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Precision Measurements and Quantum Sensing Using Cold Atoms

Alisher Duspayev

Published Jan 1, 2024• 0 citations
Publisher

Abstract

Atoms cooled and trapped using laser light have become the backbone of modern quantum physics research. Various new applications based on laser-cooled atoms are in active development. The work described in this dissertation is dedicated to this endeavor. I present experimental and numerical investigations on several fundamental properties of the laser-cooled atoms in different quantum states and how they may translate into applications within several emerging atom-based technologies. First, I describe the precision measurements of the dynamic polarizability and photo-ionization cross-section at 1064-nm light of the $5D_{3/2}$ state in rubidium, which is of interest for the portable atomic clocks. Two different spectroscopic methods are implemented to extract the polarizability from the experimental data. The results are statistically equivalent: -524(17) [-499(59)] atomic units from the first [second] method. The photo-ionization cross section of the $5D_{3/2}$ state at 1064~nm is measured to be 44(1) Mb. The aforementioned photo-ionization cross section is suitable for efficient ion generation and the subsequent application in cold-atom-based ion sources. This is demonstrated in the second study presented here. The ion source employs a novel method to monitor the electric microfields of the ions using the Stark effect of the highly-excited (Rydberg) atoms seeded within the origin of the ion source. Rydberg atoms are extremely sensitive to electric fields and serve as an embedded real-time sensor and tracker of the ion-source performance because of their quasicontinuous creation and detection. The principles of such ion-microfield sensing are discussed and analyzed in detail and can be utilized in any cold-atom-based focused-ion-beam source. The hyperfine structure of the $4D_{3/2}$ state in rubidium is investigated next. The measurement utilizes two-photon optical absorption spectroscopy and a detailed analysis of light shifts. All four hyperfine components are resolved in the experimental data. The magnetic-dipole and electric-quadrupole hyperfine constants are obtained to be 7.419(45) MHz and 4.19(19) MHz, respectively. Future applications of the $4D_{3/2}$ state in optical atomic clocks and Rydberg-atom physics are discussed. Next, I investigate a Rydberg-atom-ion molecule bound by long-range multipolar forces. The molecule is the first example of a bound state between a Rydberg atom and an ion. Strong interactions between these two particles lead to large molecular binding energy (reaching a few GHz) and large binding length (exceeding 1 $mu$m). The Born-Oppenheimer approximation does not hold for the Rydberg-atom-ion molecule because the motion of the Rydberg-atom nucleus is on the same timescale as the dynamics of the ion. The Born-Huang theory is utilized to investigate the resultant nonadiabatic dynamics of the molecule and to make the first fully quantum predictions of its lifetime as a function of the vibrational state and quantum principal number of the Rydberg atom. Lastly, I present a novel technique for atom interferometry that employs uninterrupted three-dimensional confinement and manipulation during the interferometric protocol. The atoms in the existing apparatuses have to follow several-meter-long trajectories along which various detrimental effects such as wave-packet dispersion erase useful phase information. ``Tractor atom interferometry" presented here promises to suppress the harmful effects and improve the interferometers’ sensitivity; provide flexible control over the atomic trajectories; and significantly reduce the experimental footprint. The suitability of this approach for measuring background acceleration (such as due to the gravitational field) is demonstrated. The sensitivity estimations, experimental considerations, and initial laboratory work are discussed.

