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Bulletin of the American Physical Society

Towards Photoassociation in 87RB BEC with raman light-induced synthetic gauge fields

David Blasing, Yong Chen

Published Jun 9, 2015• 0 citations
OpenAlex

Abstract

f spins in the F=1 manifold of 87Rb, with the new eigenstates being superpositions of the bare m f states. Some photoassociation channels are allowed or forbidden depending the m f spin of the colliding atoms. We will report the progress in our measurements, with the goal of investigating the role of synthetic gauge elds on the photoassociation process.

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f spins in the F=1 manifold of 87Rb, with the new eigenstates being superpositions of the bare m f states. Some photoassociation channels are allowed or forbidden depending the m f spin of the colliding atoms. We will report the progress in our measurements, with the goal of investigating the role of synthetic gauge elds on the photoassociation process.
In this thesis I will describe my experiments to study spin transport of spin Bose Einstein Condensates (BECs) with synthetic spin-orbit coupling. First I will describe procedures used to reproduce previous experiments with synthetic gauge fields in Bose-Einstein Condensates, from constant vector potentials to synthetic electric fields and synthetic spin-orbit coupling as well as all of the necessary calibration experiments. Next I will describe new experiments measuring spin transport induced by synthetic spin-dependent electric fields in spin-orbit coupled Bose-Einstein Condensates (BECs). The one dimensional (1D) spin-orbit coupling (SOC) is created with counter propagating Raman lasers which couple hyperfine sub-levels and momentum states of 87Rb, allowing us to engineer spin dependent vector potentials. Quickly lowering the Raman laser intensity, decreases the spin-orbit Raman coupling strength (Ω), separates the spin vector potentials and applies opposite synthetic electric fields to the two dressed spin BECs. We allow them to oscillate in opposite directions within the optical trap (exhibiting a spin dipole mode) and measure the time evolution of their momentum and density after time-of-flight (TOF). The damping of the oscillations as the spin BECs collide is found to drastically increase for larger final Raman coupling. Over longer time scales, the bare spins' oscillations damp and thermalize. However, with Raman coupling, dressed spins' oscillations are overdamped and accompanied by rich excitations in the BEC but less thermalization.
Coherent control of reactants remains a long-standing challenge in quantum chemistry. In particular, we have studied laser-induced molecular formation (photoassociation) in a Raman-dressed spin-orbit-coupled ^{87}Rb Bose-Einstein condensate, whose spin quantum state is a superposition of multiple bare spin components. In contrast to the notably different photoassociation-induced fractional atom losses observed for the bare spin components of a statistical mixture, a superposition state with a comparable spin composition displays the same fractional loss on every spin component. We interpret this as the superposition state itself undergoing photoassociation. For superposition states induced by a large Raman coupling and zero Raman detuning, we observe a nearly complete suppression of the photoassociation rate. This suppression is consistent with a model based upon quantum destructive interference between two photoassociation pathways for colliding atoms with different spin combinations. This model also explains the measured dependence of the photoassociation rate on the Raman detuning at a moderate Raman coupling. Our work thus suggests that preparing atoms in quantum superpositions may represent a powerful new technique to coherently control photochemical reactions.
A central aim of this research is to study the dynamics of ultracold atoms in synthetic gauge fields. In this work, we developed a method to optimize the evaporation of ultracold atoms to the Bose-Einstein condensate (BEC) phase. We implement a model of atomic evaporation in a trapping potential, and we find optimal parameters for the trap depth and stiffness during evaporation. Using this model, we achieve a high efficiency of optical evaporation (γ eff = 4.0). Using that BEC, we study the dynamics of the BEC in various light-induced synthetic gauge fields. In particular, we have studied the transition between adiabatic and diabatic transport in a spin-orbit (SO) coupled BEC, and found the behavior to be well understood by the Landau-Zener (LZ) theory. Various parameters of the SO coupled BEC were explored, and we demonstrated the ability to use such LZ transitions as the basis of an atomic transistor. Finally, we created a novel type of 3π spin-orbit coupling for ultracold atoms using modulated Raman coupling. Using the 3π SO coupling eigenlevel structure, we observed a Stueckelberg type interference of the BEC. We developed a model of Stueckelberg type interferometers, and we were able to quantitatively account for the observed interference fringes.
We use a two-photon dressing field to create an effective vector gauge potential for Bose-Einstein-condensed 87Rb atoms in the F=1 hyperfine ground state. These Raman-dressed states are spin and momentum superpositions, and we adiabatically load the atoms into the lowest energy dressed state. The effective Hamiltonian of these neutral atoms is like that of charged particles in a uniform magnetic vector potential whose magnitude is set by the strength and detuning of the Raman coupling. The spin and momentum decomposition of the dressed states reveals the strength of the effective vector potential, and our measurements agree quantitatively with a simple single-particle model. While the uniform effective vector potential described here corresponds to zero magnetic field, our technique can be extended to nonuniform vector potentials, giving nonzero effective magnetic fields.
Ultra-cold quantum gases subjected to light-induced synthetic gauge potentials have become an emergent field of theoretical and experimental studies. Because of the novel application of two-photon Raman transitions, ultra-cold neutral atoms behave like charged particles in magnetic field. The Raman coupling naturally gives rise to an effective spin-orbit interaction which couples the atom’s center-of-mass motion to its selected pseudo-spin degrees of freedom. Combined with unprecedented controllability of interactions, geometry, disorder strength, spectroscopy, and high resolution measurement of momentum distribution, etc., we are truly in an exciting era of ful- filling and going beyond Richard Feynman’s vision of realizing quantum simulators to better understand the quantum mechanical nature of the universe, manifested immensely in the ultra-cold regimes. In this dissertation, we present a collection of theoretical progresses made by the doctoral candidate and his colleagues and collaborators. From the past few years of work, we mainly address three aspects of the synthetic spin-orbit and light field induced coupling in ultracold quantum gases: a) The ground-state physics of single- particle system, two-body bound states, and many-body systems, all of which are subjected to spin-orbit coupling originated from synthetic gauge potentials; b) The symmetry breaking, topological phase transition and quench dynamics, which are conveniently offered by the realized experimental setup; c) The proposal and impli- cations of light field induced dynamical spin-orbit coupling for atoms inside optical cavity. Our work represents an important advancement of theoretical understanding to the active research frontier of ultra-cold atom physics with spin-orbit coupling.