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Canadian Journal of Physics

Calculation of the transfer efficiency between dual magneto-optical traps and simulation of a Ioffe trap for Bose–Einstein condensation

Itay Yavin, Tsoline Mikaelian, Anantharaman Kumarakrishnan

Published Apr 1, 2003• 2 citations
Publisher

Abstract

We consider the problem of transferring a cold atomic cloud from a low-vacuum chamber to an ultra-high-vacuum (UHV) chamber, where it can be recaptured and cooled to the transition temperature for Bose–Einstein condensation (BEC). Our calculation assumes an initial Maxwell–Boltzmann velocity distribution for the thermal cloud and a Gaussian spatial density distribution that is characteristic of magneto-optical traps (MOTs). Using a coordinate transformation we find the density of the recaptured atomic cloud as a function of time. This allows us to investigate the effect of experimental parameters on the transfer efficiency. These parameters include the distance of separation between the two chambers, the duration of the initial on-resonant laser used to push the thermal cloud, and the initial cloud temperature. We also present numerical simulations of the magnetic field due to a simplified Ioffe–Pritchard (IP) trap that has recently been used to obtain BEC using laser-cooling techniques. This trap converts a quadrupole magnetic field into an IP configuration using the magnetic field of a conical solenoid placed orthogonally to the axis of symmetry of a pair of quadrupole coils. Our results are suitable for small experimental groups interested in achieving BEC. PACS No.: 03.75