DOI
10.7302/23729

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Atoms cooled and trapped using laser light have become the backbone of modern quantum physics research. Various new applications based on laser-cooled atoms are in active development. The work described in this dissertation is dedicated to this endeavor. I present experimental and numerical investigations on several fundamental properties of the laser-cooled atoms in different quantum states and how they may translate into applications within several emerging atom-based technologies. First, I describe the precision measurements of the dynamic polarizability and photo-ionization cross-section at 1064-nm light of the 5D3/25D_{3/2}5D3/2​ state in rubidium, which is of interest for the portable atomic clocks. Two different spectroscopic methods are implemented to extract the polarizability from the experimental data. The results are statistically equivalent: -524(17) [-499(59)] atomic units from the first [second] method. The photo-ionization cross section of the 5D3/25D_{3/2}5D3/2​ state at 1064~nm is measured to be 44(1) Mb. The aforementioned photo-ionization cross section is suitable for efficient ion generation and the subsequent application in cold-atom-based ion sources. This is demonstrated in the second study presented here. The ion source employs a novel method to monitor the electric microfields of the ions using the Stark effect of the highly-excited (Rydberg) atoms seeded within the origin of the ion source. Rydberg atoms are extremely sensitive to electric fields and serve as an embedded real-time sensor and tracker of the ion-source performance because of their quasicontinuous creation and detection. The principles of such ion-microfield sensing are discussed and analyzed in detail and can be utilized in any cold-atom-based focused-ion-beam source. The hyperfine structure of the 4D3/24D_{3/2}4D3/2​ state in rubidium is investigated next. The measurement utilizes two-photon optical absorption spectroscopy and a detailed analysis of light shifts. All four hyperfine components are resolved in the experimental data. The magnetic-dipole and electric-quadrupole hyperfine constants are obtained to be 7.419(45) MHz and 4.19(19) MHz, respectively. Future applications of the 4D3/24D_{3/2}4D3/2​ state in optical atomic clocks and Rydberg-atom physics are discussed. Next, I investigate a Rydberg-atom-ion molecule bound by long-range multipolar forces. The molecule is the first example of a bound state between a Rydberg atom and an ion. Strong interactions between these two particles lead to large molecular binding energy (reaching a few GHz) and large binding length (exceeding 1 mumumum). The Born-Oppenheimer approximation does not hold for the Rydberg-atom-ion molecule because the motion of the Rydberg-atom nucleus is on the same timescale as the dynamics of the ion. The Born-Huang theory is utilized to investigate the resultant nonadiabatic dynamics of the molecule and to make the first fully quantum predictions of its lifetime as a function of the vibrational state and quantum principal number of the Rydberg atom. Lastly, I present a novel technique for atom interferometry that employs uninterrupted three-dimensional confinement and manipulation during the interferometric protocol. The atoms in the existing apparatuses have to follow several-meter-long trajectories along which various detrimental effects such as wave-packet dispersion erase useful phase information. ``Tractor atom interferometry" presented here promises to suppress the harmful effects and improve the interferometers’ sensitivity; provide flexible control over the atomic trajectories; and significantly reduce the experimental footprint. The suitability of this approach for measuring background acceleration (such as due to the gravitational field) is demonstrated. The sensitivity estimations, experimental considerations, and initial laboratory work are discussed.
In this dissertation, I explore several aspects of Rydberg physics with laser spectroscopy. The experiments center around the use of a very deep, cavity-generated, one-dimensional optical lattice in a cold atom sample. The optical lattice produces elongated clouds of dense, cold atoms via dipole trapping and is able to generate extreme light shifts in the atoms due to its high intensity. I describe experiments which may be divided into two main classes. In the first, I use the dipole trap to prepare a dense sample of atoms for photoassociation of Rb atom pairs into diatomic Rydberg molecules. The Rydberg molecules are formed by low-energy scattering of the Rydberg electron of one atom from the other atom, which is a ground-state atom. This novel binding mechanism also has relevance for chemical physics. I present binding energy measurements for eight different cases of Rydberg molecules. The binding energies reveal information about electron-atom collisions, such as their scattering lengths, and about the spin couplings present within the molecular system. In the second class of experiments, I introduce two proposed measurements in which the optical lattice is ramped to very high intensity during the laser excitation, creating AC Stark shifts in the transition frequencies. In one measurement, low-lying atomic states are probed. Measurements of the light shifts are expected to yield data on the dynamic polarizability and the photoionization cross section of the 5D3/2D_{3/2}D3/2​-state in rubidium in a 1064-nm light field. I show corresponding preliminary experimental data. The polarizability and photo-ionization cross-section measurements will provide tests of atomic structure theory, which is important both at a fundamental level and for applications such as atomic clocks. In another proposed measurement, Rydberg atoms in the lattice experience ponderomotive shifts and strong mixing of their angular-momentum states, allowing direct optical excitation of the lattice-mixed high-angular-momentum states. I describe the theoretical basis and experimental preparation for spectroscopically characterizing the lattice-mixed states. The ability to optically excite selected high-angular-momentum states without the use of static or RF fields may be useful for quantum information processing and simulation.
