This thesis presents experimental methods and results for comparative measurements of the photoassociation dynamics of two individual ultracold rubidium-85 atoms when geometry of the trapping potential is changed. An asymmetric trap geometry is hypothesized to change the dynamics of two-atom photoassociation, as the asymmetry increases the probability of the two-atom wavefunction forming as a peaked state. Two atoms in an optical trap, of which the symmetry is controlled via a mechanical iris, are exposed to photoassociation light. The results from the asymmetric trap are compared to the photoassociation dynamics of atoms in a symmetric trap. A paired sample t-test determines a statistically significant difference in the two measurements. This t-test yields a p-value no higher than p = 7 × 10−3 , thus it is likely that the two measurements do not come from the same distribution. This aligns with our hypothesis, and is a new, enlightening piece of research done in the field of few-atom physics.
This thesis presents experimental methods and results for comparative measurements of the photoassociation dynamics of two individual ultracold rubidium-85 atoms when geometry of the trapping potential is changed. An asymmetric trap geometry is hypothesized to change the dynamics of two-atom photoassociation, as the asymmetry increases the probability of the two-atom wavefunction forming as a peaked state. Two atoms in an optical trap, of which the symmetry is controlled via a mechanical iris, are exposed to photoassociation light. The results from the asymmetric trap are compared to the photoassociation dynamics of atoms in a symmetric trap. A paired sample t-test determines a statistically significant difference in the two measurements. This t-test yields a p-value no higher than p = 7 × 10−3 , thus it is likely that the two measurements do not come from the same distribution. This aligns with our hypothesis, and is a new, enlightening piece of research done in the field of few-atom physics.
In this thesis, we present experimental methods and results for direct measurements of the photoassociation of two individual ultra-cold rubidium-85 atoms, each prepared in the |F = 2,m_F = −2⟩ ground state. From there two individual rubidium-85 atoms are loaded into separate far-off resonance optical tweezers which are near-deterministically prepared into |F = 2,m_F = −2⟩. The two optical tweezers are then merged and the atoms are exposed to photoassociation light. To ensure the correct frequency of photoassociation light is used a dual cavity locking scheme, capable of generating frequencies with a stability of ±425.7 kHz over a large wavelength range for use in photoassociation spectroscopy, was developed. From there, a light modulation technique is implemented to isolate the photoassociation event from unwanted far-off resonance trap effects, such as the AC Stark shift. By controlling the intensity, frequency, and duration parameters of the photoassociation pulses applied to the rubidium atoms, frequency-dependent, and time-dependent atom loss is observed for a single band of the 0^{+}_{u} series; located at 377.00067±0.00006 THz. The resonance and linewidth data exhibits the same behaviour as photoassociation performed on large atom ensembles within the many-body regime. However, two distinct fast and slow photoassociation rates are observed in contrast to the many-body regime where only a single photoassociation rate constant, K_{PA}, has been reported. This marks a novel physical process which heretofore has not been reported in the field of atomic or molecular physics.
We investigate the photoassociation dynamics of exactly two laser-cooled ^{85}Rb atoms in an optical tweezer and reveal fundamentally different behavior to photoassociation in many-atom ensembles. We observe nonexponential decay in our two-atom experiment that cannot be described by a single rate coefficient and find its origin in our system's pair correlation. This is in stark contrast to many-atom photoassociation dynamics, which are governed by decay with a single rate coefficient. We also investigate photoassociation in a three-atom system, thereby probing the transition from two-atom dynamics to many-atom dynamics. Our experiments reveal additional reaction dynamics that are only accessible through the control of single atoms and suggest photoassociation could measure pair correlations in few-atom systems. It further showcases our complete control over the quantum state of individual atoms and molecules, which provides information unobtainable from many-atom experiments.
