Continuous propagation and energy filtering of a cold atomic beam in a long high-gradient magnetic atom guide

We demonstrate the continuous injection and propagation of a cold atomic beam in a high-gradient (up to $2.7\mathrm{kG}∕\mathrm{cm}$) magnetic guide of $1.7\mathrm{m}$ length. Continuous injection is accomplished using a side-loading scheme that involves a sequence of two modified magneto-optic traps. Methods are developed to measure the atomic-flow temperatures and the flux under steady-state conditions. In the high-gradient portion of the guide, the guided atomic beam has a transverse temperature of $420\mu \mathrm{K}\pm 40\mu \mathrm{K}$, a longitudinal temperature of $1\mathrm{mK}$, and an average velocity of order $1\mathrm{m}∕\mathrm{s}$. Using a radio-frequency current of a fixed frequency $\nu$ coupled directly into the guide wires, atoms exceeding a transverse energy of $h\nu$ can be continuously and selectively removed from the atomic beam.

Figure 1

(Color online) Sketch of the experimental setup. Atoms are injected from a pyramidal magneto-optical trap (PMOT) via a “moving MOT” (MMOT) into the vertical injection and launch section of a magnetic atom guide. As the injected atomic flow rises upward, it slows under in the influence of gravity and becomes magnetically compressed. The flow then propagates into a long horizontal guide section. A radio-frequency current coupled into the horizontal guide section is used to remove the most energetic atoms from the guided atomic flow. The system has three differentially pumped vacuum regions, the pressure values of which are shown here. The inset on the right shows a cross section through the horizontal section of the guide. (A) Vacuum chamber, (B) mounting screws, (C) aluminum spacers, (D) insulating polyimide film, (E) guide wires, (F) coolant, (G) alumina spacers, (H) guide rail, and (I) lock screws.

Figure 2

(Color online) Primary pyramidal MOT (PMOT). The image shows the actual atom cloud and multiple reflections thereof in the pyramidal mirror.

Figure 3

(Color online) Setup of absorption measurement used in order to characterize the (unguided) primary atomic beam emitted by the PMOT. The current in the guide wires is turned off for this measurement.

Figure 4

(Color online) Contour plot of the relative absorption $-dP∕P$ due to the primary atomic beam as a function of vertical position $y$ and velocity $v_z$.

Figure 5

(Color online) Relative absorption $-dP∕P$ of a probe beam passed through the atomic beam exiting the MMOT vs the frequency detuning of the probe laser, ${\nu }_{\mathrm{L}}-{\nu }_0$. The red circles show measured data, while the green line shows a Lorentzian of $6\mathrm{MHz}$ FWHM that has the same area as the data curve.

Figure 6

(Color online) Fluorescence images of the MMOT (guide-axis vertical). The exposure times are chosen such that the images do not saturate. Left: launch velocity $v_0=0\mathrm{m}∕\mathrm{s}$. The fluorescence rapidly diminishes above the repumper knife edge. Middle: launch velocity $v_0=1.6\mathrm{m}∕\mathrm{s}$ and depumping beam blocked. Atoms are transported upward out of the range of the MMOT, leaving a thin trail of fluorescence along the guide axis above the repumper edge. Right: launch velocity $v_0=1.6\mathrm{m}∕\mathrm{s}$ and depumping beam active. The fluorescence abruptly diminishes above the depumper edge, as the depumper optically pumps all atoms into the dark state $F=1$. The dotted lines in the middle and right images outline a narrowing of the MMOT fluorescence as a function of height.

Figure 7

(Color online) Experimental setup to probe the atomic beam with a top-hat intensity profile. The output of a fiber containing both repumper and cycling-transition light is collimated and directed onto a small aperture of $<1\mathrm{mm}$ diameter. A long-focal-length lens images the top-hat intensity profile created at the aperture into the detection region. Fluorescence from the atomic sample is then observed using a one-to-one imaging scheme with a scientific charge-coupled-device (CCD) camera.

Figure 8

(Color online) Fluorescence images (insets) and profiles transverse to the guide axis (curves) of a continuous atomic beam in the magnetic guide. In the images, the guide axis is vertical. The image obtained with continuous illumination [curve and inset (a)] is distorted and broadened due to radiation pressure and magnetic forces acting while the atoms are being imaged. The strobed image [curve and inset (b)] is much narrower and exhibits a constant fluorescence intensity along the full length of the illumination region along the guide axis. These properties of the strobed image follow from the fact that the strobe intervals are sufficiently short that no significant atomic motion occurs while the atoms are being imaged.

Figure 9

(Color online) Atomic-beam image (inset) and corresponding profile transverse to the guide axis (data points). The image is obtained at a location just below the atom-guide knee with repumping light only. In the image, the guide axis is vertical. The line through the data points represents a fit for a temperature of $620\mu \mathrm{K}$.

Figure 10

(Color online) Top row: atomic-beam images with both repumper light and probe light present in the detection region for the indicated detunings of the probe laser relative to the field-free transition frequency. The images are obtained at a location just below the atom-guide knee and with a horizontal linear polarization of the probe laser. In the images, the guide axis is vertical. Lower panel: profiles of atomic-beam images transverse to the guide axis for the indicated detunings of the probe laser relative to the field-free transition frequency. By fitting the envelope of the profiles with curves of the type of Eq. (12), a transverse temperature of $T_{\perp }\approx 420\mu \mathrm{K}$ is obtained.

Figure 11

(Color online) Time-of-flight experiment in the vertical launch section. The time-integrated atomic-beam fluorescence (triangles) is shown vs the integration time for the indicated launch velocities. At time zero, the atomic flux out of the MMOT is suddenly increased by about a factor of 5. The time-of-flight distributions (curves peaking at about $100\mathrm{ms}$) are obtained by twice differentiating the smoothed curves laid through the data points.

Figure 12

(Color online) Atomic-beam image obtained at the end of the atom guide with repumping light only (inset) and corresponding profile transverse to the guide axis (data points). In the image, the guide axis is horizontal. The line through the data points represents a fit for a temperature of $420\mu \mathrm{K}$.

Figure 13

(Color online) Profiles of atomic-beam images transverse to the guide axis with both repumper light and probe light present in the detection region for the indicated detunings of the probe laser relative to the field-free transition frequency. The estimate $T_{\perp }\approx 350\mu \mathrm{K}$ is obtained by fitting the envelope of the profiles with curves of the type from Eq. (12).

Figure 14

Longitudinal velocity distribution obtained from a time-of-flight experiment in the horizontal section of the atom guide. The MMOT launch velocity is $v_0=2.2\mathrm{m}∕\mathrm{s}$. The velocity distribution yields a longitudinal temperature of $\approx 1\mathrm{mK}$ and an average forward velocity of $\approx 1.2\mathrm{m}∕\mathrm{s}$.

Figure 15

(Color online) Atomic-beam profiles transverse to the guide axis at the end of the atom guide, obtained with repumping light only and with a continuous radio-frequency (rf) current of $9\mathrm{MHz}$ frequency. The rf current is coupled onto a guide tube in the horizontal guide section. The estimated value of the rf-field magnitude, $B_{\mathrm{RF}}$ is indicated to the right of the plot. The experimental data agree reasonably well with calculations for a thermal distribution with $T_{\perp }=500\mu \mathrm{K}$ (red curve) and the same thermal distribution truncated at the “rf knife edge” (gray curve). Both theoretical curves are scaled with the same factor to match the experimental data.