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Spectroscopy and evaporative cooling of rubidium atoms in radiofrequency dressed traps

Raghavan K. Easwaran

Published Apr 16, 2009• 0 citations
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Abstract

In this thesis, I describe the spectroscopic investigations and preliminary results of evaporative cooling of rubidium atoms done on a magnetic trap dressed with a radiofrequency field. Unlike the case of a static magnetic trap, where a single radiofrequency photon is used to outcouple atoms we have demonstrated that a multiphoton transition is necessary. This transitions are nearly resonant with the energy spacing of the radiofrequency dressed states. The evaporation efficiency is not the same for all the observed transitions: the transition involving a two photon process is more efficient compared to the others. Preliminary results of evaporative cooling in a radiofrequency dressed trap are discussed. The quantum degeneracy is not reached as it is difficult to reach thermalization due to the strong anisotropy in the trap. A way to render the evaporative cooling more efficient is to increase the horizontal oscillation frequencies. Finally, we discuss a method to load the atoms into a radiofrequency dressed quadrupole trap, leading to large oscillation frequencies in the horizontal directions.

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Spectroscopy and evaporative cooling of rubidium atoms in radiofrequency dressed traps

Raghavan K. Easwaran

In this thesis, I describe the spectroscopic investigations and preliminary results of evaporative cooling of rubidium atoms done on a magnetic trap dressed with a radiofrequency field. Unlike the case of a static magnetic trap, where a single radiofrequency photon is used to outcouple atoms we have demonstrated that a multiphoton transition is necessary. This transitions are nearly resonant with the energy spacing of the radiofrequency dressed states. The evaporation efficiency is not the same for all the observed transitions: the transition involving a two photon process is more efficient compared to the others. Preliminary results of evaporative cooling in a radiofrequency dressed trap are discussed. The quantum degeneracy is not reached as it is difficult to reach thermalization due to the strong anisotropy in the trap. A way to render the evaporative cooling more efficient is to increase the horizontal oscillation frequencies. Finally, we discuss a method to load the atoms into a radiofrequency dressed quadrupole trap, leading to large oscillation frequencies in the horizontal directions.

•Apr 16, 2009•0 citations

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We study the spectroscopy of atoms dressed by a resonant radiofrequency (RF) field inside an inhomogeneous magnetic field and confined in the resulting adiabatic potential. The spectroscopic probe is a second, weak, RF field. The observed line shape is related to the temperature of the trapped cloud. We demonstrate evaporative cooling of the RF-dressed atoms by sweeping the frequency of the second RF field around the Rabi frequency of the dressing field.

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Evaporative Cooling of Atoms to Quantum Degeneracy in an Optical Dipole Trap

Saptarishi Chaudhuri, Sanjukta Roy, C. S. Unnikrishnan

We discuss our experimental results on forced evaporative cooling of cold rubidium 87 Rb atoms to quantum degeneracy in an Optical Dipole Trap. The atoms are first trapped and cooled in a magneto-optical trap (MOT) loaded from a continuous beam of cold atoms [1]. More than 10 10 atoms are trapped in the MOT and then about 10 8 atoms are transferred to a Quasi-Electrostatic Trap (QUEST) formed by tightly focused CO2 laser (λ = 10.6μ m ) beams intersecting at their foci in an orthogonal configuration in the horizontal plane. Before loading the atoms into the dipole trap, the phase-space density of the atomic ensemble was increased making use of sub-doppler cooling at large detuning and the temporal dark MOT technique. In a MOT the phase-space density of the atomic ensemble is six orders of magnitude less than what is required to achieve quantum degeneracy. After transferring atoms into the dipole trap efficiently, phase-space density increases by a factor of 10 3 . Further increase in phase-space density to quantum degeneracy is achieved by forced evaporative cooling of atoms in the dipole trap. The evaporative cooling process involves a gradual reduction of the trap depth by ramping down the trapping laser intensity over a second. The temperature of the cold atomic cloud was measured by time-of-flight (TOF) technique. The spatial distribution of the atoms is measured using absorption imaging. We report results of evaporative cooling in a single beam and in a crossed double-beam dipole traps. Due to the large initial phase space density, and large initial number of atoms trapped, the quantum phase transition occurs after about 600 ms of evaporative cooling in our optimized crossed dipole trap.

