Search for exotic spin-dependent interactions with a spin-exchange relaxation-free magnetometer

We propose a novel experimental approach to explore exotic spin-dependent interactions using a spin-exchange relaxation-free (SERF) magnetometer, the most sensitive noncryogenic magnetic-field sensor. This approach studies the interactions between optically polarized electron spins located inside a vapor cell of the SERF magnetometer and unpolarized or polarized particles of external solid-state objects. The coupling of spin-dependent interactions to the polarized electron spins of the magnetometer induces the tilt of the electron spins, which can be detected with high sensitivity by a probe laser beam similarly as an external magnetic field. We estimate that by moving unpolarized or polarized objects next to the SERF Rb vapor cell, the experimental limit to the spin-dependent interactions can be significantly improved over existing experiments, and new limits on the coupling strengths can be set in the interaction range below ${10}^{-2}\mathrm{m}$.

Figure 1

Schematic of the experimental setup. Almost parallel pump and probe beams in the $\widehat{z}$ axis are sent to a large Rb vapor cell. An unpolarized or polarized test mass is located next to the cell. The Rb electron spins in the cell and spins of the test mass are represented by ${\overrightarrow{\sigma }}_1$ and ${\overrightarrow{\sigma }}_2$, respectively. The test mass can move with a velocity $\overrightarrow{v}$ forward and backward along the $\widehat{z}$ axis, move right and left along the $\widehat{x}$ axis, or rotate clockwise and counterclockwise around the $\widehat{z}$ axis (along the $\varphi$ angle) according to the format of the exotic spin-dependent interactions [Eqs. (1)–(11)]. There are nine cases of the combination of ${\widehat{\sigma }}_1$, ${\widehat{\sigma }}_2$, and $\overrightarrow{v}$ as listed in Table 2.

Figure 2

The constraints of the coupling strength to the interactions, from top to bottom, $V_{4+5}$, $V_{9+10}$, $V_{12+13}$ [12], as a function of the interaction range (bottom axes) and the ALP mass (top axes). The dashed curves are the estimated sensitivity of the proposed experiment to the interactions between the polarized Rb electrons and the unpolarized BGO [29] test mass for the one second measurement period. The solid curves are current limits. The axion coupling strength and range [11, 13] are shown in $V_{9+10}$. The constraints from the stellar cooling for the axion together with short-range gravity experiments with unpolarized masses are also shown in $V_{4+5}$ and $V_{9+10}$ [52].

Figure 3

The constraints of the coupling strength to the interactions, from top to bottom, $V_2$, $V_3$, $V_{11}$ [12], as a function of the interaction range (bottom axes) and the ALP mass (top axes). The dashed curves are the estimated sensitivity of the proposed experiment to the interactions between the polarized Rb electrons and the polarized DyIG [13] test mass for the one second measurement period. The solid curves are current limits. The axion coupling strength and range [11, 13] are shown in $V_3$.

Figure 4

The constraints of the coupling strength to the interactions, $V_{6+7}$, $V_8$, $V_{14}$, $V_{15}$, and $V_{16}$ [12], as a function of the interaction range (bottom axes) and the ALP mass (top axes). The dashed curves are the estimated sensitivity of the proposed experiment to the interactions between the polarized Rb electrons and the polarized DyIG [13] test mass for the one second measurement period. The solid curves are current limits. The constraints from the dark photon [56] are also shown in $V_{15}$.