Narrow-line magneto-optical trap for dysprosium atoms.

We present our technique to create a magneto-optical trap (MOT) for dysprosium atoms using the narrow-line cooling transition at 626 nm to achieve suitable conditions for direct loading into an optical dipole trap. The MOT is loaded from an atomic beam via a Zeeman slower using the strongest atomic transition at 421 nm. With this combination of two cooling transitions we can trap up to 2.0·10(8) atoms at temperatures down to 6 µK. This cooling approach is simpler than present work with ultracold dysprosium and provides similar starting conditions for a transfer to an optical dipole trap.

Spectroscopy of a narrow-line optical pumping transition in atomic dysprosium.

We present measurements of the hyperfine coefficients and isotope shifts of the Dy I 683.731 nm transition, using saturated absorption spectroscopy on an atomic beam. A King Plot is drawn resulting in an updated value for the specific mass shift δν(684,sms)(164-162)=-534±17 MHz. Using fluorescence spectroscopy, we measure the excited state lifetime τ684=1.68(5) µs, yielding a linewidth of γ684=95±3 kHz. We give an upper limit to the branching ratio between the two decay channels from the excited state showing that this transition is usable for optical pumping into a dark state and demagnetization cooling.

Effect of light assisted collisions on matter wave coherence in superradiant Bose-Einstein condensates.

We investigate experimentally the effects of light assisted collisions on the coherence between momentum states in Bose-Einstein condensates. The onset of superradiant Rayleigh scattering serves as a sensitive monitor for matter-wave coherence. A subtle interplay of binary and collective effects leads to a profound asymmetry between the two sides of the atomic resonance and provides far bigger coherence loss rates for a condensate bathed in blue detuned light than previously estimated. We present a simplified quantitative model containing the essential physics to explain our experimental data and point at a new experimental route to study strongly coupled light matter systems.

d-wave collapse and explosion of a dipolar bose-einstein condensate.

We investigate the collapse dynamics of a dipolar condensate of 52Cr atoms when the s-wave scattering length characterizing the contact interaction is reduced below a critical value. A complex dynamics, involving an anisotropic, d-wave symmetric explosion of the condensate, is observed. The atom number decreases abruptly during the collapse. We find good agreement between our experimental results and those of a numerical simulation of the three-dimensional Gross-Pitaevskii equation, including contact and dipolar interactions as well as three-body losses. The simulation indicates that the collapse induces the formation of two vortex rings with opposite circulations.

Observation of dipole-dipole interaction in a degenerate quantum gas.

We have investigated the expansion of a Bose-Einstein condensate of strongly magnetic chromium atoms. The long-range and anisotropic magnetic dipole-dipole interaction leads to an anisotropic deformation of the expanding chromium condensate which depends on the orientation of the atomic dipole moments. Our measurements are consistent with the theory of dipolar quantum gases and show that a chromium condensate is an excellent model system to study dipolar interactions in such gases.

Observation of Feshbach resonances in an ultracold gas of 52Cr.

We have observed Feshbach resonances in collisions between ultracold 52Cr atoms. This is the first observation of collisional Feshbach resonances in an atomic species with more than one valence electron. The zero nuclear spin of 52Cr and thus the absence of a Fermi-contact interaction leads to regularly spaced resonance sequences. By comparing resonance positions with multichannel scattering calculations we determine the s-wave scattering length of the lowest (2S+1)Sigma(+)(g) potentials to be 112(14) a(0), 58(6) a(0), and -7(20) a(0) for S=6, 4, and 2, respectively, where a(0)=0.0529 nm.

Determination of the s-wave scattering length of chromium.

We have measured the deca-triplet s-wave scattering length of the bosonic chromium isotopes 52Cr and 50Cr. From the time constants for cross-dimensional thermalization in ultracold atomic samples, we have determined the magnitudes |a(52Cr)|=(170+/-39)a(0) and |a(50Cr)|=(40+/-15)a(0), where a(0)=0.053 nm. By measuring the rethermalization rate of 52Cr over a wide temperature range and comparing the temperature dependence with the effective-range theory and numerical single-channel calculations, we have obtained strong evidence that the sign of a(52Cr) is positive. Rescaling our 52Cr model potential to 50Cr strongly suggests that a(50Cr) is positive also.