This thesis comprises two topics in the field of the ultracold atomic and molecular physics, both involving formation of ultracold 85Rb 2 by photoassociation (PA). Typically, these ground-state molecules are formed mainly in the very highest vibrational levels, rather than the low-v levels that will be required for most future investigations of ultracold molecular physics and chemistry. The first topic of my research is a method for circumventing this problem: resonant enhancement (RE) of photoassociative ground-state molecule production. The key idea of RE is to photoassociate pairs of colliding atoms into an excited state that has a bimodal vibrational probability distribution, with one peak at long range to enhance the photoassociation rate and another at shorter range to enhance radiative decay into deeply bound levels of the ground state. This can be arranged thanks to the strong spin-orbit perturbative interaction between the two 0+u states converging to two different asymptotes, (5s + 5p1/2) and (5s + 5 p3/2). This method enhances ground-state molecule formation in the region well below the dissociation limit. The experimental work described here was done at the University of Connecticut in collaboration with C. P. Koch at the Institut für Theoretische Physik, Freie Universität Berlin, Germany, who performed the theoretical analysis [1].^ The second part of my thesis is the investigation of ultracold molecules in an optical dipole trap. We have constructed a quasi-electrostratic optical trap (QUEST) for 85Rb using a focused CO2 laser [2]. This provides a very high density of ultracold atoms, up to 10 12 cm-3, providing much higher photoassociation rates. It also keeps the molecules trapped once they have formed. The QUEST provides an ample trapping lifetime for both trapped atoms and molecules, yielding about 4-5×106 atoms and about 9×105 high-v molecules. The long lifetime of the trapped molecules and atoms, combined with their high density, allows us to study interactions such as atom-molecule or molecule-molecule collisions in the ultracold regime. A preliminary experiment on Rb2 molecular decay is described. ^
This thesis describes the results of several experiments that studied the photoassociation of an ultracold atomic Rb gas. In the first experiment, we produced ultracold diatomic molecules from an atomic gas via single-color photoassociation. The molecules were detected with resonance-enhanced multiphoton ionization. Trapping of these molecules in a quadrupole magnetic trap, with lifetimes up to 20 seconds, was also demonstrated. In addition, the rate constant for inelastic collisions between the trapped molecules and atoms was determined from measurements of the atomic density dependence of the decay rate of the trapped molecules. In another experiment, stimulated Raman photoassociation of Rb atoms in a Mott insulator state was studied. A Bose-Einstein condensate (BEC) of 87Rb atoms was loaded into a three-dimensional optical lattice formed by the interference pattern of three orthogonal standing wave laser fields. This system constitutes a very good realization of the Bose-Hubbard model, which predicts a quantum phase transition between a superfluid state and a Mott insulator state at a particular lattice height. A time-of-flight imaging method was used to study the state of the atomic gas, and the quantum phase transition was observed at the predicted lattice height. The signature of the phase transition was the disappearance and reappearance of peaks in the image that arose from the interference of atoms originating from different lattice sites. Two coherent laser fields were applied to the gas in its Mott insulating state, and tuned close to a Raman photoassociation resonance, and this resulted in an observable loss of atoms due to the formation of molecules. This transition exhibited a double-peaked spectrum, with one of the peaks arising from photoassociation of atoms in sites containing only two atoms, and the other from sites containing three atoms. Also, the loss of atoms vs. the duration of the Raman photoassociation period was studied, with the lasers tuned to the peak corresponding to two atoms per site. It was found that a central core of the gas, containing about 40 percent of the atoms, exhibited a coherent oscillation between an atomic and molecular quantum gas.
Chemical reactions typically proceed via stochastic encounters between reactants. Going beyond this paradigm, we combined exactly two atoms in a single, controlled reaction. The experimental apparatus traps two individual laser-cooled atoms [one sodium (Na) and one cesium (Cs)] in separate optical tweezers and then merges them into one optical dipole trap. Subsequently, photoassociation forms an excited-state NaCs molecule. The discovery of previously unseen resonances near the molecular dissociation threshold and measurement of collision rates are enabled by the tightly trapped ultracold sample of atoms. As laser-cooling and trapping capabilities are extended to more elements, the technique will enable the study of more diverse, and eventually more complex, molecules in an isolated environment, as well as synthesis of designer molecules for qubits.
In the ultracold regime, where the interactions between atoms become quantum mechanical in nature, we can investigate the fundamental properties of matter. A natural progression from the catalogue of pioneering experiments using ultracold atoms is to extend the size of our quantum system by producing ultracold molecules in prescribed low-energy internal states. Techniques for cold molecule production are split into two methods: direct and indirect cooling. While direct cooling methods have yet to realize ultracold temperatures, collisional relaxation in the molecules leads to low internal energy states. By contrast, indirect cooling — the association of molecules from pre-cooled atoms—has produced a range of molecules at ultracold temperatures; the challenge with this technique is to control the internal state.This thesis concentrates on a technique that is complementary to those already in existence: ultrafast photoassociation. Key to this technique is the formation of time non-stationary wavepackets in the excited-state in order to improve FranckCondon overlap of the excited state with deeply bound ground-state vibrational levels. A pump-probe experiment was designed and built to demonstrate the formation of bound excited-state dimers. In this work we show that the initial state from which the wavepacket originates is of critical importance to the evolution of excited-state population. We find that the internuclear separation of the wavepacket produced in a rubidium magneto-optical trap is too large to observe coherent oscillations in the excited state. The implications of this are discussed along with recommendations for future ultrafast photoassociation experiments. Consequently, a new ultracold atom apparatus was built utilizing magnetic and dipole-force trapping to increase the density of the atomic sample; this apparatus will enable future experiments combining the exciting fields of ultracold matter and ultrafast light.