Compact Continuous Cold Atomic Beam from a Single Cell with 3D Cooling and Ultra-low Light Shift

TL;DR

This work addresses the need for compact, continuous cold-atom beams with low decoherence and light shifts for clocks and interferometers. It introduces a single-cell source that combines a 2D MOT with an off-axis moving optical molasses, enabling simultaneous 3D cooling within a 50 mm region and a tunable mean beam velocity. The device delivers a high flux up to 4.9×10^9 atoms/s, with transverse and longitudinal temperatures of 94 μK and 231 μK, and a mean velocity tunable from 5 to 20 m/s; Raman–Ramsey interferometry demonstrates an ultra-low light shift of -0.51 Hz and a fringe contrast of 90.85% at 100 mm separation, indicating low decoherence. These features make the source a practical building block for field-deployable atomic clocks and interferometers with reduced aliasing noise and improved sensitivity.

Abstract

We report a compact single-cell source of a continuous cold-atom beam with three-dimensional (3D) cooling. By integrating an off-axis moving optical molasses (OM) with a two-dimensional magneto-optical trap (MOT), we achieve simultaneous 3D cooling within a 50 mm interaction region. The source delivers a continuous flux up to 4.9(5)x10^9 atoms/s, with a transverse temperature of 94(5) microK, a longitudinal temperature as low as 231(65) microK, and a tunable mean velocity between 5 and 20 m/s. Custom in-vacuum mirrors integrate the reflective geometry for the off-axis OM beams with a 0.8 mm output aperture, ensuring stable alignment while suppressing stray light and fluorescence leakage. Ultra-low light shift and decoherence are verified via continuous Raman-Ramsey interferometry, yielding a light shift of -0.51(4)Hz and a typical fringe contrast of 90.85(30)% at a Raman separation of 100 mm (interrogation time of 8.70 ms). This compact continuous cold-atom beam source constitutes a practical building block for atomic-beam clocks and interferometers, enabling reduced aliasing noise together with improved sensitivity and accuracy for field applications.

Figures (11)

• Figure 1: Principles of (a) a classical MOT-based single-cell beam source and (b) an off-axis moving-OM single-cell source. Forward OM (FOM) and backward OM (BOM) beams are oriented at angle $\theta$ relative to the extraction axis. $d_{\rm ec}$ is the diameter of the extraction column defined by the unidirectional component of the pushing laser beam; $d_{\rm a}$ is the diameter of the mechanical output aperture; $l_{\rm c}$ is the cooling length defined by the MOT laser beams; $l_{\rm OM}$ is the OM interaction length defined by the OM laser beams.
• Figure 2: Simulated longitudinal temperature of a 2D$^+$ MOT beam as a function of extraction column diameter $d_{\rm ec}$, defined by the unidirectional pushing laser beam. The blue dashed line indicates the simulated longitudinal temperature for the off-axis OM configuration.
• Figure 3: Effect of output aperture size $d_{\rm a}$. (a) Theoretical atomic flux and transverse temperature as functions of $d_{\mathrm{a}}$. Dots: numerical simulations. Lines: analytical estimates from Eq. \ref{['eq:alpha']}, assuming $l_{\rm c}=50~\mathrm{mm}$ and mean velocity $11.5~\mathrm{m/s}$. (b) Predicted near-resonant optical power leaked into the downstream cell from ray-tracing simulations, and the corresponding decoherence rate estimated from the scattering rate.
• Figure 4: Apparatus schematic. (a) Vacuum cell containing the 2D MOT and the off-axis moving OM. Directions of the laser beams are indicated by the arrows. Four mirrors define the optical paths for the forward OM (FOM) and backward OM (BOM) laser beams (blue). MOT laser beams (red) counterpropagate along $x$ and $y$, with the $x$ laser beams omitted for clarity. Both the cooling length $l_{\rm c}$ and OM interaction length $l_{\rm OM}$ are about $50~\mathrm{mm}$. The cell is illustrated as transparent for clarity. (b) Energy-level diagram and laser frequencies. $\Delta_{\rm MOT}$ and $\Delta_{\rm OM}$ denote MOT and OM detunings; $\delta_{\rm OM}$ is the OM frequency shift; $\delta_{12}$ is the two-photon detuning of Raman lasers. (c) Locations of laser beams used to characterize the cold-atom beam (not to scale).
• Figure 5: Longitudinal velocity distribution measured over a $294~\mathrm{mm}$ flight distance, for different saturation parameters $s = I/I_{\rm sat}$ of the transverse MOT lasers (where $I_{\rm sat} = 1.67 ~{\rm mW/cm^2}$). The distribution obtained without the BOM is also shown at $s = 12$.