We describe the setup to create a large Bose-Einstein condensate (BEC) containing more than 120 million atoms. In the experiment a thermal beam is slowed by a Zeeman slower and captured in a dark-spot magneto-optical trap (MOT). The sample is subsequently 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. This large atom number BEC yields the possibility to enter the hydrodynamic regime by evaporative cooling at a relative low density suppressing the effect of avalanches. The collisional opacity can be tuned from the collisionless regime to a collisional opacity of more than 3 by compressing the trap after condensation. In the collisional opaque regime a significant heating of the cloud at time scales shorter than half of the radial trap period is measured. This is direct proof that the BEC is hydrodynamic. Furthermore a detailed study is presented of superradiant scattering from a BEC. The relation between the depletion of the BEC and the onset of the different side modes as well as the transition from the weak to the strong pulse regime is studied yielding a good insight in the coupling processes between the BEC and the side modes. Finally, a simplified model to describe the time evolution of the process is presented. The model gives a good quantitative description of the fraction of atoms coupled to different side modes, showing that the approximations made in the modeling are valid. Also the atom pair production by superradiant backward-scattering from a Bose-Einstein condensate is investigated. In our experiment the energy in the process is conserved by using a pulse with two frequencies. This makes the process resonant, which gives full control over the number of atoms in the different recoil modes. It is shown that resonant backward-scattering produces strongly correlated side modes in which number squeezing and phase correlation are likely. This makes resonant superradiant backward-scattering a promising candidate for many-particle entanglement.
We describe the setup to create a large Bose-Einstein condensate (BEC) containing more than 120 million atoms. In the experiment a thermal beam is slowed by a Zeeman slower and captured in a dark-spot magneto-optical trap (MOT). The sample is subsequently 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. This large atom number BEC yields the possibility to enter the hydrodynamic regime by evaporative cooling at a relative low density suppressing the effect of avalanches. The collisional opacity can be tuned from the collisionless regime to a collisional opacity of more than 3 by compressing the trap after condensation. In the collisional opaque regime a significant heating of the cloud at time scales shorter than half of the radial trap period is measured. This is direct proof that the BEC is hydrodynamic. Furthermore a detailed study is presented of superradiant scattering from a BEC. The relation between the depletion of the BEC and the onset of the different side modes as well as the transition from the weak to the strong pulse regime is studied yielding a good insight in the coupling processes between the BEC and the side modes. Finally, a simplified model to describe the time evolution of the process is presented. The model gives a good quantitative description of the fraction of atoms coupled to different side modes, showing that the approximations made in the modeling are valid. Also the atom pair production by superradiant backward-scattering from a Bose-Einstein condensate is investigated. In our experiment the energy in the process is conserved by using a pulse with two frequencies. This makes the process resonant, which gives full control over the number of atoms in the different recoil modes. It is shown that resonant backward-scattering produces strongly correlated side modes in which number squeezing and phase correlation are likely. This makes resonant superradiant backward-scattering a promising candidate for many-particle entanglement.
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.
We report the realization of a Bose-Einstein condensate (BEC) in the hydrodynamic regime. The hydrodynamic regime is reached by evaporative cooling at a relatively low density suppressing the effect of avalanches. With the suppression of avalanches a BEC containing more than 108 atoms is produced. The collisional opacity can be tuned from the collisionless regime to a collisional opacity of more than 2 by compressing the trap after condensation. In the collisional opaque regime a significant heating of the cloud at time scales shorter than half of the radial trap period is measured, which is a direct proof that the BEC is hydrodynamic.
We investigate the atom pair production by superradiant backward-scattering from a Bose-Einstein condensate. By driving the superradiant process with two frequencies we can extend both the range of pulse duration and intensity by two orders of magnitude and obtain full control over the number of scattered atoms in forward and backward direction. We show that the atoms scattered in forward direction are strongly correlated with the atoms scattered in backward direction, which makes resonant superradiant backward-scattering a promising candidate for many-particle entanglement.
Summary form only given.We have established an experimental and theoretical program focused on understanding photon atom interactions in condensates with the aim of probing the properties of condensates as well as testing our models of the interactions themselves. With a combination of laser and evaporative cooling, we are routinely creating million atom Bose-Einstein condensates of sodium atoms in the newly designed 4D magnet. Our experimental setup incorporates a candlestick atomic beam source, which provides for recirculation of uncollimated sodium atoms, The laser cooling step involves a zero-crossing Zeeman slower, magneto-optic trap, and polarization gradient cooling. The laser-cooled atom cloud is loaded into the 4D magnetic trap and adiabatically compressed to high density and an evaporative cooling process is initiated. Using a RF magnetic resonance technique, hot atoms are ejected through magnetic spin flips. After formation of the condensate in the tightly confining trap, we manipulate the condensate by adiabatically changing the trapping potential and through interaction with photons. We have developed a technique for probing the condensates directly inside the trap using near-resonant absorption imaging. This imaging technique gives high sensitivity to details of the condensate wavefunction and to its surface structure. Images of pure condensates are compared to calculations of the condensate wavefunction based on a Hartree mean field description of the two-body interaction between condensate atoms.
