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arXiv:1709.03870 · Physical Review A

Improving the lifetime in optical microtraps by using elliptically polarized dipole light

Sébastien Garcia, Jakob Reichel, Romain Long

Published Feb 5, 2018• 3 citations
arXivHTMLPDF

Abstract

Tightly focused optical dipole traps induce vector light shifts (``fictitious magnetic fields'') which complicate their use for single-atom trapping and manipulation. The problem can be mitigated by adding a larger, real magnetic field, but this solution is not always applicable; in particular, it precludes fast switching to a field-free configuration. Here we show that this issue can be addressed elegantly by deliberately adding a small elliptical polarization component to the dipole trap beam. In our experiments with single $^{87}\mathrm{Rb}$ atoms laser-cooled in a chopped trap, we observe improvements up to a factor of 11 of the trap lifetime compared to the standard, seemingly ideal linear polarization. This effect results from a modification of heating processes via spin-state diffusion in state-dependent trapping potentials. We develop Monte Carlo simulations of the evolution of the atom's internal and motional states and find that they agree quantitatively with the experimental data. The method is general and can be applied in all experiments where the longitudinal polarization component is non-negligible.

DOI
10.1103/physreva.97.023406

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Tightly focused optical dipole traps induce vector light shifts (``fictitious magnetic fields'') which complicate their use for single-atom trapping and manipulation. The problem can be mitigated by adding a larger, real magnetic field, but this solution is not always applicable; in particular, it precludes fast switching to a field-free configuration. Here we show that this issue can be addressed elegantly by deliberately adding a small elliptical polarization component to the dipole trap beam. In our experiments with single 87Rb^{87}\mathrm{Rb}87Rb atoms laser-cooled in a chopped trap, we observe improvements up to a factor of 11 of the trap lifetime compared to the standard, seemingly ideal linear polarization. This effect results from a modification of heating processes via spin-state diffusion in state-dependent trapping potentials. We develop Monte Carlo simulations of the evolution of the atom's internal and motional states and find that they agree quantitatively with the experimental data. The method is general and can be applied in all experiments where the longitudinal polarization component is non-negligible.
We propose a microtrap realized with a fictitious magnetic field by applying a circularly polarized laser beam. We calculate the potentials and demonstrate that various atom traps, such as the Ioffe-Pritchard trap, double traps, and ring trap, can easily be built by varying the intensity and the position of the laser beams. We show that even an optical lattice can be realized by adding more laser beams properly. Our trap has the advantage as compared to a general atom lattice that the properties of the traps can be manipulated individually.
We experimentally investigate ${\ensuremath{\sigma}}^{+}\text{\ensuremath{-}}{\ensuremath{\sigma}}^{\ensuremath{-}}$ polarization gradient cooling (PGC) of a single 87Rb^{87}\mathrm{Rb}87Rb atom in a tightly focused dipole trap and show that the cooling limit strongly depends on the polarization of the trapping field. For optimized cooling light power, the temperature of the atom reaches $10.4(6)\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$ in a linearly polarized trap, approximately five times lower than in a circularly polarized trap. The inhibition of PGC is qualitatively explained by the fictitious magnetic fields induced by the trapping field. We further demonstrate that switching the trap polarization from linear to circular after PGC induces only minor heating.
In our experiment, a single cesium atom prepared in a large-magnetic-gradient magneto-optical trap (MOT) can be efficiently transferred into a 1064-nm far-off-resonance microscopic optical dipole trap (FORT). The efficient transfer of the single atom between the two traps is used to determine the trapping lifetime and the effective temperature of the single atom in FORT. The typical trapping lifetime has been improved from ∼ 6.9 s to ∼ 130 s by decreasing the background pressure from ∼ 1 × 10 −10 Torr to ∼ 2 × 10 −11 Torr and applying one-shot 10-ms laser cooling phase. We also theoretically investigate the dependence of trapping lifetimes of a single atom in a FORT on trap parameters based on the FORT beam’s intensity noise induced heating. Numerical simulations show that the heating depends on the FORT beam’s waist size and the trap depth. The trapping time can be predicted based on effective temperature measurement of a single atom in the FORT and the intensity noise spectra of the FORT beam. These experimental results are found to be in agreement with the predictions of the heating model.
Ultracold cesium atoms are stored in a novel dipole-force trap, which provides long storage times of spin polarization and facilitates easy Stern-Gerlach selection. The trap consists of a far red-detuned standing light wave, oriented vertically in the field of gravity. By comparing the trapping of a single magnetic substate ( F =4, m F =0) with the simultaneous storage of all sublevels, we measure the decay of spin polarization that results from photon scattering of trap light. We furthermore observe spin precession in an optically induced "fictitious magnetic field".
Summary form only given. Optical microtraps provide a strong spatial confinement for laser-cooled atoms. They can, e.g., be realized with strongly focused trapping light beams or the optical near fields of nano-scale waveguides and photonic nanostructures. Atoms in such traps often experience strongly spatially varying AC Stark shifts which are proportional to the magnetic quantum number of the respective energy level. These inhomogeneous fictitious magnetic fields can cause a displacement of the trapping potential that depends on the Zeeman state. Hitherto, this effect was mainly perceived as detrimental. However, it also provides a means to probe and to manipulate the motional state of the atoms in the trap by driving transitions between Zeeman states. Furthermore, by applying additional real or fictitious magnetic fields, the state-dependence of the trapping potential can be controlled. Here, using laser-cooled atoms that are confined in a nanofiber-based optical dipole trap [1], we employ this control in order to tune the microwave coupling of motional quantum states. We record corresponding microwave spectra which allow us to infer the trap parameters as well as the temperature of the atoms. Finally, we reduce the mean number of motional quanta in one spatial dimension to {n}=0.3±0.1, close to the motional ground state, by using microwave sideband cooling. Our work shows that the inherent fictitious magnetic fields in optical microtraps expand the experimental toolbox for interrogating and manipulating cold atoms.
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.