The Department of Defense relies on many platforms that require inertial sensors to operate in Global Positioning System (GPS)–compromised environments to provide or verify navigational data. The current generation of sensors in use are subject to sensitivity limitations and drift errors that can compound significantly over time in GPS-contested areas. Inertial sensors based on atom interferometers could provide navigational data that is significantly more accurate and less prone to drift (and therefore navigational errors). This research builds upon previous work (Manicchia in 2020, Gervis in 2021) in which an atomic interferometer was constructed and studied. In this thesis, I demonstrate a path toward making inertially sensitive measurements with this apparatus. In contrast to the previous work, I use counter-propagating Raman fields that can achieve inertial sensitivity. However, these Raman fields are sensitive to non-trivial transverse velocities of our atom beam. I demonstrate a method to reduce these transverse velocities using an optical molasses, but do not pursue this in the apparatus due to spatial considerations. I explore the difference between Ramsey and spin echo spectroscopies, forms of atomic interference, in co- and counter-propagating configurations, the second of which is inertially sensitive. I also demonstrate that interference is still visible in the counter-propagating geometry, laying the foundation for a fully inertially sensitive interferometer.
The Department of Defense relies on many platforms that require inertial sensors to operate in Global Positioning System (GPS)–compromised environments to provide or verify navigational data. The current generation of sensors in use are subject to sensitivity limitations and drift errors that can compound significantly over time in GPS-contested areas. Inertial sensors based on atom interferometers could provide navigational data that is significantly more accurate and less prone to drift (and therefore navigational errors). This research builds upon previous work (Manicchia in 2020, Gervis in 2021) in which an atomic interferometer was constructed and studied. In this thesis, I demonstrate a path toward making inertially sensitive measurements with this apparatus. In contrast to the previous work, I use counter-propagating Raman fields that can achieve inertial sensitivity. However, these Raman fields are sensitive to non-trivial transverse velocities of our atom beam. I demonstrate a method to reduce these transverse velocities using an optical molasses, but do not pursue this in the apparatus due to spatial considerations. I explore the difference between Ramsey and spin echo spectroscopies, forms of atomic interference, in co- and counter-propagating configurations, the second of which is inertially sensitive. I also demonstrate that interference is still visible in the counter-propagating geometry, laying the foundation for a fully inertially sensitive interferometer.
Inertial navigation systems (INSs) are a redundant form of navigation for modern naval platforms and a primary form in Global Positioning System (GPS) denied environments. Current technologies like ring laser gyroscopes (RLG) are used by INSs to measure rotation and acceleration. These measurements are then used to determine position via dead reckoning. There are, however, limitations to the accuracy and stability of this technology. The fundamental sensitivity of an RLG is determined by the effective mass of the photon, which is very small. RLGs are also prone to measurement drift, leading to errors that compound over time. Atomic interferometers show increases in both accuracy and stability over time. Atomic interferometer measurements are fundamentally tied to the mass of an atom, which is ten orders of magnitude larger than the photon’s effective mass. These systems have significantly reduced measurement drift because atomic parameters, which can be tied to gyroscope parameters like scale factors, do not change over time. We discuss the complications of using atomic interferometers as gyroscopes. Two area-reversed interferometers are needed for an atomic gyroscope to distinguish between rotation and acceleration. We explore the addition of a second atomic beam to form a second interferometer in the NPS apparatus. Both atomic beams are characterized separately to verify optimal apparatus settings before a laser is used to simultaneously detect counter-propagating beams.
Current sensors used for inertial navigation are based on technology with inherent sensitivity limitations. Atom interferometry is a very promising replacement, which could yield many orders of magnitude of improvement in sensitivity, leading to more accurate navigation over longer periods of time relative to current technology. This research investigates the physics of atom interferometry through the construction and characterization of an atomic accelerometer/gyroscope combination. In contrast to current state-of-the-art atomic sensors, which use pulsed cold atom sources and pulsed laser beams, the apparatus investigated relies purely on continuous atomic and laser beams. These differences would result in a sensor that lends itself more readily to smaller sizing, lower power consumption, and reduced complexity. This change in approach to atomic interferometry also introduces challenges resulting from laser and atomic beam divergences and the velocity averaging due to both longitudinal and transverse temperature, which this dissertation shows how to overcome. The work contained herein explores many stages of construction of an atomic gyro including the demonstration and characterization of two-dimensional laser cooling of atoms, stimulated Raman transitions, and Ramsey and spin echo interference. The implications for future research are also outlined and discussed.
