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Characterization of a magneto-optical trap of 87Rb atoms

Loparco, Letizia

Published Mar 27, 2025• 0 citations
OpenAlex

Abstract

This thesis presents the optimization and characterization of a magneto-optical trap (MOT) setup for rubidium-87 atoms. The apparatus is designed to provide a source of cold atoms, with temperatures around 100 $\mu$K, near the tip of a hollow-core photonic crystal fiber. The ultimate goal is to load the cold atomic cloud into the fiber to explore the unique properties of this hybrid system. Since the number of captured atoms depends on the interplay between the magnetic field and detuning, a detailed study was conducted to investigate the number of trapped atoms as a function of different detuning values and magnetic field gradients. By comparing two measurement methods, it was determined that the MOT is capable of trapping approximately $10^7$ rubidium atoms. The optimal magnetic field gradient and laser detuning frequency were found to be $B' = 9.32 \, \text{G/cm}$ and $\delta \approx -13$ MHz, respectively. Additionally, the number of captured atoms was found to be strongly dependent on the background pressure inside the MOT chamber. In this regard, it was observed that by closing the rubidium source valve, the pressure can be reduced by up to 80\% over a 24-hour period, and one possible way to measure it could be through the background signal with the magnetic field gradient turned off. Then we analyzed the trap lifetime $\tau$, which is also inversely proportional to the partial pressure of rubidium. Under optimal conditions, we obtained a trap lifetime of $0.31 \, \text{s}$. Furthermore, a temperature estimation of the atomic cloud was performed using the time-of-flight (TOF) method. The velocity distribution along the $x$-axis, which lie in the imaging plane, was captured with a CMOS camera and yielded temperature of $T_x = (252 \pm 20) \, \mu\text{K}$. These results provide essential insights into the optimization of the MOT parameters and lay the foundation for future studies involving the loading of cold atoms into hollow-core photonic crystal fibers.

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Setup of a magneto-optical trap of cold 87Rb atoms in proximity to an hollow core photonic crystal fiber

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In this thesis work the results of optimization and characterization of a magneto-optical trap (MOT) setup for rubidium 87 atoms are reported. The apparatus is aimed to provide a source of cold (about 100 μK) atoms in the proximity of the tip of an hollow core photonic crystal fiber. The final goal is loading the cold atomic cloud into the fiber to study the peculiar properties of this combined system. In particular, the thesis focuses on the achievements obtained in frequency and polarization stability of MOT beams, to determine the most critical aspects of the apparatus that could hinder the reproducible realization of the MOT, in terms of temperature and number of trapped atoms. The final results show that the setup is able to provide sub-MHz frequency stability of the MOT beams, while still lacking of good polarization stability. Indeed, relative fluctuations in number and temperature of trapped atoms, due to the measured polarization instability, are expected to be up to 50% and 70%, respectively. This is thus the critical aspect of the setup which needs improvements to achieve good reproducibility. Based on this analysis, possible solutions to mitigate these effects are proposed for next developments in the experimental apparatus.

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ENGLISH ABSTRACT: A Magneto Optical Trap (MOT) is a configuration formed by three orthogonal pairs of counterpropagating circularly polarized laser beams and a magnetic field gradient. A MOT is used to cool, capture and trap large numbers of atoms in vacuum at very low temperature in K range. In this thesis the development of an experimental setup for realising a MOT of 87Rb atoms is presented. The atomic structure of Rb and the theoretical background of laser cooling and magneto optical trapping was reviewed. The influence of rubidium background pressure in the vacuum system, the laser beam size and the power and frequency on the number of the trapped atoms were studied in literature. The trapping and repumping lasers were characterised experimentally. Six circularly polarised trapping beams with equal power were formed and properly aligned to intersect at the center of the trapping cell. Two optical setups were designed and exploited to investigate and optimise the trapping beam polarisation. The repumping laser beam was successfully aligned and colinearly combined into all the trapping beams. Three different experimental setups for saturated absorption spectroscopy were developed. Saturated absorption spectra showing the hyperfine structure of both 85Rb and 87Rb isotopes were measured and are discussed. Using two saturated absorption spectroscopy setups, the frequencies of both lasers were successfully locked to the trapping and repumping transitions of 87Rb respectively. A rectangular trapping cell was designed and attached to the vacuum system. A pressure of about 10­¯7 mbar was achieved. The magnetic field coils were characterised and affixed on both sides of the cell in an anti-Helmholtz configuration. Setups for imaging and quantification of the 87Rb atoms in the MOT were designed. Finally, the procedures for demonstrating a MOT are presented. In conclusion, the current status of the project is reported, with recommendations for the future work.

