Magneto-optical trapping of a group-III atom

We realize the first magneto-optical trap of an atom in main group III of the Periodic Table. Our atom of choice (indium) does not have a transition out of its ground state suitable for laser cooling; therefore, laser cooling is performed on the $|5P_{3/2},F=6\rangle \to |5D_{5/2},F=7\rangle$ transition, where $|5P_{3/2},F=6\rangle$ is a long-lived metastable state. Optimization of our trap parameters results in atoms numbers as large as $5\times {10}^8$ atoms with temperatures of order 1 mK. Additionally, through trap decay measurements, we infer a one-body trap lifetime of $12.3\mathrm{s}$. This lifetime is consistent with background gas collisions and indicates that our repumpers have closed all leakage pathways. We also infer a two-body loss rate of $1.6\times {10}^{-11}{\mathrm{cm}}^3/\mathrm{s}$, which is comparable to those measured in alkali atoms. The techniques demonstrated in this Letter can be straightforwardly applied to other group-III atoms, and our results pave the way for realizing quantum degenerate gases of these particles.

Figure 1

(a) Energy levels of ${}^{115}\mathrm{In}$ [24, 28, 29, 30, 31]. Relevant transition wavelengths and natural linewidths are denoted with $\lambda$ and $\gamma =\mathrm{\Gamma }/2\pi$, respectively. Laser cooling of indium is based on the cycling transition $|5P_{3/2},F=6\rangle \to |5D_{5/2},F=7\rangle$, where the lower-energy cooling state is long-lived ($\sim 10\text{−}\mathrm{s}$ lifetime) [32]. We drive the two $|5P_{1/2},F=4,5\rangle \to |6S_{1/2},F=5\rangle$ transitions at 410 nm and the two $|5P_{3/2},F=4,5\rangle \to |6S_{1/2},F=5\rangle$ transitions at 451 nm for initial-state preparation just after the atomic beam emerges from the effusion cell [25]. Lasers on these four transitions are also used for repumping during the Zeeman slowing and MOT stages. (b) The magneto-optical trap setup. Three pairs of orthogonal 326-nm cooling beams in ${\sigma }^{+}-{\sigma }^{-}$ configuration are sent through the center of the vacuum chamber, which overlaps with the center of the quadrupole magnetic field generated by a pair of coils in anti-Helmholtz configuration. Two 410-nm and two 451-nm repumper lasers are coaligned with the $z$ direction MOT beam. The MOT fluorescence signals are collected by an electron multiplying CCD (EMCCD) camera via a $2f$ imaging system.

Figure 2

Optimization of the MOT atom number. We vary the MOT detuning $\mathrm{\Delta }$, the MOT laser intensity (expressed in the figure as the saturation parameter $s_0=I/\mathcal{M}{I}_{\mathrm{sat}}$), and the magnetic-field gradient $\partial B/\partial z$ at the trap center projected along the $z$ axis. The measurement is taken by fixing the detuning and then varying $s_0$ and $\partial B/\partial z$ until the atom number maximizes. Each bar represents the best atom number observed for a given detuning. The trapped atom number fluctuations are estimated from the standard deviation of multiple atom number measurements for the same MOT parameters.

Figure 3

Atom number decay for optimal MOT parameters. The red solid curve is generated by fitting Eq. (4) to the data. The black dashed curve is the solution to Eq. (3) with $\beta$ taken to be zero and fit to the data for times larger than $15\mathrm{s}$. The $\beta =0$ case is a pure one-body model, which is expected to be valid at longer times after two-body processes have decayed. The discrepancy between the data and the black dashed line at short times illustrates the extent of two-body effects. The inset: Observed MOT loading curve and a false color MOT fluorescence image. The fitted loading rate for these data is $1.30\left(2\right)\times {10}^8\mathrm{atoms}/\mathrm{s}$. A two-dimensional Gaussian fit of the steady-state MOT fluorescence image yields root-mean-square radii ${\sigma }_x=1.0$ and ${\sigma }_z=1.3\mathrm{mm}$.

Figure 4

MOT lifetime as a function of repumper power. These data are collected under the optimal MOT parameters mentioned above. The $x$ axis is the combined power of both repumpers for a given wavelength. With the repumper lasers, the MOT lifetime extends up to two orders of magnitude. The inset: Trapped atom number as a function of repumper power under optimal MOT parameters. The trapped atom number increases up to two orders of magnitude with the application of repumpers.