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Characterizing the Potential Energy of an Atom Trap Through Tomographic Fluorescence Imaging

Tanwa Arpornthip

Published Nov 23, 2016• 0 citations
Publisher

Abstract

We have developed a technique to fully characterize an arbitrary potential energy of an atom trap.The characterization allows one to measure the symmetry of the trap, a determining factor in many trapped atom applications.Created through a magneto-optical trap and evaporative cooling, a cold atom cloud is first loaded into our weakly-confined time-orbiting potential (TOP) trap.The whole atom cloud is then optically pumped into a dark state, an energy level that does not interact with the probe laser light.A selected part of the atom cloud is reactivated by a repump light which optically pump the atoms back into a state that can be probed.The repump light is shaped into a light sheet 168 µm thick.The reactivated region interacts with the probe laser light to create a fluorescence image.Since the light sheet is much thinner than the atom cloud, which is roughly 2 mm wide, the fluorescence image obtained is a cross-section of the atom cloud.A movable light sheet allows us to generate crosssection images of the cloud at different positions.A composite image of all the crosssection images shows the complete potential profile of our atom trap.This is similar to tomographic imaging used in medical imaging.We have verified the technique with two other methods: direct oscillation measurement at varying amplitude and numerical simulation of the atom trap.The technique is able to measure the potential up to the fourth-order terms in spatial coordinates.A complete characterization of the atom trap's symmetry will allow us to develop an atomic Foucault pendulum, a novel application for trapped atoms.5.3 Acceptable level of anharmonicity . . . . . . .