DOI
10.1139/p03-050

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We consider the problem of transferring a cold atomic cloud from a low-vacuum chamber to an ultra-high-vacuum (UHV) chamber, where it can be recaptured and cooled to the transition temperature for Bose–Einstein condensation (BEC). Our calculation assumes an initial Maxwell–Boltzmann velocity distribution for the thermal cloud and a Gaussian spatial density distribution that is characteristic of magneto-optical traps (MOTs). Using a coordinate transformation we find the density of the recaptured atomic cloud as a function of time. This allows us to investigate the effect of experimental parameters on the transfer efficiency. These parameters include the distance of separation between the two chambers, the duration of the initial on-resonant laser used to push the thermal cloud, and the initial cloud temperature. We also present numerical simulations of the magnetic field due to a simplified Ioffe–Pritchard (IP) trap that has recently been used to obtain BEC using laser-cooling techniques. This trap converts a quadrupole magnetic field into an IP configuration using the magnetic field of a conical solenoid placed orthogonally to the axis of symmetry of a pair of quadrupole coils. Our results are suitable for small experimental groups interested in achieving BEC. PACS No.: 03.75
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
We report the transport of magnetically trapped cold atoms over a large distance. Atoms are captured in a magneto-optical trap (MOT) and loaded into a magnetic quadrupole trap. The quadrupole potential is then moved over a distance of 33 cm into an ultra-high-vacuum (UHV) chamber using a chain of quadrupole coils. By running suitable currents through the quadrupole coil pairs the trapping geometry of the potential is maintained during the transport process, thus minimizing heating of the trapped atom cloud. Magnetically trapped 87Rb{}^{87}\mathrm{Rb}87Rb atoms at an initial temperature of $T=125 \ensuremath{\mu}\mathrm{K}$ were heated by less than $20 \ensuremath{\mu}\mathrm{K}$ during the transport process, and up to 109{10}^{9}109 atoms were transferred into the UHV chamber. Due to the spatial separation of the final magnetic trap and the MOT we have been able to capture atoms in the MOT, while storing magnetically trapped atoms in the UHV chamber.
Since its first observation in 1995, Bose-Einstein condensation (BEC) experiments in dilute gases have been performed with only a few different kinds of magnetic traps. We have performed a BEC experiment with /sup 87/Rb atoms in a Ioffe-Pritchard magnetic potential, which opens up the possibility to study, e.g., Bose-Einstein condensates released in only one dimension. Our trap consists of a clip-like coil, which generates a pure two-dimensional (2D) quadrupole field for radial confinement. The axial confinement is provided by two plane pinch coils inside the clip-coil. These two coils create a bottle field. Two additional Helmholtz coils on the same symmetry axis are used to compensate the homogeneous part of the bottle field. By continuously varying the currents through the coils, the trapping potential can be changed from a 3D spherical harmonic to a 2D linear potential. The change can be done adiabatically or nonadiabatically by switching off the axial confinement slowly, or instantaneously respectively. This opens up the possibility to study 1D nonlinear atom optics. In combination with, e.g., far detuned light sheets, a Fabry-Perot type interferometer for a condensate might be realizable. Our experiments are performed in a vacuum system with two magneto-optical traps, which serves as the source of cold atoms for the magnetic trap. 10/sup 9/ atoms are captured in less than one minute. RF-induced evaporative cooling is used to increase the phase space density of the trapped atomic cloud. The BEC phase transition occurs at a temperature of 550 nK with 10/sup 6/ particles in the trap. Pure condensates with more than 10/sup 5/ atoms were produced.
Summary form only given.Since its first observation in 1995, Bose-Einstein condensation (BEC) experiments in dilute gases have been performed with only a few different kinds of magnetic traps. We have performed a BEC experiment with /sup 87/Rb atoms in a Ioffe-Pritchard magnetic potential, which opens up the possibility to study, e.g., Bose-Einstein condensates released in only one dimension. Our trap consists of a clip-like coil, which generates a pure two-dimensional (2D) quadrupole field for radial confinement. The axial confinement is provided by two plane pinch coils inside the clip-coil. These two coils create a bottle field. Two additional Helmholtz coils on the same symmetry axis are used to compensate the homogeneous part of the bottle field. By continuously varying the currents through the coils, the trapping potential can be changed from a 3D spherical harmonic to a 2D linear potential. The change can be done adiabatically or nonadiabatically by switching off the axial confinement slowly, or instantaneously respectively. This opens up the possibility to study 1D nonlinear atom optics. In combination with, e.g., far detuned light sheets, a Fabry-Perot type interferometer for a condensate might be realizable. Our experiments are performed in a vacuum system with two magneto-optical traps, which serves as the source of cold atoms for the magnetic trap. 10/sup 9/ atoms are captured in less than one minute. RF-induced evaporative cooling is used to increase the phase space density of the trapped atomic cloud. The BEC phase transition occurs at a temperature of 550 nK with 10/sup 6/ particles in the trap. Pure condensates with more than 10/sup 5/ atoms were produced.
We report a novel approach for preparing a Bose–Einstein condensate (BEC) of 87 Rb atoms using an electro-pneumatically driven transfer of atoms into a quadrupole-Ioffe magnetic trap (QUIC trap). More than 5 × atoms from a magneto-optical trap are loaded into a spherical quadrupole trap and then transferred into an Ioffe trap by moving the Ioffe coil towards the center of the quadrupole coil thereby changing the distance between the quadrupole trap center and the Ioffe coil. The transfer efficiency is more than 80%. This approach is different from the conventional approach of loading the atoms into a QUIC trap wherein the spherical quadrupole trap is transformed into a QUIC trap by changing the currents in the quadrupole and the Ioffe coils. The phase space density is then increased by forced rf evaporative cooling to achieve Bose–Einstein condensation of more than 10 5 atoms.
We report the ultra-high efficiency transport of cold 87 Rb atoms using a moving magnetic quadrupole potential generated by three overlapping pairs of fixed coils. The transfer efficiency is better than 97%, which is the highest ever reported to our knowledge. The temperature increase due to heating is less than 10 when the initial cloud temperature is 110 μK. Our setup is similar to the magnetic transferring belt design [ Phys. Rev. A 63 (2001) 031401(R) ], although it is simpler because the push coil is not required. We use it to transport atoms away from a magneto-optical trap to very close to the wall of the glass cell, facilitating future experiments employing three-dimensional optical lattices, high resolution in-situ imaging, and magnetic Feshbach resonances.