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Tirumalasetty Panduranga Mahesh, Nobuyuki Takei Supervisors Dr. Kenji Ohmori, Ayan Banerjee

This thesis reports on the generation of MOT for laser cooling and trapping of Rubidium 87 atoms. Probe-pump technique have been used in Rb gas cell and doppler free absorption peaks has been observed. Peak at the crossover frequency will be locked for laser cooling and repump beams. This locked freuqency used in MOT to trap Rb atoms. The technique of optical molasses is used to cool the atoms and quadrupole magnetic field applied to confine or trap atoms at the center of MOT. Two chambers are constructed for 2D and 3D MOT with different pressure. Rb atoms loaded efficiently from the high flux cold atom beam generated in a compact 2D MOT. This is the first step of Bose Einstein Condensate (BEC). A performance of 2D MOT was tested by another main chamber of 3D MOT contains 3 pairs of counter-propagating beams. Over 5x10⁶ atoms trapped in a 3D MOT with loading rate of 1:8x10⁶ atoms/sec. This experiment provided an excellent starting point for future experiment on Rydberg atoms in dense ultracold quantum gases.

•Apr 1, 2018•0 citations
This thesis presents the optimization and characterization of a magneto-optical trap (MOT) setup for rubidium-87 atoms. The apparatus is designed to provide a source of cold atoms, with temperatures around 100 μ\muμK, near the tip of a hollow-core photonic crystal fiber. The ultimate goal is to load the cold atomic cloud into the fiber to explore the unique properties of this hybrid system. Since the number of captured atoms depends on the interplay between the magnetic field and detuning, a detailed study was conducted to investigate the number of trapped atoms as a function of different detuning values and magnetic field gradients. By comparing two measurement methods, it was determined that the MOT is capable of trapping approximately 10710^7107 rubidium atoms. The optimal magnetic field gradient and laser detuning frequency were found to be B′=9.32 G/cmB' = 9.32 \, \text{G/cm}B′=9.32G/cm and δ≈−13\delta \approx -13δ≈−13 MHz, respectively. Additionally, the number of captured atoms was found to be strongly dependent on the background pressure inside the MOT chamber. In this regard, it was observed that by closing the rubidium source valve, the pressure can be reduced by up to 80% over a 24-hour period, and one possible way to measure it could be through the background signal with the magnetic field gradient turned off. Then we analyzed the trap lifetime τ\tauτ, which is also inversely proportional to the partial pressure of rubidium. Under optimal conditions, we obtained a trap lifetime of 0.31 s0.31 \, \text{s}0.31s. Furthermore, a temperature estimation of the atomic cloud was performed using the time-of-flight (TOF) method. The velocity distribution along the xxx-axis, which lie in the imaging plane, was captured with a CMOS camera and yielded temperature of Tx=(252±20) μKT_x = (252 \pm 20) \, \mu\text{K}Tx​=(252±20)μK. These results provide essential insights into the optimization of the MOT parameters and lay the foundation for future studies involving the loading of cold atoms into hollow-core photonic crystal fibers.
In this thesis work the results of optimization and characterization of a magneto-optical trap (MOT) setup for rubidium 87 atoms are reported. The apparatus is aimed to provide a source of cold (about 100 μK) atoms in the proximity of the tip of an hollow core photonic crystal fiber. The final goal is loading the cold atomic cloud into the fiber to study the peculiar properties of this combined system. In particular, the thesis focuses on the achievements obtained in frequency and polarization stability of MOT beams, to determine the most critical aspects of the apparatus that could hinder the reproducible realization of the MOT, in terms of temperature and number of trapped atoms. The final results show that the setup is able to provide sub-MHz frequency stability of the MOT beams, while still lacking of good polarization stability. Indeed, relative fluctuations in number and temperature of trapped atoms, due to the measured polarization instability, are expected to be up to 50% and 70%, respectively. This is thus the critical aspect of the setup which needs improvements to achieve good reproducibility. Based on this analysis, possible solutions to mitigate these effects are proposed for next developments in the experimental apparatus.
ENGLISH ABSTRACT: A Magneto Optical Trap (MOT) is a configuration formed by three orthogonal pairs of counterpropagating circularly polarized laser beams and a magnetic field gradient. A MOT is used to cool, capture and trap large numbers of atoms in vacuum at very low temperature in K range. In this thesis the development of an experimental setup for realising a MOT of 87Rb atoms is presented. The atomic structure of Rb and the theoretical background of laser cooling and magneto optical trapping was reviewed. The influence of rubidium background pressure in the vacuum system, the laser beam size and the power and frequency on the number of the trapped atoms were studied in literature. The trapping and repumping lasers were characterised experimentally. Six circularly polarised trapping beams with equal power were formed and properly aligned to intersect at the center of the