Steady-State Magneto-Optical Trap with 100-Fold Improved Phase-Space Density

We demonstrate a continuously loaded ${}^{88}\mathrm{Sr}$ magneto-optical trap (MOT) with a steady-state phase-space density of $1.3(2)\times {10}^{-3}$. This is 2 orders of magnitude higher than reported in previous steady-state MOTs. Our approach is to flow atoms through a series of spatially separated laser cooling stages before capturing them in a MOT operated on the 7.4-kHz linewidth Sr intercombination line using a hybrid $\text{slower}+\mathrm{MOT}$ configuration. We also demonstrate producing a Bose-Einstein condensate at the MOT location, despite the presence of laser cooling light on resonance with the 30-MHz linewidth transition used to initially slow atoms in a separate chamber. Our steady-state high phase-space density MOT is an excellent starting point for a continuous atom laser and dead-time free atom interferometers or clocks.

Figure 1

(a) Schematic of our setup, showing the main cooling stages, and the position of the three red MOT configurations (see text). (b) Electronic level scheme of strontium, with the blue and red transitions used for laser cooling. (c) In situ absorption picture of a steady-state ${}^{88}\mathrm{Sr}$ MOT with a PSD of $1.3(2)\times {10}^{-3}$ [Red MOT II(b) configuration].

Figure 2

Working principle of the hybrid $\text{slower}+\mathrm{MOT}$ in Red MOT I configuration. (a) Energy diagram of the ${}^1{\mathit{S}}_0$, $m_J$, and the ${}^3{P}_1$, $m_{J^{\prime }}$ states in dependence of $y$ for $x=z=0$, where the coordinate origin lays in the quadrupole field center. An atom with velocity $v\ne 0$ along the $y$ axis is addressed by the upward-propagating Red MOT I beam in a region delimited by the pair of red dashed arrows on the right. The range of this region is determined by the span of the laser frequency modulation $\mathrm{\Delta }{\nu }_L$ and the magnetic field gradient. An atom with $v=0$ is addressed in the region delimited by the pair of dashed arrows on the left. (b) $y$ deceleration from scattering of photons on the red transition (in units of earth acceleration $g$) in dependence of the initial velocity of atoms 20 cm above the quadrupole field center, obtained by an idealized 1D calculation. A horizontal path across the diagram corresponds to a 1D time-dependent trajectory along $y$. Annotations indicate regions dominated by Zeeman type slowing or red MOT type trapping. Atoms eventually captured in the MOT exhibit a time-independent non-zero force (yellow region at $\sim 1g$). The white regions show decelerations smaller than $0.25g$. The “Bounce” and “Fall” regions are undesirable cases described in Ref. [45]. (c) Equivalent deceleration data obtained by a full 3D Monte Carlo calculation with a realistic geometry.