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arXiv:1301.4439 · Physical Review A

Near-resonant optical forces beyond the two-level approximation for a continuous source of spin-polarized cold atoms

Thomas Vanderbruggen, Morgan W. Mitchell

Published Mar 8, 2013• 6 citations
arXivHTMLPDF

Abstract

We propose a method to generate a source of spin-polarized cold atoms which are continuously extracted and guided from a magneto-optical trap using an atom-diode effect. We show that it is possible to create a pipe-like potential by overlapping two optical beams coupled with the two transitions of a three-level system in a ladder configuration. With alkali-metal atoms, and in particular with ${}^{87}$Rb, a proper choice of transitions enables both the potential generation and optical pumping, thus polarizing the sample in a given Zeeman state. We extend the Dalibard and Cohen-Tannoudji dressed-atom model of radiative forces to the case of a three-level system. We derive expressions for the average force and the different sources of momentum diffusion in the resonant, nonperturbative regime. We show using numerical simulations that a significant fraction of the atoms initially loaded can be guided over several centimeters with output velocities of a few meters per second. This would produce a collimated continuous source of slow spin-polarized atoms suitable for atom interferometry.

DOI
10.1103/physreva.87.033410

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We propose a method to generate a source of spin-polarized cold atoms which are continuously extracted and guided from a magneto-optical trap using an atom-diode effect. We show that it is possible to create a pipe-like potential by overlapping two optical beams coupled with the two transitions of a three-level system in a ladder configuration. With alkali-metal atoms, and in particular with 87{}^{87}87Rb, a proper choice of transitions enables both the potential generation and optical pumping, thus polarizing the sample in a given Zeeman state. We extend the Dalibard and Cohen-Tannoudji dressed-atom model of radiative forces to the case of a three-level system. We derive expressions for the average force and the different sources of momentum diffusion in the resonant, nonperturbative regime. We show using numerical simulations that a significant fraction of the atoms initially loaded can be guided over several centimeters with output velocities of a few meters per second. This would produce a collimated continuous source of slow spin-polarized atoms suitable for atom interferometry.
Summary form only given. We propose a new method to produce a continuous source of spin-polarized cold atoms which are all-optically guided after their extraction from a magneto-optical trap (MOT), as depicted in Fig. 1 (a). The techniques combines several physical effects and relies on light-shift engineering [Fig. 1 (b)], implemented using two coaxially overlapped optical beams each one driving a given transition of a three-level atom in a ladder configuration (Ξ-system). In a well chosen configuration, the light-shift creates a state-dependent potential which implements the atom-diode [1] responsible for the continuous extraction of the atoms from a MOT into an all-optical guide.We performed a theoretical study by extending the Dalibard and Cohen-Tannoudji dressed-atom model [2] to the case of a doubly-driven Ξ-system. We analysed and quantified the dipole forces and the various sources of momentum diffusion in the resonant, non-perturbative, regime. In particular, from the Markovian evolution of the internal state, we obtain a general formula for the diffusion coefficient associated with the dipole force fluctuation. We proposed and studied in detail the implementation of the method for 87Rb atoms. We show that, using σ+polarized fields driving the transitions 52S1/2 → 52P3/2 and 52P3/2 → 42D5/2 at 780 nm and 1529 nm respectively, a closed Ξ-system can be isolated within the complicated structure of 87Rb. Moreover, with this choice of transitions, the atoms will be optically pumped in a given Zeeman sub-state thus polarizing the atomic sample. The theoretical results have then been used in a numerical simulation of Langevin-like equations [Fig. 1 (c)] to estimate the performance of the system. From the simulation of 10000 trajectories, we show that a large fraction of the atoms (~ 64%) is guided over at least 5 cm, the mean velocity at this distance being 5.6 m/s with a dispersion of 2.2 m/s. This guided distance allows to deliver the atoms inside a magnetic shield that cancels the continuously operated MOT magnetic field. The proposed method thus creates a continuous source of guided spin-polarized cold atoms. Such a source can be of particular interest for atom interferometry.
We present an intense source of 87 Rb atoms that has been set up to produce a continuous, slow and cold beam in a magnetic guide. It consists of a two-dimensional magneto-optical trap whose cooling laser power is provided by a master-oscillator tapered-amplifier system. This trap produces an atomic beam with a flux of over 10 10 atoms/s and a mean velocity of 40 m/s. The beam is recaptured by a second trap whose purpose consists in reducing the beam's velocity, in further cooling the atoms and injecting them into the magnetic guide. The article focusses on the first stage of the process described above.
