Imaging trapped quantum gases by off-axis holography.

We present a dispersive imaging method for trapped quantum gases based on digital off-axis holography. Both phase delay and intensity of the probe field are determined from the same image. Due to the heterodyne gain inherent to the holographic method, it is possible to retrieve the phase delay induced by the atoms at probe beam doses two orders of magnitude lower than phase-contrast imaging methods. Using the full field of the probe beam, we numerically correct for image defocusing.

Observation of a Space-Time Crystal in a Superfluid Quantum Gas.

Time crystals are a phase of matter, for which the discrete time symmetry of the driving Hamiltonian is spontaneously broken. The breaking of discrete time symmetry has been observed in several experiments in driven spin systems. Here, we show the observation of a space-time crystal using ultracold atoms, where the periodic structure in both space and time is directly visible in the experimental images. The underlying physics in our superfluid can be described ab initio and allows for a clear identification of the mechanism that causes the spontaneous symmetry breaking. Our results pave the way for the usage of space-time crystals for the discovery of novel nonequilibrium phases of matter.

Quantum Enhancement of the Index of Refraction in a Bose-Einstein Condensate.

We study the index of refraction of an ultracold bosonic gas in the dilute regime. Using phase-contrast imaging with light detuned from resonance by several tens of linewidths, we image a single cloud of ultracold atoms for 100 consecutive shots, which enables the study of the scattering rate as a function of temperature and density using only a single cloud. We observe that the scattering rate is increased below the critical temperature for Bose-Einstein condensation by a factor of 3 compared to the single-atom scattering rate. We show that current atom-light interaction models to second order of the density show a similar increase, where the magnitude of the effect depends on the model that is used to calculate the pair-correlation function. This confirms that the effect of quantum statistics on the index of refraction is dominant in this regime.

Enhanced heat flow in the hydrodynamic collisionless regime.

We study the heat conduction of a cold, thermal cloud in a highly asymmetric trap. The cloud is axially hydrodynamic, but due to the asymmetric trap radially collisionless. By locally heating the cloud we excite a thermal dipole mode and measure its oscillation frequency and damping rate. We find an unexpectedly large heat conduction compared to the homogeneous case. The enhanced heat conduction in this regime is partially caused by atoms with a high angular momentum spiraling in trajectories around the core of the cloud. Since atoms in these trajectories are almost collisionless they strongly contribute to the heat transfer. We observe a second, oscillating hydrodynamic mode, which we identify as a standing wave sound mode.

Damping of superfluid flow by a thermal cloud.

One of the principal signatures of superfluidity is the frictionless flow of a superfluid through another substance. Here, we study the flow of a Bose-Einstein condensate through a thermal cloud and study its damping for different harmonic confinements and temperatures. The damping rates close to the collisionless regime are found to be in good agreement with Landau damping and become smaller for more homogeneous systems. In the hydrodynamic regime, we observe additional damping due to collisions, and we discuss the implications of these findings for superfluidity in this system.

Large atom number Bose-Einstein condensate of sodium.

We describe the setup to create a large Bose-Einstein condensate containing more than 120 x 10(6) atoms. In the experiment a thermal beam is slowed by a Zeeman slower and captured in a dark-spot magneto-optical trap (MOT). A typical dark-spot MOT in our experiments contains 2.0 x 10(10) atoms with a temperature of 320 microK and a density of about 1.0 x 10(11) atoms/cm(3). The sample is spin polarized in a high magnetic field before the atoms are loaded in the magnetic trap. Spin polarizing in a high magnetic field results in an increase in the transfer efficiency by a factor of 2 compared to experiments without spin polarizing. In the magnetic trap the cloud is cooled to degeneracy in 50 s by evaporative cooling. To suppress the three-body losses at the end of the evaporation, the magnetic trap is decompressed in the axial direction.

Magnetic nanodots from atomic Fe: can it be done?

Laser focusing of Fe atoms offers the possibility of creating separate magnetic structures on a scale of 10 nm with exact periodicity. This can be done by using the parabolic minima of the potential generated by a standing light wave as focusing lenses. To achieve the desired 10-nm resolution, we need to suppress chromatic and spherical aberrations, as well as prevent structure broadening caused by the divergence of the incoming beam. Chromatic aberrations are suppressed by the development of a supersonic Fe beam source with speed ratio S = 11 +/- 1. This beam has an intensity of 3 x 10(15) atoms sr(-1) s(-1). The spherical aberrations of the standing light wave will be suppressed by aperturing with beam masks containing 100-nm slits at 744-nm intervals. The beam divergence can be reduced by application of laser cooling to reduce the transverse velocity. We have constructed a laser system capable of delivering over 500 mW of laser light at 372 nm, the wavelength of the (5)D(4) --> (5)F(5) atomic transition of (56)Fe we intend to use for laser cooling. Application of polarization spectroscopy to a hollow cathode discharge results in a locking system holding the laser continuously within 2 MHz of the desired frequency.

Photoassociation spectroscopy of cold He(2 3S) atoms

We observe vibrational states by photoassociation spectroscopy of cold He(2 3S) atoms. Photoassociation resonances are detected as peaks in the Penning ionization rate over a frequency range of 20 GHz below the atomic 2 3S1-2 3P2 transition frequency. We have observed three vibrational series, of which two can be identified. A possible mechanism to explain the observed increase of the Penning ionization rate is discussed.

Photoassociation spectroscopy of cold he(2

We observe vibrational states by photoassociation spectroscopy of cold He(2 ^{3}S) atoms. Photoassociation resonances are detected as peaks in the Penning ionization rate over a frequency range of 20 GHz below the atomic 2 ^{3}S_{1}-2 ^{3}P_{2} transition frequency. We have observed three vibrational series, of which two can be identified. A possible mechanism to explain the observed increase of the Penning ionization rate is discussed.