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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

rf-induced magnetic degeneracy and dipolar relaxation in a chromium Bose-Einstein Condensate

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

Published Jun 1, 2009• 0 citations
Publisher

Abstract

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.

DOI
10.1109/cleoe-eqec.2009.5194130

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Magnetic dipole-dipole interactions (MDDIs) are long-ranged and anisotropic. They induce a coupling between spin and angular momentum degrees of freedom paving the way for new interesting physics in the fields of quantum gases and magnetism. This paper thoroughly studies dipolar relaxation (DR) processes in a chromium BEC. Atoms are transferred from the absolute ground state with m = -3 to the excited m = +3 Zeeman state and are loaded in a 3D optical lattice; the magnetic state distribution of this system under various conditions is monitored. In a previous publication, it was demonstrated how the confinement of a quantum gas in 1D and 2D optical lattices deeply modifies the DR processes. In particular, it was shown that DR is inhibited at low magnetic fields when the released energy becomes smaller than the vibrational quantum of energy at the bottom of the lattice wells. In the present work, instability (or decay) of the “excited state BEC” appears as a resonant process as the released energy matches band excitation. Analyzing these resonances as a function of the magnetic field orientation reveals a new manifestation of the anisotropy of MDDIs. The spectroscopy of these resonances as a function of the field intensity is found to be sensitive to the lattice site number occupancy as demonstrated by performing it after various system preparation schemes including number-squeezed Mott states. Decays from sites housing three atoms produce entangled spin and orbital states.

Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•May 1, 2013•1 citation
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.
We study the effect of strong radio-frequency (rf) fields on a Bose-Einstein condensate (BEC) of chromium atoms, in a regime where the rf frequency is much larger than the Larmor frequency. We use the modification of the Land\'e factor by the rf field to bring all Zeeman states to degeneracy, despite the presence of a static magnetic field of up to 100mG100\phantom{\rule{0.3em}{0ex}}\mathrm{mG}100mG. 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 rf-assisted two-body loss process induced by dipole-dipole interactions.
We study the effect of strong radio-frequency (rf) fields on a chromium Bose-Einstein condensate, in a regime where the rf frequency is much larger than the Larmor frequency. We show that the Lande factor of the atoms is lowered by the presence of the rf, and can be even set to zero for a proper combination of the rf power and frequency. The trajectories of the atoms under the influence of magnetic potentials are thus greatly modified. We study the criteria for adiabaticity of the rf dressing process leading to such a modification of the atom magnetic susceptibility. We also measure the lifetime of the rf dressed BECs.
Full text: The study of quantum gases made of chromium atoms is compelling for several reasons. Being accessible to laser manipulation, chromium has a most abundant bosonic isotope {sup 52}Cr and a 9-percent abundant fermionic isotope {sup 53}Cr. Most importantly, Cr atoms carry an exceptionally large magnetic moment of 6 {mu}{sub B}. Consequently, Cr provides a valuable tool to study the physics of dipolar quantum gases as demonstrated elsewhere. We present our recent achievement of a chromium Bose-Einstein Condensation (Cr-BEC) using an all-optical procedure along with two innovative techniques: continuous accumulation of metastable {sup 52}Cr atoms in a mixed optical and magnetic trap; fast and intense rf sweeps to average to zero the magnetic potential and optimize the transfer efficiency from the Cr-MOT to the optical trap. We also report on the rf spectroscopy and association of weakly bound Cr{sub 2} molecules in the decatriplet {sup 13}{sigma}{sub g}{sup +} state. These latter experiments are performed in the vicinity of a d-wave Feshbach resonance at low magnetic field. Though the association rate is at present fairly low, we can study the spectroscopic properties of these cold trapped high-spin chromium molecules. (author)
In this work a gas of ultra-cold chromium atoms in a magnetic trap has been prepared and its elastic and inelastic scattering properties have been investigated with regard to Bose-Einstein condensation. Bose-Einstein condensation of dilute atomic gases is achieved using different cooling and trapping techniques. Deviating from the standard way we were able to devise a continuous loading mechanism for a magnetic trap as a result of the spectroscopic properties and the large magnetic dipole moment of chromium. It not only allows us to trap more atoms but also facilitates the subsequent preparation steps. After loading, the magnetic trap is compressed to increase the atomic density. In doing so we also increases the temperature of the atomic cloud. Doppler cooling of the atoms in the compressed trap reduces the temperature and increases the density. Subsequent evaporative cooling further reduces the temperature. The efficiency of the cooling mechanism is determined by the elastic and inelastic scattering properties of atomic species. Ultra-cold collisions between ground state atoms are dominated by s-wave collisions. They are characterized by a single parameter, the scattering length a. We were able to determine the temperature dependence of the elastic collision rate for the two bosonic isotopes 52Cr and 50Cr in a relaxation experiment. Comparing our results with the effective range theory allowed us not only to extract the magnitude of the scattering length, but also its sign. The sign of the scattering length is important since only condensates with a positive scattering length are stable. Our efforts to achieve Bose-Einstein condensation in chromium by evaporative cooling in a magnetic trap resulted in a maximum phase space density of 0.04. Further cooling reduced the phase space density due to increased loss of atoms from dipolar relaxation. We were able to obtain preliminary results on the magnetic field dependence of dipolar relaxation. Our experimental data is in excellent agreement with theory. These experiments confirm the theoretical prediction that the dipolar relaxation rate is independent from the details of the interaction potential but rather scales with the magnetic offset field and the magnetic dipole moment of the atoms. The findings of this work mark an important step towards the realization of a Bose-Einstein condensate with chromium atoms. Especially the knowledge of the elastic and inelastic scattering properties allow now to devise a successful strategy.
Temperature dependence of the cross-section for elastic ground-state collisions of /sup 52/Cr- and /sup 50/Cr-atoms using the method of cross-dimensional relaxation, respectively is determined. The corresponding absolute values of the scattering length are on the order of 100a/sub 0/ for /sup 52/Cr and /sup 50/a/sub 0/ for /sup 50/Cr, respectively. However, to use evaporative cooling to increase the phase space density and achieve Bose-Einstein condensation with chromium atoms, have been limited to values below 10/sup -2/. Unusuually high inelastic collisio rates due to dipolar relaxation cause atom loss and heating of the sample. These losses are investigated and found similar magnetic-field-dependent rate coefficients (/spl ap/10/sup -11/ cm/sup 3//s @ 50 G offset field) for the two isotopes /sup 50/Cr and /sup 52/Cr indicating that the strongly enhanced dipolar relaxation is a generic feature of the dipole-dipole interaction. This conclusion is supported by theoretical calculations. To circumvent these losses, an optical dipole trap is currently being set up. Here, chromium can be trapped in the energetically lowest state where dipolar relaxation is absent.
Magnetic dipole-dipole interactions (MDDIs) are long-ranged and anisotropic. They induce a coupling between spin and angular momentum degrees of freedom paving the way for new interesting physics in the fields of quantum gases and magnetism. This paper thoroughly studies dipolar relaxation (DR) processes in a chromium BEC. Atoms are transferred from the absolute ground state with m = -3 to the excited m = +3 Zeeman state and are loaded in a 3D optical lattice; the magnetic state distribution of this system under various conditions is monitored. In a previous publication, it was demonstrated how the confinement of a quantum gas in 1D and 2D optical lattices deeply modifies the DR processes. In particular, it was shown that DR is inhibited at low magnetic fields when the released energy becomes smaller than the vibrational quantum of energy at the bottom of the lattice wells. In the present work, instability (or decay) of the “excited state BEC” appears as a resonant process as the released energy matches band excitation. Analyzing these resonances as a function of the magnetic field orientation reveals a new manifestation of the anisotropy of MDDIs. The spectroscopy of these resonances as a function of the field intensity is found to be sensitive to the lattice site number occupancy as demonstrated by performing it after various system preparation schemes including number-squeezed Mott states. Decays from sites housing three atoms produce entangled spin and orbital states.