trapping cell. Two optical setups were designed and exploited to investigate and optimise the trapping beam polarisation. The repumping laser beam was successfully aligned and colinearly combined into all the trapping beams. Three different experimental setups for saturated absorption spectroscopy were developed. Saturated absorption spectra showing the hyperfine structure of both 85Rb and 87Rb isotopes were measured and are discussed. Using two saturated absorption spectroscopy setups, the frequencies of both lasers were successfully locked to the trapping and repumping transitions of 87Rb respectively. A rectangular trapping cell was designed and attached to the vacuum system. A pressure of about 10­¯7 mbar was achieved. The magnetic field coils were characterised and affixed on both sides of the cell in an anti-Helmholtz configuration. Setups for imaging and quantification of the 87Rb atoms in the MOT were designed. Finally, the procedures for demonstrating a MOT are presented. In conclusion, the current status of the project is reported, with recommendations for the future work.
Ultracold atoms are a desirable substrate for experiments in quantum optics due to their highly coherent nature. Magneto-Optical Traps (MOTs) are a commonly used apparatus for producing clouds of cold, neutrally charged atoms such as Rubidium. The method relies on the effect of Doppler cooling, where laser light that has been detuned slightly below an atomic resonance results in a net cooling effect on the atoms, and magnetic trapping, where a linear magnetic field gradient results in a position dependent restoring force that traps the desired atoms at a central point. In the UVic AMO lab, we aim to build a MOT that is capable of producing ultracold rubidium clouds with temperatures on the order of 10μK.
A magneto-optical trap (MOT) for rubidium (Rb) atoms has been in operation at the University of San Carlos Optoelectronics Research Laboratory in Cebu City since 1998. The experimental design for this MOT is based on the system described by Wieman et al. (1995). This system formed the backbone of the research which resulted in the first observation of Bose-Einstein condensation (BEC) by the same group in 1995. An essential step for realizing this remarkable result was the accuracy in the timing mechanism of the experiment. To obtain an MOT in which we can switch the laser and magnetic fields with sub-millisecond accuracy, we have put considerable efforts to realize a computer-controlled system. Previous measurements have been used to validate our new results. The realization of a fast switch for the trapping laser creates the opportunity to measure time-of-flight (TOF) signals. These ballistic measurements can be used to infer the temperature of the cloud of trapped atoms. We hope to continue working on this automated system of the MOT towards BEC as a long-term goal of our research group.
We theoretically investigate the process of coupling cold atoms into the core of a hollow-core photonic-crystal optical fiber using a blue-detuned Laguerre-Gaussian beam. In contrast to the use of a red-detuned Gaussian beam to couple the atoms, the blue-detuned hollow beam can confine cold atoms to the darkest regions of the beam, thereby minimizing shifts in the internal states and making the guide highly robust to heating effects. This single optical beam is used as both a funnel and a guide to maximize the number of atoms into the fiber. In the proposed experiment, Rb atoms are loaded into a magneto-optical trap (MOT) above a vertically oriented optical fiber. We observe a gravito-optical trapping effect for atoms with high orbital momentum around the trap axis, which prevents atoms from coupling to the fiber: these atoms lack the kinetic energy to escape the potential and are thus trapped in the laser funnel indefinitely. We find that by reducing the dipolar force to the point at which the trapping effect just vanishes, it is possible to optimize the coupling of atoms into the fiber. Our simulations predict that by using a low-power (2.5 mW) and far-detuned (300 GHz) Laguerre-Gaussian beam with a 20-$\ensuremath{\mu}$m-radius core hollow fiber, it is possible to couple 11% of the atoms from a MOT 9 mm away from the fiber. When the MOT is positioned farther away, coupling efficiencies over 50% can be achieved with larger core fibers.
This thesis reports on the generation of MOT for laser cooling and trapping of Rubidium 87 atoms. Probe-pump technique have been used in Rb gas cell and doppler free absorption peaks has been observed. Peak at the crossover frequency will be locked for laser cooling and repump beams. This locked freuqency used in MOT to trap Rb atoms. The technique of optical molasses is used to cool the atoms and quadrupole magnetic field applied to confine or trap atoms at the center of MOT. Two chambers are constructed for 2D and 3D MOT with different pressure. Rb atoms loaded efficiently from the high flux cold atom beam generated in a compact 2D MOT. This is the first step of Bose Einstein Condensate (BEC). A performance of 2D MOT was tested by another main chamber of 3D MOT contains 3 pairs of counter-propagating beams. Over 5x10⁶ atoms trapped in a 3D MOT with loading rate of 1:8x10⁶ atoms/sec. This experiment provided an excellent starting point for future experiment on Rydberg atoms in dense ultracold quantum gases.