N.J. van Druten

Evaporative Cooling of Trapped Atoms

Publisher Summary This chapter focuses on the concept of evaporative cooling of trapped neutral atoms. The recent observations of Bose–Einstein condensation have shown dramatically the potential of evaporative cooling. Through evaporative cooling, phase-space density could be increased by six orders of magnitude in these experiments. In such experiments, evaporative cooling was used to reach temperature and densities that are unprecedented for trapped atoms and greatly exceeded what had been reached before by laser cooling. Laser cooling has recently broken the recoil limit in three dimensions (3D) and reached extremely cold temperatures of 3 nK in 1D. However, none of these optical sub-recoil techniques has been realized so far at high densities. The current density limitations are caused by the absorption of light, radiation trapping, and excited state collisions. An often-mentioned disadvantage of evaporative cooling is the loss of atoms. However, as discussed in the chapter, the efficiency of evaporative cooling is quite high.

Direct, Nondestructive Observation of a Bose Condensate

The spatial observation of a Bose condensate is reported. Dispersive light scattering was used to observe the separation between the condensed and normal components of the Bose gas inside a magnetic trap. This technique is nondestructive, and about a hundred images of the same condensate can be taken. The width of the angular distribution of scattered light increased suddenly at the phase transition.

Yang-Yang thermodynamics on an atom chip

We investigate the behavior of a weakly interacting nearly one-dimensional trapped Bose gas at finite temperature. We perform in situ measurements of spatial density profiles and show that they are very well described by a model based on exact solutions obtained using the Yang-Yang thermodynamic formalism, in a regime where other, approximate theoretical approaches fail. We use Bose-gas focusing [I. Shvarchuck, Phys. Rev. Lett. 89, 270404 (2002)] to probe the axial momentum distribution of the gas and find good agreement with the in situ results.

Field emission from individual multiwalled carbon nanotubes prepared in an electron microscope

Individual multiwalled carbon nanotube field emitters were prepared in a scanning electron microscope. The angular current density, energy spectra, and the emission stability of the field-emitted electrons were measured. An estimate of the electron source brightness was extracted from the measurements. The results show that carbon nanotubes are promising candidates to replace existing sources in high-resolution electron beam instruments.

Transverse mode formation in microlasers by combined gain- and index-guiding

The formation of transverse modes in end-pumped microlasers with combined index- and gain-guiding is investigated theoretically, We study the case of a planoconcave cavity pumped by means of a Gaussian optical field in the frame of the effective index method and the mean-field limit. We find crossings of the modal gains as the cavity losses are varied. Ring patterns are observed in both the near- and far-field profiles in qualitative agreement with the experiments. We study the bad-cavity region and observe one compact trapped gain-guided mode with large modal gain, and many other deformed index-guided modes with much less gain.

Stable field emission from W tips in poor vacuum conditions

We report a stable field emission (FE) current from sharp tungsten (W) tips in relatively poor vacuum (up to 10−2 mbar) conditions. We use small tip–anode spacing to keep the extraction voltage low. A simple current regulator circuit, with a bandwidth of ∼1.6 kHz, was designed, which controls the voltage applied according to the emission current measured. Without a current regulator circuit, an uncleaned W tip in unbaked 6×10−7 mbar system pressure cannot emit stable FE current and short-term fluctuations at a few nA current were found to be more than 300%. The current regulator circuit improves the FE current stability dramatically. It was observed that at current level of ∼3.5 nA and regulation voltage of ∼120 V the short-term fluctuations in the current were ∼5% at 6×10−7 mbar unbaked system pressure. Subsequently, the system pressure was increased in steps up to 10−3 mbar of argon gas and it was observed that the current regulator circuit worked at almost the same efficiency. Around 10−2 mbar of Ar ga...

Box traps on an atom chip for one-dimensional quantum gases

We present the implementation of tailored trapping potentials for ultracold gases on an atom chip. We realize highly elongated traps with box-like confinement along the long, axial direction combined with conventional harmonic confinement along the two radial directions. The design, fabrication and characterization of the atom chip and the box traps are described. We load ultracold ( 1 µK) clouds of 87Rb in a box trap, and demonstrate Bose-gas focusing as a means to characterize these atomic clouds in arbitrarily shaped potentials. Our results show that box-like axial potentials on atom chips are very promising for studies of one-dimensional quantum gases.

Longitudinal character of atom-chip-based rf-dressed potentials

We experimentally investigate the properties of radio-frequency-dressed potentials for Bose-Einstein condensates on atom chips. The three-dimensional potential forms a connected pair of parallel waveguides. We show that rf-dressed potentials are robust against the effect of small magnetic-field variations on the trap potential. Long-lived dipole oscillations of condensates induced in the rf-dressed potentials can be tuned to a remarkably low damping rate. We study a beam splitter for Bose-Einstein condensates and show that a propagating condensate can be dynamically split in two vertically separated parts and guided along two paths. The effect of gravity on the potential can be tuned and compensated for using a rf-field gradient.

Adsorbate dynamics on a silica-coated gold surface measured by Rydberg Stark spectroscopy

Trapping a Rydberg atom close to a surface is an important step towards the realisation of many proposals for quantum information processing or hybrid quantum systems. One of the challenges in these experiments is posed by the electric field emanating from contaminations on the surface. Here we report on measurements of an electric field created by 87Rb atoms adsorbed on a 25 nm thick layer of SiO2, covering a 90 nm layer of Au. The electric field is measured using a two-photon transition to the and states. The electric field value that we measure is higher than typical values measured above metal surfaces, but is consistent with a recent measurement above a SiO2 surface. In addition, we measure the temporal behaviour of the field and observe that we can reduce it in a single experimental cycle, using ultraviolet light or by mildly locally heating the surface with one of the excitation lasers, whereas the buildup of the field takes thousands of cycles. We explain these results by a change in the adatom distribution on the surface. These results indicate that, while the stray electric field can be reduced, achieving field-free conditions above a silica-coated gold chip remains challenging.

Yang-Yang thermometry and momentum distribution of a trapped one-dimensional Bose gas

We describe the use of the exact Yang-Yang solutions for the one-dimensional Bose gas to enable accurate kinetic-energy thermometry based on the root-mean-square width of an experimentally measured momentum distribution. Furthermore, we use the stochastic projected Gross-Pitaevskii theory to provide a quantitative description of the full momentum distribution measurements of Van Amerongen et al. [Phys. Rev. Lett. 100, 090402 (2008)]. We find the fitted temperatures from the stochastic projected Gross-Pitaevskii approach are in excellent agreement with those determined by Yang-Yang kinetic-energy thermometry.