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Canadian Journal of Physics

Transfer of ultracold 87Rb from a QUIC magnetic trap into a far off resonance optical trap

H Ming, William Arie van Wijngaarden

Published Mar 1, 2007• 2 citations
Publisher

Abstract

Ultracold 87Rb atoms were transferred from a QUIC (quadrupole and Ioffe configuration) magnetic trap into a far off resonance optical trap (FORT). FORTs were created by focusing a 150 mW laser beam having a wavelength of 852 nm to a spot having a radius of 20 and 30 µm. A probe laser then passed through the ultracold atom cloud after the magnetic trap was turned off to study the temporal evolution of the optically trapped atoms. Nearly 106 atoms could be transferred into the FORT at temperatures as low as 1 µK with an efficiency as high as 50%. PACS No.: 32.80.Pj

DOI
10.1139/p07-046

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Chinese Physics B•Jul 31, 2009•10 citations
Ultracold 87Rb atoms were transferred from a QUIC (quadrupole and Ioffe configuration) magnetic trap into a far off resonance optical trap (FORT). FORTs were created by focusing a 150 mW laser beam having a wavelength of 852 nm to a spot having a radius of 20 and 30 µm. A probe laser then passed through the ultracold atom cloud after the magnetic trap was turned off to study the temporal evolution of the optically trapped atoms. Nearly 106 atoms could be transferred into the FORT at temperatures as low as 1 µK with an efficiency as high as 50%. PACS No.: 32.80.Pj
A useful tool that can achieve a high level of spin polarization is a circularly polarized far off-resonance dipole force trap (CFORT). This work is the first step in its development; building the necessary components to successfully load stable 39K into a linearly polarized far off-resonance trap (FORT) from a magneto-optical trap, where the atoms are precooled to a temperature of 400 μK. The FORT consists of 700 mW of light from a titanium–sapphire ring laser, focused with a 200 mm achromatic lens to a waist of 35 μm. The light is detuned between 1 and 20 nm to the red of the D1 transition and this gives rise to trap depths in the region of a few milliKelvin. A characterization of the FORT's loading efficiency and lifetime as a function of FORT beam detuning, power, and ellipticity are studied. PACS Nos.: 32.80.Pj, 23.40.–s, 39.25.+k
This dissertation reports on the design, construction, and investigation of a pulsed optical dipole force trap which uses laser light to confine ultracold rubidium (Rb) atoms. Because the laser frequency is detuned far from the atomic resonance frequency, the optical dipole force trap is also called a "far-off-resonance trap" (FORT). The use of pulsed laser light to create an optical trap may find application in expanding the number of atomic species which can be confined. The experiments reported here are principally aimed, however, at understanding the physics of pulsed FORT dynamics in anticipation of using the free electron laser (FEL) at Jefferson Lab (Jlab) to construct a FORT. The Jlab FEL will provide a tunable, high power laser light source enabling the realization of spatially large and/or energetically deep traps which are not presently accessible with table-top laser sources. Here, a mode-locked Nd:YAG laser is used as a pulsed laser source. Since the conservative optical dipole force trap does not cool Rb atoms, ultracold atoms must be loaded into the FORT. The optical dipole force trap is formed at the focus of a Gaussian, 1.06 μm Nd:YAG laser beam which is located at the center of a high vacuum chamber and superimposed onto the center of a pre-cooled Rb atom cloud that has been previously accumulated in a magneto-optic trap (MOT). The performance of the pulsed FORT is compared to a continuous wave mode (cw) FORT which is built using the same laser beam; the operation of the two kinds of FORTs can be switched easily without disturbing the experimental alignment. The dependencies of FORT loading efficiency to FORT and MOT parameters such as FORT laser power, loading time, storage/holding time, detuning of the primary MOT laser frequency, and repump laser intensity are investigated. There are about 1.5×107 ultracold rubidium atoms in the MOT. At 7 Watts FORT laser power, about 8% of the atoms are loaded successfully into the cw FORT and about 5% into the pulsed FORT under similar, but not identical, conditions. In most respects both the cw and pulsed FORTs show comparable behavior. The behavior of both FORTs depends strongly on FORT laser power. As expected, at higher power, more atoms can be loaded into the FORT. Both FORTs also depend on the loading time and holding time.
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 create a Bose-Einstein condensate (BEC) of 87 Rb atoms by runaway evaporative cooling in an optical trap.Two crossed infrared laser beams with a wavelength of 1064 nm are used to form an optical dipole trap.After precooling the atom samples in a quadrupole-Ioffe configuration (QUIC) trap under 1.5 µK by radio-frequency (RF) evaporative cooling, the samples are transferred into the center of the glass cell, then loaded into the optical dipole trap with 800 ms.The pure condensate with up to 1.5×10 5 atoms is obtained over 1.17 s by lowering the power of the trap beams.
Based on our work on single cesium atoms trapped in a large-magnetic-gradient vapour-cell magneto-optical trap (MOT), the signal-to-noise ratio (SNR) is remarkably improved. Also a far-off-resonance optical dipole trap (FORT) formed by a strongly-focused 1064 nm single frequency Nd:YVO 4 laser beam is introduced. One cesium atom is prepared in the MOT, and then it can transfer successfully between the MOT and the FORT which is overlapped with the MOT. Utilizing the effective transfer, the lifetime of single atoms trapped in the FORT is measured to be 6.9 ± 0.3 s. Thus we provide a system where the atomic qubit can be coherently manipulated.