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arXiv:1002.0222 · Physical Review A

Control of dipolar relaxation in external fields

Benjamin Pasquiou, G. Bismut, Quentin Beaufils, A. Crubellier, É. Maréchal, P. Pedri, L. Vernac, O. Gorceix, B. Laburthe-Tolra

Published Apr 29, 2010• 84 citations
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Abstract

We study dipolar relaxation in both ultracold thermal and Bose-condensed Cr atom gases. We show three different ways to control dipolar relaxation, making use of either a static magnetic field, an oscillatory magnetic field, or an optical lattice to reduce the dimensionality of the gas from three-dimensional (3D) to two-dimensional (2D). Although dipolar relaxation generally increases as a function of a static magnetic-field intensity, we find a range of nonzero magnetic-field intensities where dipolar relaxation is strongly reduced. We use this resonant reduction to accurately determine the $S=6$ scattering length of Cr atoms: ${a}_{6}=103\ifmmode\pm\else\textpm\fi{}4{a}_{0}$. We compare this new measurement to another new determination of ${a}_{6}$, which we perform by analyzing the precise spectroscopy of a Feshbach resonance in $d$-wave collisions, yielding ${a}_{6}=102.5\ifmmode\pm\else\textpm\fi{}0.4{a}_{0}$. These two measurements provide, by far, the most precise determination of ${a}_{6}$ to date. We then show that, although dipolar interactions are long-range interactions, dipolar relaxation only involves the incoming partial wave $l=0$ for large enough magnetic-field intensities, which has interesting consequences on the stability of dipolar Fermi gases. We then study ultracold Cr gases in a one-dimensional (1D) optical lattice resulting in a collection of independent 2D gases. We show that dipolar relaxation is modified when the atoms collide in reduced dimensionality at low magnetic-field intensities, and that the corresponding dipolar relaxation rate parameter is reduced by a factor up to $7$ compared to the 3D case. Finally, we study dipolar relaxation in the presence of rf oscillating magnetic fields, and we show that both the output channel energy and the transition amplitude can be controlled by means of the rf frequency and Rabi frequency.

DOI
10.1103/physreva.81.042716

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We study dipolar relaxation in both ultracold thermal and Bose-condensed Cr atom gases. We show three different ways to control dipolar relaxation, making use of either a static magnetic field, an oscillatory magnetic field, or an optical lattice to reduce the dimensionality of the gas from three-dimensional (3D) to two-dimensional (2D). Although dipolar relaxation generally increases as a function of a static magnetic-field intensity, we find a range of nonzero magnetic-field intensities where dipolar relaxation is strongly reduced. We use this resonant reduction to accurately determine the S=6S=6S=6 scattering length of Cr atoms: a6=103\ifmmode±\else\textpm\fi4a0{a}_{6}=103\ifmmode\pm\else\textpm\fi{}4{a}_{0}a6​=103\ifmmode±\else\textpm\fi4a0​. We compare this new measurement to another new determination of a6{a}_{6}a6​, which we perform by analyzing the precise spectroscopy of a Feshbach resonance in ddd-wave collisions, yielding a6=102.5\ifmmode±\else\textpm\fi0.4a0{a}_{6}=102.5\ifmmode\pm\else\textpm\fi{}0.4{a}_{0}a6​=102.5\ifmmode±\else\textpm\fi0.4a0​. These two measurements provide, by far, the most precise determination of a6{a}_{6}a6​ to date. We then show that, although dipolar interactions are long-range interactions, dipolar relaxation only involves the incoming partial wave l=0l=0l=0 for large enough magnetic-field intensities, which has interesting consequences on the stability of dipolar Fermi gases. We then study ultracold Cr gases in a one-dimensional (1D) optical lattice resulting in a collection of independent 2D gases. We show that dipolar relaxation is modified when the atoms collide in reduced dimensionality at low magnetic-field intensities, and that the corresponding dipolar relaxation rate parameter is reduced by a factor up to 777 compared to the 3D case. Finally, we study dipolar relaxation in the presence of rf oscillating magnetic fields, and we show that both the output channel energy and the transition amplitude can be controlled by means of the rf frequency and Rabi frequency.
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Dipole-dipole interactions are long-ranged and anisotropic. They induce a coupling between spin and angular momentum degrees of freedom yielding new interesting physics in the field of dilute quantum gases. We have thoroughly studied dipolar relaxation DR processes in a chromium BEC transferred into an excited magnetic m = +3 sublevel and loaded in 1D and 2D optical lattices. We demonstrate and quantify how the confinement of a quantum gas in optical lattices deeply modifies the DR processes. In particular, we show that DR is strongly inhibited at low magnetic fields when the released energy becomes smaller than the vibrational quantum of energy at the bottom of the lattice wells. We show that interband transitions are induced above the corresponding threshold and that metastability of the "excited state BEC" is reached below the 2D threshold.
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We observe interband transitions mediated by dipole-dipole interactions for an array of 1D quantum gases of chromium atoms, trapped in a 2D optical lattice. Interband transitions occur when dipolar relaxation releases an energy larger than the lattice band gap. For symmetric lattice sites, and a magnetic field parallel to the lattice axis, we compare the measured dipolar relaxation rate with a Fermi golden rule calculation. Below a magnetic field threshold, we obtain an almost complete suppression of dipolar relaxation, leading to metastable 1D gases in the highest Zeeman state.
We observe several Feshbach resonances in magnetic fields below 40 G for Cs atoms trapped in a 1D optical lattice. One resonance occurs in the lowest-energy ground state F=3F\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3F=3, mF=3{m}_{F}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3mF​=3 which is stable against inelastic binary collisions. This opens new possibilities for Bose condensation of Cs. When the elastic collision rate far exceeds the radial vibration frequency, we enter a 2D hydrodynamic regime where the thermalization rate is independent of density and temperature. Other resonances are also observed in the F=3F\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3F=3, ${m}_{F}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}3$ state through a dramatic increase in the dipolar relaxation rate.
We create Bose-Einstein Condensates of52Cr atoms in an optical trap. Intense radio-frequency fields are used to induce degeneracy between the Zeeman sub-states of the ground state atoms in the BEC despite an ambient static field of 100 mG. We demonstrate this effect by analyzing atom trajectories under the influence of dressed magnetic potentials. We investigate the problem of adiabaticity in the dressing process. In a further set of experiments, we transfer the atoms from the absolute ground state into the excited Zeeman sub-state and study the dipolar relaxation in ultra-cold gases of chromium atoms above and below the condensation threshold. We observe a minimum in the dipolar relaxation rate for a finite magnetic field value which enables a new determination of the scattering length.