Journal of Physics Conference Series•Sep 1, 2007•12 citations

Trapping and cooling of rf-dressed atoms in a quadrupole magnetic field

Olivier Morizot, Carlos L. Garrido Alzar, Paul-Éric Pottie +2 more

We observe the spontaneous evaporation of atoms confined in a bubble-like radio frequency (rf)-dressed trap (Zobay and Garraway 2001 Phys. Rev. Lett. 86 1195; 2004 Phys. Rev. A 69 023605). The atoms are confined in a quadrupole magnetic trap and are dressed by a linearly polarized rf field. The evaporation is related to the presence of holes in the trap, at the positions where the rf coupling vanishes, due to its vectorial character. The final temperature results from a competition between residual heating and evaporation efficiency, which is controlled via the height of the holes with respect to the bottom of the trap. The experimental data are modelled by a Monte Carlo simulation predicting a small increase in phase-space density limited by the heating rate. This increase was within the phase-space density determination uncertainty of the experiment.

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Evaporative cooling in a radio-frequency trap

Carlos L. Garrido Alzar, Hélène Perrin, B. M. Garraway +1 more

A theoretical investigation for implementing a scheme of forced evaporative cooling in radio-frequency (rf) adiabatic potentials is presented. Supposing the atoms to be trapped in a combination of a dc magnetic field and a rf field at frequency ${\ensuremath{\omega}}_{1}$, the cooling procedure is facilitated using a second rf source at frequency ${\ensuremath{\omega}}_{2}$. This second rf field produces a controlled coupling between the spin states dressed by ${\ensuremath{\omega}}_{1}$. The evaporation is then possible in a pulsed or continuous mode. In the pulsed case, atoms with a given energy are transferred into untrapped dressed states by abruptly switching off the ${\ensuremath{\omega}}_{2}$ coupling. In the continuous case, it is possible for energetic atoms to adiabatically follow the doubly dressed states and escape out of the trap. Our results also show that when ${\ensuremath{\omega}}_{1}$ and ${\ensuremath{\omega}}_{2}$ are separated by at least the Rabi frequency associated with ${\ensuremath{\omega}}_{1}$, additional evaporation zones appear which can make this process more efficient.