Rydberg atoms are highly excited atoms that have large sensitivities to external fields, lending these atoms extraordinary characteristics for fundamental high-precision spectroscopy as well as for electromagnetic-field sensing applications. Recent results on high-precision spectroscopy with cold Rydberg atoms in modulated optical lattices are presented. The discussed approach can serve as a platform for fundamental-physics studies, which include a model-insensitive measurement of the Rydberg constant and a search for wave-like dark matter. As an example for the utility of Rydberg atoms in field sensing applications, an experiment is presented in which the linear Stark effect of cold Rydberg atoms is employed to measure electric fields in cold ion clouds, demonstrating that Rydberg laser spectroscopy allows electrode-free, non-invasive plasma diagnostics.
In this dissertation we report the results of two experimental projects with laser-cooled rubidium atoms: I. Application of Bessel beams for atom optics, and II. Spectroscopic measurements of Rydberg blockade effect. The first part of the thesis is devoted to the development of new elements of atom optics based on blue-detuned high-order Bessel beams. Properties of a 4th order Bessel beam as an atomic guide were investigated for various parameters of the hollow beam, such as the detuning from an atomic resonance, size and the order of the Bessel beam. We extended its application to create more complicated interferometer-type structures by demonstrating a tunnel lock, a novel device that can split an atomic cloud, transport it, delay, and switch its propagation direction between two guides. We reported a first-time demonstration of an atomic beam switch based on the combination of two crossed Bessel beams. We achieved the 30% efficiency of the switch limited by the geometrical overlap between the cloud and the intersection volume of the two tunnels, and investigate the heating processes induced by the switch. We also showed other applications of crossed Bessel beams, such as a 3-D optical trap for atoms confined in the intersection volume of two hollow beams and a splitter of the atomic density. The second part of this dissertation is devoted to the spectroscopic measurements of the Rydberg blockade effect, a conditional suppression of Rydberg excitations depending on the state of a control atom. We assembled a narrow-linewidth, tunable, frequency stabilized laser system at 480 nm to excite laser-cooled rubidium atoms to Rydberg states with a high principal quantum number n ~ 50 through a two-photon transition. We applied the laser system to observe the Autler-Townes splitting of the intermediate 5p state and used the broadening of the resonance features to investigate the enhancement of Rydberg-Rydberg interactions in the presence of an external electric field.
Quantum sensing exploits the sensitivity of quantum systems to their environment, translating external fields and forces into measurable shifts in quantum-mechanical observables. Neutral atoms constitute a highly effective and scalable sensing platform owing to their identical nature and extended coherence times. This dissertation investigates two implementations of atom-based quantum sensing: electric field diagnostics in low-pressure plasma discharges using Rydberg electromagnetically induced transparency (EIT), and inertial sensing via tractor atom interferometry (TAI). Non-invasive electric field measurement is crucial for understanding plasma dynamics without disturbing the local environment, for which high-resolution, coherent spectroscopy of Rydberg states via EIT is a promising candidate. Deploying EIT in plasma environments requires a systematic characterization of collisional effects. To this end, Rydberg-EIT is analyzed in rubidium vapor interacting with plasma precursor gases such as argon, neon, and nitrogen at pressures up to 5 Torr. The observed frequency shifts and collisional broadening are consistent with semiclassical predictions from the Fermi pseudopotential model. Crucially, collisional broadening remains narrower than typical EIT linewidths below 100 mTorr, identifying this pressure range as an operational window for EIT-based plasma field diagnostics. Rydberg-EIT is subsequently applied to a low-pressure, inductively coupled plasma (ICP). In a custom ultra-high-vacuum plasma chamber, Rydberg Stark shifts of rubidium tracers seeded into a continuous argon discharge are mapped via EIT. Prior to plasma ignition, Rydberg spectra exhibit modulation sidebands driven by both electric and magnetic near-fields of the ICP coil. Upon ignition, the bulk plasma screens the rf drive field, suppressing the sidebands. The resulting in-plasma lineshapes are instead governed by V/cm-scale plasma microfields and collisional broadening. Modeling the microfield with a Holtsmark distribution enables the extraction of plasma electron densities as a function of pressure and rf power. By resolving a Rydberg resonance whose lineshape is governed by the plasma environment rather than by spectroscopic limits, these measurements establish a framework for sensitive and non-invasive plasma field diagnostics. The second part of the dissertation investigates inertial sensing via a tractor atom interferometer (TAI). Unlike free-space propagation, TAI utilizes continuous three-dimensional confinement and coherent atomic transport. This mitigates wave packet dispersion, guarantees path closure, permits variable holding times, and enables complex spatial trajectories. The proposed experimental framework employs a Laguerre-Gaussian ``pinwheel" optical lattice to drive multi-loop interferometric sequences. Numerical simulations confirm that TAI implementations may achieve rotation sensitivities comparable to those of established free-space interferometers. In the fast-transport regime required for large enclosed areas, where nonadiabatic excitations would otherwise degrade contrast, quantum optimal control is used to design acceleration sequences that remain fast while preserving coherence. These results illustrate the versatility of neutral atoms as a sensing platform, probing electromagnetic fields through their internal structure and inertial forces through their center-of-mass motion.
