Dipolar relaxation in an ultra-cold gas of magnetically trapped chromium atoms

Hensler, S.; Werner, J.H.; Griesmaier, Axel; Schmidt, Piet O.; Görlitz, Axel; Pfau, Tilman; Giovanazzi, S.; Rza̧żewski, Kazimierz

doi:10.1007/s00340-003-1334-0

Cited by 97 publications

(169 citation statements)

References 21 publications

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“…Averaging over the inter-particle velocities we consider that the atomic depolarization can occur via single or double spin flip. The measurement of β dr ∼ 10 −13 cm 2 /s results in good agreement with theoretical predictions based on 1 st order Born approximation for dipole-dipole interaction [11].…”

supporting

confidence: 86%

“…Magnetic dipoledipole interaction can induce inelastic relaxations with a rate that drastically increase with the atomic magnetic moments. In particular the cross section for the inelastic single spin-flip (only one atom changes its m J value by one) is proportional to J 3 [11]. In [10], particular attention has been paid to chromium atomic gases [12] because grounded Cr 7 S 3 atoms possess very large magnetic dipole moments of six Bohr magnetons.…”

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confidence: 99%

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Demagnetization cooling of a gas

et al. 2006

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We demonstrate demagnetization cooling of a gas of ultracold 52 Cr atoms. Demagnetization is driven by inelastic dipolar collisions which couple the motional degrees of freedom to the spin degree. By that kinetic energy is converted into magnetic work with a consequent temperature reduction of the gas. Optical pumping is used to magnetize the system and drive continuous demagnetization cooling. Applying this technique, we can increase the phase space density of our sample by one order of magnitude, with nearly no atom loss. This method can be in principle extended to every dipolar system and could be used to achieve quantum degeneracy via optical means.

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confidence: 86%

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confidence: 99%

Demagnetization cooling of a gas

et al. 2006

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“…In this situation, the energy shift is given by 5) where I(r) is the intensity of the laser. In the dressed atom picture, the energy shift (2.5) is interpreted as an effective potential V opt (r) = ∆E(r), that follows the spatial pattern of the laser field intensity, in which the atom moves.…”

Section: Optical Latticesmentioning

confidence: 99%

Ultracold dipolar gases in optical lattices

Trefzger¹,

Menotti²,

Capogrosso-Sansone³

et al. 2011

J. Phys. B: At. Mol. Opt. Phys.

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Abstract. This tutorial is a theoretical work, in which we study the physics of ultra-cold dipolar bosonic gases in optical lattices. Such gases consist of bosonic atoms or molecules that interact via dipolar forces, and that are cooled below the quantum degeneracy temperature, typically in the nK range. When such a degenerate quantum gas is loaded into an optical lattice produced by standing waves of laser light, new kinds of physical phenomena occur. These systems realize then extended Hubbard-type models, and can be brought to a strongly correlated regime. The physical properties of such gases, dominated by the long-range, anisotropic dipole-dipole interactions, are discussed using the meanfield approximations, and exact Quantum Monte Carlo techniques (the Worm algorithm).

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“…Therefore, the two-body loss parameter that we expect for this rf-assisted mechanism is on the same order of magnitude as K rel 2 , the twobody loss parameter for dipolar relaxation in a static field B eq ≈h ω gJ µB . Given the known value for K rel 2 [13], and our typical BEC density, a typical lifetime of a few tens of ms is expected at large rf power. Although a full quantitative analysis is beyond the scope of this paper, we represent in Fig 4 the evolution of the gap we have calculated as a function of rf power.…”

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confidence: 89%

Radio-frequency-induced ground-state degeneracy in a Bose-Einstein condensate of chromium atoms

et al. 2008

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We study the effect of strong radio-frequency (rf) fields on a chromium Bose-Einstein condensate (BEC), in a regime where the rf frequency is much larger than the Larmor frequency. We use the modification of the Landé factor by the rf field to bring all Zeeman states to degeneracy, despite the presence of a static magnetic field of up to 100 mG. This is demonstrated by analyzing the trajectories of the atoms under the influence of dressed magnetic potentials in the strong field regime. We investigate the problem of adiabaticity of the rf dressing process, and relate it to how close the dressed states are to degeneracy. Finally, we measure the lifetime of the rf dressed BECs, and identify a new rf-assisted two-body loss process induced by dipole-dipole interactions. [2]. These systems are known as spinor quantum gases. To explore these new features, it is important that differences in interaction energies between different total spin states are larger than their relative Zeeman energy, which requires magnetically shielded environments.Up to now experiments on spin-1 [3] and spin-2 [4] spinor condensates were typically performed starting with atoms in the |m = 0 magnetic state, with emphasis on spin dynamics and coherent oscillations between the spin components. Since spin dynamics is driven by spin exchange collisions, which do not modify the total spin angular momentum, one therefore works in a subspace insensitive to first order Zeeman effect, but the spinor ground state is not obtained [5]. A subspace insensitive to magnetic fields to first order is also used for quantum computing purposes with cold atoms in optical lattices, to reduce decoherence during quantum gate operations [6]. More generally, a very accurate control of the magnetic fields is required for precision measurements (e.g. atomic clocks use both magnetic shielding and a transition insensitive to the Zeeman effect to first order).To ease the constraints on magnetic field control, we suggest to use strong off resonant linearly polarized rf fields, to bring all Zeeman states to degeneracy despite a non-zero magnetic field. We demonstrate this idea by sending strong rf fields to optically trapped Bose condensed chromium atoms. We analyze the trajectories of atoms in dressed magnetic potentials and we show that, as expected from [7], the Landé factor is modifed, and can even be set to zero. At this point, all Zeeman states are degenerate. We show that the adiabaticity criterion for ramping up the rf power strongly depends on such degeneracy. Finally, we discuss inelastic losses measured in the dressed sample, and attribute them to an exoenergetic rf-assisted dipolar coupling to higher partial waves.Before describing our experimental results, let us give a physical insight into the modification of the atoms eigenenergies by the rf field. As shown in [7] using first order perturbation theory, when the rf frequency ω is much larger than the Larmor frequency ω ⊥ the Landé factor g J perpendicular to the rf field axis is modified by the rf dressing of t...

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Dipolar relaxation in an ultra-cold gas of magnetically trapped chromium atoms

Cited by 97 publications

References 21 publications

Demagnetization cooling of a gas

Demagnetization cooling of a gas

Ultracold dipolar gases in optical lattices

Radio-frequency-induced ground-state degeneracy in a Bose-Einstein condensate of chromium atoms

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