor as a function of alkali-metal polarization, which are further verified by the theoretical models. The reduction of SE relaxation and improvement of scale factor by using 87Rb atoms is also studied.
Alkali metal-noble gas comagnetometers operated in the regime of spin-exchange relaxation-free (SERF) have been widely used in fundamental physics tests and rotation sensing as gyroscope. Spin-exchange relaxation is an important systematic bias of the comagnetometer, which could limit sensitivity significantly. However, the suppression effect of the spin-exchange relaxation has not been examined quantitatively so far. Herein, we systematically investigate the relaxation of the alkali metal atoms and find that the spin-exchange relaxation is not completely eliminated under the normal operating condition of the K-Rb-21Ne comagnetometer. In addition to the optical pumping rate, the spin-exchange relaxation still plays a dominant role in the broadening of electron Zeeman resonances due to the large electron magnetic field. This is the first demonstration that the alkali metal atoms are operated in the near-SERF regime in the alkali metal-noble gas comagnetometer with a large election magnetic field, which will shed light on the way to reduce the relaxation of the alkali metal atoms and enhance the sensitivity of the comagnetometer.
Due to high sensitivity and compactness, spin-exchange relaxation-free (SERF) atomic magnetometers have found widespread applications. These devices rely on alkali atoms to sense magnetic fields, with87Rb being the commonly used isotope. However,87Rb must be extracted from natural abundance rubidium (natRb) through isotopic separation, making it expensive. Replacing87Rb with natRb reduces the cost to approximately just 1%. Nevertheless, it is generally believed that the isotopic mixture of87Rb and85Rb in natRb affects the coherence of the atomic ensemble. Using a dual-beam SERF magnetometer, we systematically compared 3 mm inner-size cubic vapor cells filled with natRb and87Rb. By optimizing the buffer gas pressure and optical parameters, we demonstrate that the natRb magnetometer can achieve performance comparable to that of the87Rb magnetometer. Experimental results show that both cells achieved a sensitivity of 4 1T/Hz1/2. These findings demonstrate that natRb can effectively replace87Rb in miniaturized dual-beam SERF magnetometers, enabling low-cost, high-sensitivity magnetic field measurements.
Spin manipulation and detection of alkali-metal and noble-gas ensembles in spin-exchange-relaxation-free (SERF) comagnetometers are widely used in fundamental physical research, and have potential for inertial navigation. The dynamic behavior of spin ensembles is a vital characteristic that determines the applicability of comagnetometers, which significantly differs from various combinations of alkali-metal and noble-gas species. We comprehensively study the critical dynamics of strongly interacting ensembles in SERF comagnetometers with different species combinations. Different from the self-compensation point, a model of strong coupling point at which the dynamics is nonlinear because two ensembles influence each other strongly is established. We verify it by comparing the transient and steady-state dynamics of K-Rb-21Ne{}^{21}\mathrm{Ne}21Ne, K-Rb-3He{}^{3}\mathrm{He}3He, and K-3He{}^{3}\mathrm{He}3He comagnetometers with various input signals. The K-Rb-21Ne{}^{21}\mathrm{Ne}21Ne comagnetometer has a better potential for bandwidth and sensitivity in inertial measurement. Besides, we propose a critical point where the response rate of coupling spin ensembles is optimal among all polarization, and the critical dynamics is verified experimentally to be in agreement with theoretical simulations. The damping rate and precession frequency at the critical point can be more than 5 times larger than that at typical optimal spin polarization. Enhancement of spin-ensemble coupling strength improves the response rate of SERF comagnetometers and paves the way for efficient manipulation of noble-gas atoms via alkali-metal atoms in quantum information.
In this paper we calculate the effect of spin-exchange collisions in alkali-metal vapors. In the framework of the high-energy approximation, we evaluate the spin-exchange cross sections related to the line broadening and to the frequency shift of the ground state hyperfine transition. We do the calculation for the four isotopes, 23Na^{23}\mathrm{Na}23Na, 39K^{39}\mathrm{K}39K, 87Rb^{87}\mathrm{Rb}87Rb, and 133Cs^{133}\mathrm{Cs}133Cs. The results are used in particular to evaluate the spin-exchange frequency shift in Rb vapor cell frequency standards used in many applications. It turns out that, due to possible fluctuations in the atomic density, spin exchange may affect significantly the medium and long term frequency stability of the frequency standard.
Hyperpolarized K-3He^{3}\mathrm{He}3He comagnetometers have been used for tests of fundamental physics and inertial rotation sensing. These experiments will demonstrate increased sensitivity if the 3He^{3}\mathrm{He}3He buffer gas is replaced with 21Ne^{21}\mathrm{Ne}21Ne. We measure the parameters necessary to realize implementation of an alkali-metal--21Ne^{21}\mathrm{Ne}21Ne comagnetometer for two promising alkali-metal species, Rb and K. The spin-exchange rate constant for K with 21Ne^{21}\mathrm{Ne}21Ne and Rb with 21Ne^{21}\mathrm{Ne}21Ne are measured using two different methods. We also make the first measurement of the K-21Ne^{21}\mathrm{Ne}21Ne contact spin interaction. Measurements for the Rb-21Ne^{21}\mathrm{Ne}21Ne contact spin interaction, as well as the spin-destruction coefficients for 21Ne^{21}\mathrm{Ne}21Ne with both alkali-metal species are consistent with previous measurements. Additionally the quadrupolar spin-relaxation rate of 21Ne^{21}\mathrm{Ne}21Ne is measured. Finally, the feasibility of utilizing polarized 21Ne^{21}\mathrm{Ne}21Ne for operation in a comagnetometer is discussed.