In this dissertation I investigate cold Rydberg atoms and molecules in which the angular-momentum character of the quantum states involved strongly influences their properties and dynamics. In the first part I focus on long-range diatomic Rydberg molecules formed by a rubidium D-state Rydberg atom and a second rubidium atom in its ground state. Spectroscopic measurements of molecular binding energies are presented showing the effect of the Rydberg atom size and fine-structure coupling on the molecular potentials. A theoretical model is introduced that takes into account all relevant angular-momentum couplings between the molecular constituents, successfully reproducing experimental observations. Calculations of adiabatic potentials and binding energies, molecular-state lifetimes, electric and magnetic dipole moments are also presented. In the second part, I describe the production and magnetic trapping of cold circular Rydberg atoms. The circular Rydberg atoms are generated out of a cold gas of rubidium using the crossed-fields method and magnetically trapped. The trapping force is employed to induce center-of-mass oscillations of the trapped atom sample. Trap parameters and observed oscillation frequencies are used to measure the magnetic moments of the circular Rydberg atoms. Trap losses and the atomic internal-state evolution in the 300 Kelvin thermal background are also investigated.
Atom optics has been a highly active field of research with many scientific breakthroughs over the past two decades, largely due to successful advances in laser technology, microfabrication techniques, and the development of laser cooling and trapping of neutral atoms. This dissertation details several atom optics experiments with the motivation to develop tools and techniques for precision atom wave interferometry. It provides background information about atom optics and the fundamentals behind laser cooling and trapping, including basic techniques for cold gas thermometry and absorptive detection of atoms. A brief overview of magnetic trapping and guiding in tight wire-based traps is also provided before the experimental details are presented. ❧ We developed a novel laser source of 780 nm light using frequency-doubled 1560 nm fiber amplifier. This laser system provided up to a Watt of tunable frequency stabilized light for two Rb laser cooling and trapping experiments. One system generates Bose-Einstein condensates in an optical trap while the second is based on atom chip magnetic traps. The atom chip system, detailed in this thesis, was designed and built to develop the tools necessary for transport and loading large numbers of cold atoms and explore the potential for guided atom interferometry. Techniques and results from this experiment are presented, including an efficient magnetic transport and loading method to deliver cold atom to atom chip traps. We also developed a modeling tool for the magnetic fields formed by coiled wire geometries, as well as planar wire patterns. These models helped us design traps and determine adiabatic transportation of cold atoms between macro-scale traps and micro-traps formed on atom chips. Having achieved near unity transfer efficiency, we demonstrated that this approach promises to be a consistent method for loading large numbers of atoms into micro-traps. Furthermore, we discuss an in situ imaging technique to investigate magnetic field contours of the traps and the dynamics of atoms within those confining potentials. We also controlled the propagation along the atom chip guides by accelerating atoms with longitudinal magnetic gradients, and investigated an atom focusing scheme. ❧ While the atom chip wire guides perform a role analogous to optical fibers guiding light waves, ""free space"" cold atoms offer great opportunity for precision interferometry. We describe a second on-going atom optics experiment that measures gravity gradients using a pair of atom fountain interferometers separated by one meter. We have demonstrated Gravity Gradiometer resolution down to 4x10⁻⁹ g/m using a 40 kg test mass. The atomic physics subsystem is described in detail, including the vacuum, cold atom source, optics, magnetic coils and shields, and vibration isolation and compensation. The system is designed to be a compact, robust, transportable instrument, taking strides towards future gravity gradient measurements in the field. In the realm of space applications, there has been interest for micro-gravity science experiments aboard the International Space Station, along with instrument development for gravity mapping of Earth and planetary bodies with satellite-based instruments. Furthermore, there are ground-based applications for gravity imaging of local density distributions, precision measurement of gravity, as well as proposals for redefining the kilogram, detecting gravitational waves and determining the Gravitational constant.