We present a compact, two-stage atomic beam source that produces a continuous, narrow, collimated, and high-flux beam of rubidium atoms with sub-Doppler temperatures in three dimensions, which features very low emission of near-resonance fluorescence along the atomic trajectory. The atom-beam source originates in a pushed two-dimensional magneto-optical trap (2D+2{\mathrm{D}}^{+}2D+ MOT) feeding a slightly off-axis three-dimensional moving optical-molasses stage that continuously cools and redirects the atom beam. The capture velocity of the moving optical molasses is deliberately chosen to be low, approximately 3m/s3\phantom{\rule{0.2em}{0ex}}\mathrm{m}/\mathrm{s}3m/s, to reduce fluorescence, and the cooling light is detuned by several atomic linewidths from resonance to reduce the absorption cross section of cooling-induced fluorescence. Near-resonance light from the 2D+2{\mathrm{D}}^{+}2D+ MOT and the push beam does not propagate to the output atomic trajectory due to a ${10}^{\ensuremath{\circ}}$ bend in the atomic trajectory. The atomic beam emitted from the two-stage source has a flux up to $1.6(3)\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{0.2em}{0ex}}\mathrm{atoms}/\mathrm{s}$, with an optimized temperature of $15.0(2)\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{K}$. We employ continuous Raman-Ramsey interference measurements at the atom-beam output to study the sources of decoherence in the presence of continuous cooling, and demonstrate that the atom-beam source effectively preserves high fringe contrast even during cooling. This cold-atom-beam source is appropriate for use in atom interferometers and clocks, where continuous operation eliminates dead time, the slow atom-beam velocity (6--16 m/s) improves sensitivity, the narrow 3D velocity distribution improves fringe contrast, and the low reabsorption of scattered light mitigates decoherence caused by the continuous cooling process.
We have constructed a tapered magnetic atom guide that is integrated with a magneto-optical trap. We have observed atom guiding over distances of up to 20 cm. This integrated strategy is useful for loading atoms into wire guides. We have shown that tapered sections in atom guides. i.e. sections with a variable steepness of the transverse guiding potential, can reflect atoms propagating in the guide. The reflection arises from the adiabatic conservation of the action of the transverse motion; the latter can be used as a control parameter for the atom reflectivity. A method of producing a slow, dark atomic beam of atoms is demonstrated. Atoms that are extracted from a magneto-optical trap using a push beam are quickly pumped into a dark state, which does not interact with the push laser beam. This method is capable of producing atoms with a velocity of order 1 m/s. The slow atoms can be coupled into atom guides. In related research, we observe tunneling resonances of atoms in a 1D optical lattice through the measurement of the magnetization. These resonances manifest themselves as modulations in the magnetization curves. The resonances are shown to be related to the coherence properties of the atoms, and exist only under conditions where the coherence time exceeds the tunneling period of the atoms.
The capture of a moving atom by a non-dissipative trap, such as an optical dipole trap, requires the removal of the excessive kinetic energy of the atom. In this paper, we develop a mechanism to harvest ultra-cold atoms from a guided atom beam into an optical dipole trap by removing their directed kinetic energy. We propose a continuous loading scheme where this is accomplished via deceleration by a magnetic potential barrier followed by optical pumping to the energetically lowest Zeeman sublevel. We theoretically investigate the application of this scheme to the transfer of ultra-cold chromium atoms from a magnetically guided atom beam into a deep optical dipole trap. We discuss the realization of a suitable magnetic field configuration. Based on numerical simulations of the loading process, we analyse the feasibility and efficiency of our loading scheme.
We report on the realization of a transversely loaded two-dimensional magneto-optical trap serving as a source for cold strontium atoms. We analyze the dependence of the source's properties on various parameters, in particular the intensity of a pushing beam accelerating the atoms out of the source. An atomic flux exceeding 109atoms/s{10}^{9}\phantom{\rule{0.28em}{0ex}}\mathrm{atoms}/\mathrm{s}109atoms/s at a rather moderate oven temperature of $500{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ is achieved. The longitudinal velocity of the atomic beam can be tuned over several tens of m/s by adjusting the power of the pushing laser beam. The beam divergence is around 60 mrad, determined by the transverse velocity distribution of the cold atoms. The slow atom source is used to load a three-dimensional magneto-optical trap realizing loading rates up to 109atoms/s{10}^{9}\phantom{\rule{0.16em}{0ex}}\mathrm{atoms}/\mathrm{s}109atoms/s without indication of saturation of the loading rate for increasing oven temperature. The compact setup avoids undesired effects found in alternative sources like, e.g., Zeeman slowers, such as vacuum contamination and black-body radiation due to the hot strontium oven.