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Bose Einstein Condensation of Sodium Atoms in a TOP Trap

L. Deng, E. W. Hagley, Mikio Kozuma, Robert Lutwak, Yu. B. Ovchinnikov, Jiping Wen, Kristian Helmerson, William D. Phillips, S. L. Rolston

Published May 3, 1998• 3 citations

Abstract

We have achieved Bose-Einstein condensation (BEC) [1,2] of Na atoms in an anisotropic TOP trap [3]. Our TOP trap has a bias field that rotates (at 20 kHz) in a plane containing the symmetry access of the quadrapole field, resulting in an anisotropic, time-averaged TOP potential with spring constants in the ratio of 1:2:4. A dark spot MOT is loaded from a Zeeman-cooled Na beam and the cloud of atoms are cooled in dark optical molasses. This produces a sample of −1010 atoms, mainly in the F = 1 hyperfine ground state at a temperature of −100 pK. We transfer about 1/3 of these atoms into a static quadrapole trap, trapping the F = 1, mF=−1 state. The quadrapole trap is stiffened to 1000 G/cm to increase the Na density and evaporated for 3 seconds with a linearly swept rf field. The TOP field is then turned on quickly to 150 G, producing a weak TOP trap. Further evaporation is performed by reducing the TOP field to 20 G, bringing in the circle of death. This 15 second evaporation period is followed by 10 seconds of rf induced evaporation, with BEC occuring at a temperature of ~1 μK, producing a condensate of ~3×106 atoms. By subsequently adiabatically expanding the trapping potential, we have reduced the temperature to −50 nK.

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The construction of an apparatus capable of producing Bose-Einstein condensates marks a significant milestone in every experimental cold atom laboratory. In this thesis I describe the development of a system to create a Bose-Einstein condensate of $^{87}Rb$ in a Time-Orbiting Potential trap.\n\nI review the optical and magnetic techniques required to trap and cool an atomic sample under vacuum, motivating our decision to build a double MOT system comprised of a high-pressure ($10^{-9}$ torr) chamber to gather atoms and a low-pressure ($10^{-11}$ torr) chamber to cool atoms to degeneracy.\n\nBy theoretically modeling the atom number and temperature inside the magnetic trap during evaporative cooling I demonstrate a simple approach to determining a cooling path that reaches the transition temperature. By making use of the condensates produced under these non-optimized conditions I determine the heating rate of the condensate in the TOP trap to be $300$ nK/s. I further use the condensates to make a more precise measurement of the TOP trap bias field.\n\nI improve upon the conventional evaporation path used in TOP trap experiments by introducing and optimizing additional bias field compression stages in between RF evaporation ramps. I demonstrate how, by adding these additional stages, the system is capable of reaching the BEC phase transition with a final atom number of $2\\times 10^{5}$. In contrast, RF evaporation after only a single bias field ramp has yielded condensates with only $30\\times 10^3$ atoms.

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Bose–Einstein condensation in a QUIC trap

B Lu, William Arie van Wijngaarden

The apparatus and procedure required to generate a pure Bose-Einstein condensate (BEC) consisting of about half a million 87Rb atoms at a temperature of <60 nK with a phase density of >54 is described. The atoms are first laser cooled in a vapour cell magneto-optical trap (MOT) and subsequently transferred to an ultra-low pressure MOT. The atoms are loaded into a QUIC trap consisting of a pair of quadrupole coils and a Ioffe coil that generates a small finite magnetic field at the trap energy minimum to suppress Majorana transitions. Evaporation induced by an RF field lowers the temperature permitting the transition to BEC to be observed by monitoring the free expansion of the atoms after the trapping fields have been switched off.PACS Nos.: 03.75.Fi, 05.30.Jp, 32.80.Pj, 64.60.i