Bose-Einstein CondensationAtoms are normally considered as particles but, according to quantum mechanics, they also have wave-like properties.Indeed, an atom has an equivalent wavelength, the deBroglie wavelength, which is inversely proportional to the atom's momentum.As atoms are cooled they slow down and their deBroglie wavelength increases.Satyendra Bose and Albert Einstein predicted 70 years ago that at a low enough temperature the wavelength exceeds the interparticle spacing and the atoms begin to overlap.The atoms become indistinguishable, effectively entering -by a process called Bose-Einstein condensation (BEC) -a coherent state where the laws of quantum mechanics govern the behaviour of the macroscopic system.BEC has been observed in superfluid helium-4 and superconductivity, both being states of matter in which bosons (integral-spin particles) condense into macroscopic quantum states.But the bosons in these systems interact with each other, so to better understand BEC, physicists have tried for sometime to bring about condensation in an ideal gas of noninteracting atoms. The JILA ExperimentThe JILA experiment consists of three stages: A -optical loading, (pre)cooling & polarization; B -magnetic trapping & evaporative cooling in the absence of light; C -optical detection.A -laser cooling stage.For 300 s, about 107 atoms are gathered from the low-velocity tail of a room-temperature Rb-vapour at some KT-11 mbar into a so-called "dark" magneto-optical trap (MOT), which is most efficient in collecting a large number of atoms by optical forces.The gas is rapidly (pre)cooled In the MOT to about 20 µK and subsequently magnetically polarized in a small magnetic bias field by optical pumping.B -trapping and cooling.Polarized atoms are magnetically trapped in a rapidly applied timeorbiting potential (TOP) trap, which consists of a set of coils In anti-Helmholtz configuration with a small, uniform transverse field, rotating at 7.5 kHz, superimposed.The TOP provides an effectively harmonic potential with an axial frequency of about 120 Hz and radial frequency of about 42 Hz.In the TOP, the gas is adiabatically compressed to reach the starting conditions for evaporative cooling at about 90 pK with a number density of about 2 x 1010 cm-3, sufficiently large to have the elastic collision rate dominate over the loss rate in the gas.The evaporation takes 70 s and BEC is achieved with 2 x 104 atoms at approximately 170 nK.The evaporation is induced by a RF transition to an untrapped state of the atoms at the edge of the sample.C -optical detection.At the start of the detection stage the trap is expanded adiabatically to a larger size to allow a fast (destructive) absorption measurement in which the sample Is Imaged on a CCD camera.The appearance of a diffraction ring marks the growth of a partially resolved structure in the sample.This is consistent with BEC In an inhomogeneous sample, where the condensate is expected to appear as a very small, dense gas cloud at the centre of the potential well.To confirm this interpretation, the momentum distribution in the sample was measured by suddenly switching-off the trap and observing the expanding gas cloud.In this process both Isotropic expansion of the (thermal) non-condensate fraction was observed as well as an anisotropic expansion, as is to be expected for a suddenly released anisotropic single quantum state. INELASTIC X-RAY SCATTERING Great Potential Demonstrated F. Sette and G. Ruocco describe why the confirmation, at the European Synchro tron Radiation Facility, of fast sound in water at momentum transfers of from 4 to 14 nm-1 using inelastic X-ray scattering with a resolution in the meV range demonstrates the technique's important capabilities.
The field of Bose-Einstein condensation (BEC) in dilute atomic gases provides a fruitful playground to test well-developed theories of quantum fluids. Research using BECs can address open questions relating to the many-body aspects of two-component quantum liquids, namely the interaction between the hydrodynamic normal and the superfluid component at finite temperatures. After the first realization of BEC some pilot experiments have been carried out, but detailed experiments are missing. This has to be compared to the case of liquid helium II, where many experiments since the 1950s have added to our understanding of novel phenomena in quantum liquids, like collective excitations, first and second sound, and others. One of the drawbacks of liquid helium is that the interactions are so strong that a clear distinction between the two components is difficult and the interpretation of the phenomena remains ambiguous. Most experimental studies of dilute Bose-Einstein condensed gases use clouds, which are in the collisionless regime. The reason for the lack of detailed experiments in BECs to study quantum liquids and in particular the hydrodynamical aspects of it, is the limited number of atoms (typically 1--10 million) in the experiments leaving the thermal atoms virtually collisionless. Efforts to decrease the mean free path by increasing the confinement limits the lifetime of the sample, since the density is limited by three-body decay. Experiments on dilute clouds of cold atoms are generally conducted in highly asymmetric traps. In these elongated, cigar-shaped geometries the mean free path of the atoms can become much shorter than the size of the cloud in the long, axial direction, but at the same time exceeds the size in the other, radial directions. In this so-called hydrodynamic-collisionless regime the system can be considered radially collisionless and axially hydrodynamic. In our setup we have created BECs containing up to 300 million sodium atoms by evaporation of atoms in an axially strongly decompressed trap with an aspect ratio of 1:65. Hot atoms created in three-body collisions are able to leave the sample in this highly asymmetric trap, before they can heat other atoms in an avalanche. The sample is axially hydrodynamic, but due to the large aspect ratio collisionless in the radial direction. Such samples are ideal for the observation of the interaction between the superfluid and normal fluid component of the cloud. In this thesis we describe the setup which is used to reach the hydrodynamic regime and we describe a novel implementation of the well-known phase contrast imaging technique. Next, we study collective excitations in the crossover from the collisionless toward the hydrodynamic regime, we determine the heat conduction in a cold, hydrodynamic thermal cloud and observe the propagation of a thermal wave in the two-fluid regime. Furthermore, we study the friction between the superfluid and normal fluid component of the cloud. In the final chapters of this thesis we study the propagation of second sound, as well as the formation and propagation of shock waves.