Continuous Guided Strontium Beam with High Phase-Space Density

A continuous guided atomic beam of ${}^{88}\mathrm{Sr}$ with a phase-space density exceeding ${10}^{-4}$ in the moving frame and a flux of $3\times {10}^7\mathrm{ at. }{\mathrm{s}}^{-1}$ is demonstrated. This phase-space density is around 3 orders of magnitude higher than previously reported for steady-state atomic beams. We detail the architecture necessary to produce this ultracold-atom source and characterize its output after propagation for approximately $4\mathrm{cm}$. With radial temperatures of less than $1\mu \mathrm{K}$ and a velocity of $8.4\mathrm{cm}{\mathrm{s}}^{-1}$, this source is ideal for a range of applications. For example, it could be used to replenish the gain medium of an active optical superradiant clock or be used to overcome the Dick effect, which can limit the performance of pulsed-mode atom interferometers, atomic clocks, and ultracold-atom-based sensors in general. Finally, this result represents a significant step toward the development of a steady-state atom laser.

Figure 1

Production of a high-PSD atomic beam. An oven emits strontium atoms that are transversely cooled and slowed by a Zeeman slower, the output from which is caught by a two-dimensional (2D) MOT, all operating on the ${}^1{S}_0\text{−}{}^1{P}_1$ transition (beams in blue). The ${}^1{S}_0\text{−}{}^3{P}_1$ transition (beams in red) is then used to radially cool the atoms by an optical molasses, before they fall under gravity to a baffled second chamber. Here atoms are recaptured in a steady-state five-beam MOT, again operated on the ${}^1{S}_0\text{−}{}^3{P}_1$ transition. The MOT is overlapped with an optical guide, into which atoms are launched with a mean velocity of approximately $10\mathrm{cm}{\mathrm{s}}^{-1}$. This guided beam is radially cooled along the way by three low-intensity molasses beams. The ROI where we characterize the atomic beam is located $37\mathrm{mm}$ from the MOT.

Figure 2

MOT-beam shadow-casting structure. To protect the guided atomic beam from the copropagating MOT light, two cylinders are imaged at the guide location, one for each MOT beam along the $z$ axis. This structure consists of a capillary suspended with the use of three threaded 50-$\mu \mathrm{m}$-diameter wires. The picture shows the MOT-light attenuation due to one cylinder, imaged at the plane corresponding to one end of a single cylinder. A dark cylinder achieves an attenuation of approximately $30–40$ throughout its imaged length. The lines toward the edges are from the imaged suspension wires.

Figure 3

Axial velocity and velocity spread of the atomic beam. (a) Steady-state atomic beam after the application of a 1-ms pulse of light resonant with the ${}^1{S}_0\text{−}{}^1{P}_1$ transition. The ejected wavepacket is in free flight along $z$ for time $t$, after which an absorption image is taken. (b) Center-of-mass position and (c) width $\mathrm{\Delta }z$ of the ejected wavepacket for three different intensities of the launch beam (see Sec. 2). The lines are linear fits, from which we extract (b) the axial velocity ${\overline{v}}_z$ and (c) the velocity spread $\mathrm{\Delta }{v}_z$. Error bars on data points in (b),(c) show the standard deviation from Gaussian fits of the ejected wavepacket.

Figure 4

Effect of transverse cooling on the beam radial velocity spread. (a) Absorption images of the steady-state atomic beam, taken after switching the transport guide and laser cooling beams off and letting the cloud expand for various times of flight ${\tau }_{\mathrm{TOF}}$. The beam is shown both without (left) and with (right) transverse cooling applied (see Sec. 2). For clarity, we set the maximum optical density to 0.5 on the color scale, which means that some pictures are saturated. (b) Expansion of the beam size ${\mathrm{\Delta }}_r$ in free flight, from which we extract the radial velocity spread. The lines are linear fits, giving $\mathrm{\Delta }{v}_r=1.36(2)\mathrm{cm}{\mathrm{s}}^{-1}$ without transverse cooling and $\mathrm{\Delta }{v}_r=0.92(2)\mathrm{cm}{\mathrm{s}}^{-1}$ with transverse cooling. This corresponds to radial temperature $T_r=1.93(5)\mu \mathrm{K}$ and $T_r=0.89(4)\mu \mathrm{K}$, respectively. The small error bars in (b) give the standard deviation from Gaussian fits of density profiles integrated along the whole ROI.