Polarized atoms in a far-off-resonance yttrium-aluminum-garnet-laser optical dipole trap

Optical trapping of radioactive atoms has a great potential in precision measurements for testing fundamental physics such as electric-dipole moment, atomic parity nonconservation, and parity-violating $\beta$-decay correlation coefficients. One challenge that remains is to polarize the atoms to a high degree, and to measure the polarization of the sample and its evolution over time. In this paper we report on the polarization study of Rb atoms in a yttrium-aluminum-garnet (YAG)-laser optical dipole trap using both Faraday rotation polarimetry and resolved Zeeman spectroscopy techniques. We have prepared a cold cloud of polarized atoms and observed that its spin relaxation due to light scattering is suppressed in the YAG dipole trap. The spin polarization is further purified and maintained when the two-body collision loss rate between atoms in mixed spin states is greater than the one-body trap loss. These advancements are an important step toward a new generation of precision measurement with polarized trapped atoms.

Figure 1

(Color) The schematic of the experiment and ${}^{85}\text{R}\text{b}$ energy levels. (a) A layout of the setup to measure the FR signal of trapped atoms. Atoms are trapped in a focused YAG laser and polarized along the $Z$ direction by OP and RP lasers. The average FORT beam intensity is $2.7\times {10}^5\text{W}/{\text{cm}}^2$ and the calculated trap depth is $400\mu \text{K}$. The magnification of the imaging system is two. (b) Relevant energy levels of ${}^{85}\text{R}\text{b}$ atoms. OP and RP lasers are ${\sigma }^{+}$ polarized light on resonance with $F=3\to F^{\prime }=3$ and $F=2\to F^{\prime }=3$ transitions, respectively. FR probe laser is linear polarized light 900 MHz blue detuned from the $F=3\to F^{\prime }=4$ transition with a power of 1.4 mW. (c) The illustration of the Faraday rotation. Atoms are spin polarized along the $Z$ direction as defined by the optical pumping bias magnetic field $\left(B_{\text{op}}\sim 2\text{G}\right)$. When the field is suddenly turned off, the atomic spin precesses around the residual magnetic field $B_{\text{FR}}$ at the Larmor frequency ${\omega }_L=\frac{g_F{\mu }_{B}B}{\hslash }$, where ${\mu }_B$ is the Bohr magneton.

Figure 2

A single shot Faraday rotation signal of $3\times {10}^4$ polarized atoms $\left(F=3,m_F=3\right)$ in the dipole trap. YAG laser power of 2 W corresponds to $\sim 400\mu \text{K}$ trap depth. Probe beam is 1.4 mW collimated beam with 1.5 mm beam waist and 900 MHz blue detuned from the $F=3\to F^{\prime }=4$ transition. The Faraday oscillation frequency is 9.3 kHz which corresponds to $\sim 20\text{mG}$ residual $B$ field. The FORT beam remains on throughout the experiment. The decay of the Faraday signal is mainly due to the residual magnetic field gradient over the trapped atoms. The background noise with faraday probe beam on but no trapped atoms is $16\text{pW}/\sqrt{\text{Hz}}$.

Figure 3

(Color) The lifetimes of ${}^{85}\text{R}\text{b}$ atoms in different spin states trapped in the FORT. The trap loss of atoms in the lower hyperfine states (red plus sign) is dominated by the one-body decay with a lifetime of $\sim 38\text{s}$ (red line fit), in which the slight two-body effect observed at earlier times is likely due to the evaporation from the dipole trap. Trap loss for unpolarized atoms (the atoms are illuminated by repumping light for $100\mu \text{s}$ after being loaded into the dipole trap) is dominated by a two-body process (green triangle and green line fit). We fit both of these data with one- and two-body decays to get the trap lifetime and the coefficients for the two-body decay. For atoms polarized in the upper hyperfine stretched states with optical pumping, the trap loss shows fast two-body decay initially then evolves to the one-body decay (blue circle). The solid blue line is calculated with no free parameter, and uses the measured initial signal, lifetime, two-body decay coefficients, and an initial atomic polarization of $\sim 90\%$.

Figure 4

The polarization evolution of ${}^{85}\text{R}\text{b}$ atoms in the FORT. The $y$ axis is the normalized signal of Faraday rotation signal magnitude/retrap signal. Polarization rises initially and then levels off to stay in nearly fully polarized level. The error bar for the data at 2 and 7 s are estimated while all other data are limited by statistics. The inset shows the calculated 10 s evolution of samples with 95%, 90%, and 80% initial polarizations.