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High-flux source of laser-cooled rubidium atoms for quantum technology applications

Christopher J. Foot, Sean Ravenhall

Published Oct 5, 2020• 0 citations
Publisher

Abstract

We have used a pyramid magneto-optical trap (MOT) to create a compact source of cold rubidium atoms. The four dielectric-coated metal mirrors in vacuum are arranged in the form of a pyramid that creates the light field for a MOT from a single ingoing laser beam. The atoms are pushed out through a hole (aperture) in the apex of the pyramid, in a direction determined by the laser light, to give a flux of Rb-87 atoms greater than 2 × 10^10 per second with a mean velocity of 33 m/s (FWHM of 28 m/s) and a divergence of 58 mrad. The pyramid MOT has a unique mechanism for adjusting aperture size and it is highly suitable for portable quantum technology devices.

DOI
10.1117/12.2584620

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We have used a pyramid magneto-optical trap (MOT) to create a compact source of cold rubidium atoms. The four dielectric-coated metal mirrors in vacuum are arranged in the form of a pyramid that creates the light field for a MOT from a single ingoing laser beam. The atoms are pushed out through a hole (aperture) in the apex of the pyramid, in a direction determined by the laser light, to give a flux of Rb-87 atoms greater than 2 × 10^10 per second with a mean velocity of 33 m/s (FWHM of 28 m/s) and a divergence of 58 mrad. The pyramid MOT has a unique mechanism for adjusting aperture size and it is highly suitable for portable quantum technology devices.
Magneto-optical traps (MOTs) are widely used for laser cooling of atoms. We have developed a high-flux compact cold-atom source based on a pyramid MOT with a unique adjustable aperture that is highly suitable for portable quantum technology devices, including space-based experiments. The adjustability enabled an investigation into the previously unexplored impact of aperture size on the atomic flux, and optimisation of the aperture size allowed us to demonstrate a higher flux than any reported cold-atom sources that use a pyramid, LVIS, 3D-MOT or grating MOT. We achieved 2.1(1) × 1010 atoms/s of 87Rb with a mean velocity of 32(1) m/s, FWHM of 27.6(9) m/s and divergence of 59(4) mrad. Halving the total optical power to 195 mW caused only a 20% reduction of the flux, and a 30% decrease in mean velocity. Methods to further decrease the velocity as required have been identified. The low power consumption and small size make this design suitable for a wide range of cold-atom technologies.
The ongoing drive to transfer quantum technologies from research laboratories to practical, portable devices requires the development of multiple key enabling technologies. The field of cold atoms, which includes high-precision metrology and quantum simulation, has various real-world applications including time-keeping and navigation. A crucial component in these systems is a cold-atom source that generates a flux of low-velocity atoms suitable for subsequent capture. A large flux enables fast repetition rates, high duty cycles, long measurement times, and resistance to long-term performance degradation with a good signal-to-noise ratio, all of which are essential to produce cold-atom devices and technologies that are capable of out-performing the classical alternatives. This thesis presents the design, fabrication and characterisation of a novel cold-atom source using a compact pyramidal magneto-optical trap (MOT) with a unique adjustable-aperture mechanism. This allowed an investigation of the dependence of the flux characteristics on the aperture size, a parameter space that has not previously been explored. The measurements showed a strong dependence of the flux on the aperture size, with a negligible flux below 0.4mm, then an approximately linear increase up to 1.0mm, at which point the relationship showed possible signs of saturation. Using 87Rb, a flux of 1.18×1010 atoms/s was produced using a 1.27mm aperture, for which the mean velocity of the atoms was 21.6(3) m/s with a FWHM of 11.2(2) m/s, and the FWHM divergence of the beam was 86(2) mrad. This is the highest recorded flux from a pyramid or 2D+-MOT source, and amongst the highest reported fluxes from any MOT sources, and avenues for improvement still remain. This work fulfils the aim of demonstrating a compact and robust cold-atom source generating a high flux of laser-cooled atoms for real-world quantum technology applications.
Summary form only given. A simple and robust design has been implemented based on the pyramidal trap geometry which allows use of single large diameter (/spl les/20 cm) laser beam to obtain large capture rates of atoms from the background vapor. The four 45/spl deg/ mirrors are truncated just before the apex of the pyramid, and the 1 cm/sup 2/ region at the center of the incident laser beam is retro-reflected by a /spl lambda//4 plate with a high-reflectance gold coating on the second surface. A small (1 mm diameter) hole in this retro-optic forms an extraction column for the atoms while maintaining a low conductance between the source region and an adjacent UHV chamber. The characterization of this large pyramidal low-velocity intense source (LVIS) is reported, including an investigation of scaling to very large (/spl ges/10 cm) high power (/spl sim/1 W) laser beams which should allow an improvement by a factor of 2-3 in beam flux over previous reports.
An intense cold-atom beam source based on a modified pyramidal magneto-optical trap has been developed and characterized. We have produced a slow beam of cold cesium atoms with a continuous flux of 2.2× 109 atoms/s at a mean velocity of 15 m/s and with a divergence of 15 mrad. The corresponding radiant intensity is 1.2×1013 atom s-1 sr-1. We have characterized the performance of our beam source over a range of operating conditions, and the measured values for atom flux, mean velocity, and divergence are in good agreement with results from detailed Monte Carlo numerical simulations.
A three-dimensional magneto-optical trap (MOT), acting as an efficient method for producing cold atoms from room-temperature atomic vapor, has been widely used to develop atomic sensors. Various compact MOTs using a single laser beam have been reported, simplifying apparatuses and leading to miniaturized devices. Here, we propose single-beam MOTs based on back-to-back pyramidal and conical mirrors. In such back-to-back mirrors, a MOT trapping volume is formed by an incident laser beam, a retroreflected beam, and multiple reflections from the mirror surfaces. We present the design of back-to-back mirrors and a series of compact MOT configurations, with the potential of increasing access to the MOT and simultaneously creating multiple MOTs. We demonstrate a MOT in a back-to-back conical mirror, loading \ensuremath{\sim}1 \ifmmode\times\else\texttimes\fi{} 107{10}^{7}107 87Rb{}^{87}\mathrm{Rb}87Rb atoms from background vapor and cooling the atoms to \ensuremath{\sim}7 \textmu{}K using polarization gradients. Single-beam MOTs based on back-to-back mirrors contribute to building compact and scalable cold-atom-based sensors.