Canadian Journal of Physics•Feb 1, 2004•17 citations

We have achieved Bose-Einstein condensation (BEC) [1,2] of Na atoms in an anisotropic TOP trap [3]. Our TOP trap has a bias field that rotates (at 20 kHz) in a plane containing the symmetry access of the quadrapole field, resulting in an anisotropic, time-averaged TOP potential with spring constants in the ratio of 1:2:4. A dark spot MOT is loaded from a Zeeman-cooled Na beam and the cloud of atoms are cooled in dark optical molasses. This produces a sample of −1010 atoms, mainly in the F = 1 hyperfine ground state at a temperature of −100 pK. We transfer about 1/3 of these atoms into a static quadrapole trap, trapping the F = 1, mF=−1 state. The quadrapole trap is stiffened to 1000 G/cm to increase the Na density and evaporated for 3 seconds with a linearly swept rf field. The TOP field is then turned on quickly to 150 G, producing a weak TOP trap. Further evaporation is performed by reducing the TOP field to 20 G, bringing in the circle of death. This 15 second evaporation period is followed by 10 seconds of rf induced evaporation, with BEC occuring at a temperature of ~1 μK, producing a condensate of ~3×106 atoms. By subsequently adiabatically expanding the trapping potential, we have reduced the temperature to −50 nK.

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.

Summary form only given. We report the observation of Bose-Einstein condensation of a very large number of /sup 87/Rb atoms in a time orbiting potential (TOP) trap. We found that for such large samples, efficient cooling in the TOP trap is complicated to some extent by undesired losses of atoms at the orbiting magnetic field zero point, but that these difficulties can be overcome. In our experiments, we first load /sup 87/Rb atoms from a slow beam into a magneto-optic atom trap (MOT). Second, we then compress the atoms with a forced dark MOT, further cool them with optical molasses, and optically pump them into the desired state.

The study of BEC in weakly interacting system holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles, and may advance present understanding of superconductivity and superfluidity. By employing several new experimental techniques, the dark SPOT trap, rf induced evaporation, and the optically plugged magnetic trap, the authors were able to observe BEC in a gas of sodium atoms. The striking signature of Bose condensation was the sudden appearance of a bimodal velocity distribution below the critical temperature of {approximately} 2 {mu}K. The distribution consisted of an isotropic thermal distribution and an elliptical core attributed to the expansion of a dense condensate. Condensates contained up to 5 x 10{sup 5} atoms at densities exceeding 10{sup 14} cm{sup {minus}3}. Such high densities are unprecedented in cold atomic gases and open up new possibilities for studying a weakly interacting Bose gas over a broad range of densities and therefore strengths of interaction.

The construction of an apparatus capable of producing Bose-Einstein condensates marks a significant milestone in every experimental cold atom laboratory. In this thesis I describe the development of a system to create a Bose-Einstein condensate of

^{87}Rb

in a Time-Orbiting Potential trap.\n\nI review the optical and magnetic techniques required to trap and cool an atomic sample under vacuum, motivating our decision to build a double MOT system comprised of a high-pressure (

10^{-9}

torr) chamber to gather atoms and a low-pressure (

10^{-11}

torr) chamber to cool atoms to degeneracy.\n\nBy theoretically modeling the atom number and temperature inside the magnetic trap during evaporative cooling I demonstrate a simple approach to determining a cooling path that reaches the transition temperature. By making use of the condensates produced under these non-optimized conditions I determine the heating rate of the condensate in the TOP trap to be

300

nK/s. I further use the condensates to make a more precise measurement of the TOP trap bias field.\n\nI improve upon the conventional evaporation path used in TOP trap experiments by introducing and optimizing additional bias field compression stages in between RF evaporation ramps. I demonstrate how, by adding these additional stages, the system is capable of reaching the BEC phase transition with a final atom number of

2\\times 10^{5}

. In contrast, RF evaporation after only a single bias field ramp has yielded condensates with only

30\\times 10^3

atoms.

The apparatus and procedure required to generate a pure Bose-Einstein condensate (BEC) consisting of about half a million 87Rb atoms at a temperature of <60 nK with a phase density of >54 is described. The atoms are first laser cooled in a vapour cell magneto-optical trap (MOT) and subsequently transferred to an ultra-low pressure MOT. The atoms are loaded into a QUIC trap consisting of a pair of quadrupole coils and a Ioffe coil that generates a small finite magnetic field at the trap energy minimum to suppress Majorana transitions. Evaporation induced by an RF field lowers the temperature permitting the transition to BEC to be observed by monitoring the free expansion of the atoms after the trapping fields have been switched off.PACS Nos.: 03.75.Fi, 05.30.Jp, 32.80.Pj, 64.60.i