arXiv:physics/0608088•Physical Review A•Nov 17, 2006•38 citations
In this thesis, I describe the spectroscopic investigations and preliminary results of evaporative cooling of rubidium atoms done on a magnetic trap dressed with a radiofrequency field. Unlike the case of a static magnetic trap, where a single radiofrequency photon is used to outcouple atoms we have demonstrated that a multiphoton transition is necessary. This transitions are nearly resonant with the energy spacing of the radiofrequency dressed states. The evaporation efficiency is not the same for all the observed transitions: the transition involving a two photon process is more efficient compared to the others. Preliminary results of evaporative cooling in a radiofrequency dressed trap are discussed. The quantum degeneracy is not reached as it is difficult to reach thermalization due to the strong anisotropy in the trap. A way to render the evaporative cooling more efficient is to increase the horizontal oscillation frequencies. Finally, we discuss a method to load the atoms into a radiofrequency dressed quadrupole trap, leading to large oscillation frequencies in the horizontal directions.
In this thesis, I describe the spectroscopic investigations and preliminary results of evaporative cooling of rubidium atoms done on a magnetic trap dressed with a radiofrequency field. Unlike the case of a static magnetic trap, where a single radiofrequency photon is used to outcouple atoms we have demonstrated that a multiphoton transition is necessary. This transitions are nearly resonant with the energy spacing of the radiofrequency dressed states. The evaporation efficiency is not the same for all the observed transitions: the transition involving a two photon process is more efficient compared to the others. Preliminary results of evaporative cooling in a radiofrequency dressed trap are discussed. The quantum degeneracy is not reached as it is difficult to reach thermalization due to the strong anisotropy in the trap. A way to render the evaporative cooling more efficient is to increase the horizontal oscillation frequencies. Finally, we discuss a method to load the atoms into a radiofrequency dressed quadrupole trap, leading to large oscillation frequencies in the horizontal directions.
We discuss our experimental results on forced evaporative cooling of cold rubidium 87 Rb atoms to quantum degeneracy in an Optical Dipole Trap. The atoms are first trapped and cooled in a magneto-optical trap (MOT) loaded from a continuous beam of cold atoms [1]. More than 10 10 atoms are trapped in the MOT and then about 10 8 atoms are transferred to a Quasi-Electrostatic Trap (QUEST) formed by tightly focused CO2 laser (λ = 10.6μ m ) beams intersecting at their foci in an orthogonal configuration in the horizontal plane. Before loading the atoms into the dipole trap, the phase-space density of the atomic ensemble was increased making use of sub-doppler cooling at large detuning and the temporal dark MOT technique. In a MOT the phase-space density of the atomic ensemble is six orders of magnitude less than what is required to achieve quantum degeneracy. After transferring atoms into the dipole trap efficiently, phase-space density increases by a factor of 10 3 . Further increase in phase-space density to quantum degeneracy is achieved by forced evaporative cooling of atoms in the dipole trap. The evaporative cooling process involves a gradual reduction of the trap depth by ramping down the trapping laser intensity over a second. The temperature of the cold atomic cloud was measured by time-of-flight (TOF) technique. The spatial distribution of the atoms is measured using absorption imaging. We report results of evaporative cooling in a single beam and in a crossed double-beam dipole traps. Due to the large initial phase space density, and large initial number of atoms trapped, the quantum phase transition occurs after about 600 ms of evaporative cooling in our optimized crossed dipole trap.
We observe the spontaneous evaporation of atoms confined in a bubble-like radio frequency (rf)-dressed trap (Zobay and Garraway 2001 Phys. Rev. Lett. 86 1195; 2004 Phys. Rev. A 69 023605). The atoms are confined in a quadrupole magnetic trap and are dressed by a linearly polarized rf field. The evaporation is related to the presence of holes in the trap, at the positions where the rf coupling vanishes, due to its vectorial character. The final temperature results from a competition between residual heating and evaporation efficiency, which is controlled via the height of the holes with respect to the bottom of the trap. The experimental data are modelled by a Monte Carlo simulation predicting a small increase in phase-space density limited by the heating rate. This increase was within the phase-space density determination uncertainty of the experiment.
A theoretical investigation for implementing a scheme of forced evaporative cooling in radio-frequency (rf) adiabatic potentials is presented. Supposing the atoms to be trapped in a combination of a dc magnetic field and a rf field at frequency ${\ensuremath{\omega}}_{1}$, the cooling procedure is facilitated using a second rf source at frequency ${\ensuremath{\omega}}_{2}$. This second rf field produces a controlled coupling between the spin states dressed by ${\ensuremath{\omega}}_{1}$. The evaporation is then possible in a pulsed or continuous mode. In the pulsed case, atoms with a given energy are transferred into untrapped dressed states by abruptly switching off the ${\ensuremath{\omega}}_{2}$ coupling. In the continuous case, it is possible for energetic atoms to adiabatically follow the doubly dressed states and escape out of the trap. Our results also show that when ${\ensuremath{\omega}}_{1}$ and ${\ensuremath{\omega}}_{2}$ are separated by at least the Rabi frequency associated with ${\ensuremath{\omega}}_{1}$, additional evaporation zones appear which can make this process more efficient.