DOI
10.18130/v3jw3c

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We have developed a technique to fully characterize an arbitrary potential energy of an atom trap.The characterization allows one to measure the symmetry of the trap, a determining factor in many trapped atom applications.Created through a magneto-optical trap and evaporative cooling, a cold atom cloud is first loaded into our weakly-confined time-orbiting potential (TOP) trap.The whole atom cloud is then optically pumped into a dark state, an energy level that does not interact with the probe laser light.A selected part of the atom cloud is reactivated by a repump light which optically pump the atoms back into a state that can be probed.The repump light is shaped into a light sheet 168 µm thick.The reactivated region interacts with the probe laser light to create a fluorescence image.Since the light sheet is much thinner than the atom cloud, which is roughly 2 mm wide, the fluorescence image obtained is a cross-section of the atom cloud.A movable light sheet allows us to generate crosssection images of the cloud at different positions.A composite image of all the crosssection images shows the complete potential profile of our atom trap.This is similar to tomographic imaging used in medical imaging.We have verified the technique with two other methods: direct oscillation measurement at varying amplitude and numerical simulation of the atom trap.The technique is able to measure the potential up to the fourth-order terms in spatial coordinates.A complete characterization of the atom trap's symmetry will allow us to develop an atomic Foucault pendulum, a novel application for trapped atoms.5.3 Acceptable level of anharmonicity . . . . . . .
Quantum computing exploits the laws of quantum mechanics to exponentially increase computing rate for certain processes. A realized quantum computer could break encryptions and simulate large quantum systems previously unbreakable and unattainable with classical computers. Neutral atom quantum computing is a viable candidate for building these devices that satisfies four of the five criteria for a successful quantum computer. We are exploring a novel method in creating neutral atom qubits that involves a magneto-optical trap and a dipole trap created in the diffraction pattern behind an array of pinholes. The magneto-optical trap works to cool the atoms and centralize them into a cold-atom cloud in an ultra-high vacuum chamber. Laser light is then projected into the chamber through the array of pinholes, trapping the cooled atoms in light traps. In order to test the efficiency of our trapping method, I have developed an imaging system that monitors the atoms. The system is comprised of a high-speed camera that takes images of the trap while a photodiode measures the fluorescence signal from the cold-atom cloud. These measurements are used to determine the number of atoms successfully trapped, how long they remain trapped, and the trap frequency. This paper presents the details and process of developing the complete imaging system.
We use a near-concentric optical cavity at 1064 nm to generate trapping potentials for cold atoms. The cavity exhibits nondegenerate Hermite-Gaussian modes. Using just a few-milliwatt trap laser power, the cavity readily generates one- and higher-dimensional optical traps that replicate the mode functions. We spectroscopically characterize the optical trapping potentials and laser-cooling limits under continuous loading conditions. We use absorption images to measure atom densities and to compare the spatial profiles of trapped-atom samples with calculated mode functions. Steady-state fluorescence images reveal bright radiation emerging from the ends of the elongated, cavity-trapped atom clouds, providing evidence for radiation guiding.
"In this thesis, laser cooling and trapping techniques are implemented to forrn a cold atom cloud of 85Rb atoms in a magneto-optical trap (Mon configuration. The system produces an atom cloud of~ 1.5 mm in diameter, containing ~ 108 atoms and an average temperature in the sub-Doppler range. Tapered optical fibres (TOFs) are also detailed here, including their guiding properties and the process used for their fabrication. Many possible applications for TOFs have been proposed and one such application is investigated, i.e. the use of TOFs as highly sensitive atom probes. The research results contained within this thesis describe two techniques whereby TOFs can be used as probes. By overlapping the TOF and a cold atom cloud spontaneously emitted photons from the trapped atoms are coupled into the guided mode of the TOF. This then allows characteristics of the cold atom cloud, such as the cloud profile, loading time, lifetime and temperature to be determined. The second technique uses light guided by, but external to, the fibre, i.e. within the evanescent field, to probe the surrounding environment, which may be a cold atom cloud or a background vapour. The work presented also describes a theoretical basis for quantum state preparation using multi-level-atom optics, allowing for the creation of spatial superposition states in which one has perfect control over the population distribution and also the phase relation."
Summary form only given.We have established an experimental and theoretical program focused on understanding photon atom interactions in condensates with the aim of probing the properties of condensates as well as testing our models of the interactions themselves. With a combination of laser and evaporative cooling, we are routinely creating million atom Bose-Einstein condensates of sodium atoms in the newly designed 4D magnet. Our experimental setup incorporates a candlestick atomic beam source, which provides for recirculation of uncollimated sodium atoms, The laser cooling step involves a zero-crossing Zeeman slower, magneto-optic trap, and polarization gradient cooling. The laser-cooled atom cloud is loaded into the 4D magnetic trap and adiabatically compressed to high density and an evaporative cooling process is initiated. Using a RF magnetic resonance technique, hot atoms are ejected through magnetic spin flips. After formation of the condensate in the tightly confining trap, we manipulate the condensate by adiabatically changing the trapping potential and through interaction with photons. We have developed a technique for probing the condensates directly inside the trap using near-resonant absorption imaging. This imaging technique gives high sensitivity to details of the condensate wavefunction and to its surface structure. Images of pure condensates are compared to calculations of the condensate wavefunction based on a Hartree mean field description of the two-body interaction between condensate atoms.
This thesis covers experimental work conducted within two distinct fields of cold-atom physics. The first part describes an experiment capable of spatially-selective dispersive measurements of a cold-atomic system in part or in whole, by virtue of a digital micromirror device (DMD). Atom clouds are trapped in optical tweezers made and controlled by an acousto-optical deflector. Two types of high-precision magnetometers are realised. One is vectorial and reaches a single-shot precision of dB = (100, 200) µG for the field components parallel and transversal to the probe light, exceeding a previous realisation by two orders of magnitude. The other is a scalar magnetometer based on Larmor precession and reaches dB = 30 µG, putting it on equal footing with other state-of-the-art cold-atom magnetometers. The second part is devoted to the characterisation and the first results of a new quantum gas microscope experiment. It features a 0.69 NA microscope objective for high-resolution fluorescence imaging of individual atoms trapped in deep optical lattices. The imaging system is shown to perform near the diffraction limit. By means of DMD-generated off-resonant tight optical tweezers, projected through the high-resolution optics, we can load only a few planes of a co-propagating 1D lattice. In a single realisation of the experiment we acquire multiple fluorescence images, where the objective is translated between images, bringing different planes of the optical lattices in focus. In this way we can tomographically reconstruct the atom distribution in 3D.
Laser cooling and trapping techniques are foundational to modern atomic physics, with applications including atomic clocks and sensors as well as quantum simulators and computers. These applications require stable, well-calibrated laser cooling and accurate determination of atom number and temperature. We develop machine learning techniques for estimating physical properties of laser-cooled potassium-39 atoms in a magneto-optical trap using only the scattered light—i.e., fluorescence—that is intrinsic to the cooling process. In situ snapshot images of fluorescing atomic ensembles directly reveal the spatial structure of these millimeter-scale objects but contain no obvious information regarding internal properties such as the temperature. We first assembled and labeled a balanced dataset sampling 8 × 10 3 different experimental parameters that includes examples with large and dense atomic ensembles, a complete absence of atoms, and everything in between. We describe a range of models trained to predict atom number and temperature solely from fluorescence images. These run the gamut from a poorly performing linear regression model based only on integrated fluorescence to deep neural networks that give number and temperature with fractional uncertainties of 0.1 and 0.2, respectively. These results show that fluorescence images can provide fast, nondestructive diagnostics for cold-atom experiments and may enable real-time experimental feedback.