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Dissipative Control and Imaging of Cold Atoms

Jeremy J. Thorn

Published Jan 1, 2012• 2 citations
OpenAlex

Abstract

We present an all-optical one-way barrier for cold rubidium 87 atoms. Along with the basic theory on how the barrier works, we describe the experimental setup and demonstrate an actual realization of the barrier. Such a barrier appears at first glance to violate the second law of thermodynamics; we examine that law and show explicitly that it is not violated for a general class of one-way barriers including our particular realization. As our lab is going to continue on to a different set of experiments requiring very sensitive imaging techniques, we finish with the development and application of a theory for comparing electron-multiplying charge-coupled device (EMCCD) cameras.

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We present an all-optical one-way barrier for cold rubidium 87 atoms. Along with the basic theory on how the barrier works, we describe the experimental setup and demonstrate an actual realization of the barrier. Such a barrier appears at first glance to violate the second law of thermodynamics; we examine that law and show explicitly that it is not violated for a general class of one-way barriers including our particular realization. As our lab is going to continue on to a different set of experiments requiring very sensitive imaging techniques, we finish with the development and application of a theory for comparing electron-multiplying charge-coupled device (EMCCD) cameras.
We implemented an optical one-way potential barrier that allows ultracold 87Rb^{87}\text{R}\text{b}87Rb atoms to transmit through when incident on one side of the barrier but reflect from the other. This asymmetric barrier is a realization of Maxwell's demon, which can be employed to produce phase-space compression and has implications for cooling atoms and molecules not amenable to standard laser-cooling techniques. The barrier comprises two focused Gaussian laser beams that intersect the focus of a far-off-resonant single-beam optical dipole trap that holds the atoms. The main barrier beam presents a state-dependent potential to incident atoms, while the repumping barrier beam optically pumps atoms to a trapped state. We investigated the robustness of the barrier asymmetry to changes in the barrier-beam separation, the initial atomic potential energy, the intensity of the second beam, and the detuning of the first beam. We performed simulations of the atomic dynamics in the presence of the barrier, showing that the initial three-dimensional momentum distribution plays a significant role, and that light-assisted collisions are likely the dominant loss mechanism. We also carefully examined the relationship to Maxwell's demon and explicitly accounted for the apparent decrease in entropy for our particular system.
We demonstrate an asymmetric optical potential barrier for ultracold 87Rb atoms using laser light tuned near the D2 optical transition. Such a one-way barrier, where atoms incident on one side are transmitted but reflected from the other, is a realization of Maxwell's demon and has important implications for cooling atoms and molecules not amenable to standard laser-cooling techniques. In our experiment, atoms are confined to a far-detuned dipole trap consisting of a single focused Gaussian beam, which is divided near the focus by the barrier. The one-way barrier consists of two focused laser beams oriented almost normal to the dipole-trap axis. The first beam is tuned to present either a potential well or barrier, depending on the state of the incident atoms. On the reflecting side of the barrier, the second beam optically pumps the atoms to the reflecting (barrier) state, thus producing the asymmetry.
This thesis describes the progress achieved towards two goals. One is the construction of a compact magneto-optical trap (MOT) apparatus, used both as a standalone undergraduate laboratory experiment and as part of a public engagement demonstration of cold atom physics. The setup, which utilises a dichroic atomic vapour laser lock (DAVLL) system plus homemade sideband generation electronics, is capable of trapping and cooling up to 8x10⁷ ⁸⁵Rb atoms in a standard six-beam laser cooling scheme. The second goal is the loading of an ultracold ensemble of ⁸⁷Rb atoms into novel optical trap geometries using a phase-only liquid crystal spatial light modulator (LC SLM) for applications to atomtronics and quantum simulation. Details of a double vacuum chamber apparatus, designed to produce the first Bose-Einstein condensates (BECs) at St Andrews, are provided. The setup incorporates a hybrid trap evaporative cooling scheme towards quantum degeneracy and reliably produces BECs of 1.9 10⁵ ⁸⁷Rb atoms. Two computational techniques, developed during the course of this project, are presented with the aim of creating flexible, smooth holographic optical traps. The first method illuminates a single SLM with overlapped, co-propagating light beams of different wavelengths to create a composite optical pattern for atom manipulation. The second uses conjugate gradient minimisation to create exceptionally high fidelity light patterns in both amplitude and phase for light in the output plane. These techniques, combined with the implementation of a light sheet into the experimental setup, are used to trap atoms in both a ring profile and in a set of narrow waveguides connected with a tunnelling barrier.
Different laser devices working as ``atom diodes'' or ``one-way barriers'' for ultra-cold atoms have been proposed recently. They transmit ground state level atoms coming from one side, say from the left, but reflect them when they come from the other side. We combine a previous model, consisting of the stimulated Raman adiabatic passage (STIRAP) from the ground to an excited state and a state-selective mirror potential, with a localized quenching laser which produces spontaneous decay back to the ground state. This avoids backwards motion, provides more control of the decay process and therefore a more compact and useful device.