Atom-interferometric quantum inertial sensors are promising candidates for inertial navigation systems owing to their high sensitivity and intrinsically low drift. However, their application to next-generation inertial navigation, targeting a drift below 100 m/month, requires exceptionally stringent inertial-sensor bias stability. In conventional Raman atom interferometers based on ground-state hyperfine transitions, suppressing inertial-sensor biases arising from quadratic Zeeman shifts requires highly optimized magnetic shielding. In this work, we propose a Raman atom interferometer based on fine-structure transitions between metastable states of barium, which exhibit a quadratic Zeeman sensitivity approximately three orders of magnitude lower than that of conventional alkali-atom Raman transitions. We show that the proposed scheme increases the allowable spatial gradient of the magnetic-field magnitude by a factor of several tens, thereby relaxing the magnetic-field-control requirements from highly optimized shielding and compensation systems to standard magnetic-field-control techniques. Furthermore, we experimentally demonstrate Mach--Zehnder interference between metastable fine-structure states of barium, validating a key building block of the proposed inertial-sensing scheme.
We present an interferometric inertial sensor that utilizes two counter-propagating atomic beams with transverse two-dimensional cooling. By employing three parallel and spatially aligned Raman laser beams for Doppler-sensitive Raman transitions, we successfully generate inertia-sensitive Mach-Zehnder interference fringes with an interrogation length of 2L = 54 cm. The sensor’s capability to measure rotation and acceleration simultaneously in dynamic environments is validated through comparative analysis with classical sensors under force oscillation in different directions. Additionally, we conduct experiments on a turntable to calibrate the gyroscope’s scaling factor and address nonlinearity. The angular random walk (ARW) and velocity random walk (VRW) of the sensor are 3×10−4 °/√h and 107 μg/√Hz, respectively, with the long-term stability reaching 9 × 10−4 °/h for rotation and 10 μg for acceleration at an integration time of 1000 s.
The presented thesis describes the setting up of an inertial sensitive atom interferometer for navigation using Rubidium 87 atoms, which is able to perform horizontal acceleration measurement. An ultra-high vacuum interferometer chamber is designed and built. The cold atoms are prepared in a compact 2D MOT setup and loaded into the main MOT. These atoms are then cooled down to few K and prepared in a magnetically insensitive ground state using a sequence of optical and microwave pulses, after which atoms are selected in a narrow band of the velocity. A three-pulse Mach-Zehnder ( pi/2 -pi-pi/2) interferometer using stimulated Raman transitions is used to measure the horizontal acceleration. To improve the sensitivity to acceleration, the fringe contrast of the interferometer requires to be optimised. By investigating the intensity profile of the Raman beams using atoms, a procedure is developed to operate the interferometer in an improved condition, which is able to enhance the fringe contrast by a factor of 3. This procedure improves the shot-to-shot sensitivity of the interferometer below 5 um/s^2. In addition, the atom interferometer is compared with a classical state-of-the-art accelerometer.
This dissertation describes the development of a suite of interlinked techniques for precision metrology with matter wave lattices realized in a ceiling-to-floor non-magnetic atom trapping apparatus. This work involves the development of a single-state atom interferometer that relies on the observation of density gratings formed in the atomic ground state of a laser-cooled sample. These gratings, the result of Kapitza-Dirac diffraction of momentum states following a standing wave optical excitation pulse, typically dephase according to their velocity distribution, and motional properties of the sample, including the local value of gravitational acceleration g, are imprinted on the grating contrast and phase. These features can be extracted by coherently backscattering a travelling wave readout field applied within the thermal coherence time of the sample and measuring the grating free-induction decay. Alternatively, a grating echo, resulting from the application of two standing wave pulses separated by t=T_{21}, can be detected using the same backscattering method in the vicinity of the echo time t=2T_{21} on time scales defined by the transit of cold atoms out of the standing wave beam volume. This dissertation presents five main experimental results that impact a new generation of cold atom quantum sensors. Firstly, we investigate a technique for measuring the magnetic moments and sublevel distributions of laser-cooled gases. Secondly, we study optical channell-ing which can be used to increase the scattering efficiency of optical lattices and the sensitivity of grating-echo interferometers. Thirdly, we describe Bragg scattering of a readout pulse from optical lattices and develop a technique to optimize this process in dilute gas lattices, leading to increased signal strengths in echo interferometers. Fourthly, we demonstrate a novel "Ramsey" velocimeter which can measure sample center-of-mass velocity with a precision of 600 µm/s. Finally, we present a measurement of g which represents the first realization of a frequency domain echo-type gravimeter, with a sensitivity to g of 2 parts per million. We outline the basis for improving the precision of these sensors to realize devices capable of measuring g with a sensitivity of 10 parts per billion and probing velocities more precisely than all other atomic velocimeters.