Using cold atoms, I probe the energy-level intervals between Rydberg states to high spectroscopic precision. The focus of this work is to lay the groundwork for an updated, highly-controlled measurement of the Rydberg constant using circular-state Rydberg atoms, needed to resolve a recent conflict in proton-radius values that arose in 2010 (the so-called 'proton radius puzzle'). Toward this end, I have demonstrated a fundamentally-new mechanism with which to drive atomic (Rydberg-Rydberg) transitions, utilizing an inelastic (state-changing) ponderomotive interaction. Until now, the multipole-field interaction has been the sole mechanism on which spectroscopy of atomic transitions has been based. In this work, the state-changing ponderomotive interaction ('ponderomotive spectroscopy') is demonstrated by amplitude-modulating an optical standing wave at a transition resonance of a Rydberg atom. At the core of this new spectroscopic technique is the requirement that the light intensity must be modulated in space within the atomic volume, and in time at the desired atomic transition frequency. A Rydberg atom in a modulated optical lattice is very well-suited for a demonstration of this novel type of spectroscopy. Magic-wavelength conditions of this interaction are also identified, as well as a method to drive transitions at harmonics of the modulation. Both of these features of ponderomotive spectroscopy will be utilized to simultaneously trap and probe Rydberg atoms to perform the Rydberg constant measurement. I also present a high-precision measurement of the quantum defect of a high-angular-momentum state of rubidium, the atomic species that will be used in the Rydberg constant measurement. The quantum defects of high-angular-momentum states are needed to determine the effect of the rubidium core polarizability on the Rydberg constant measurement, which is anticipated to be a leading uncertainty. Finally, I present the background and design work for a measurement of the Rydberg constant specifically tailored to help resolve the proton radius puzzle, thereby completing the major preliminary work needed for the performance of that high-precision measurement.
Fundamental physics is concerned with the problems of defining fundamental constants, units of measurement, and searching for new forces and aspects of matter. Usually, the best way to resolve these inquiries is to have a precise knowledge of atomic energy levels and the transition frequencies in order for their quantum states to be manipulated. Spectroscopy is a tool that probes these atomic energy levels by observing the atom's response as the energy of the applied radiation is varied. Over the past century, spectroscopy using electromagnetic radiation has involved direct interactions with the atom's electric and magnetic multipole moments via the
textbfAcdottextbfp term of the minimal coupling Hamiltonian. The work presented in this thesis utilizes such interactions with laser fields and goes a step further by exploring a novel light-matter interaction via the
A2 term. Leading problems in the field of fundamental physics include the commercialization of optical frequency standards with short-term stabilities beating
10−11/sqrttau and the discovery of new science arising from competitive precision measurements of the Rydberg constant and the detection of axion dark matter. Ultracold rubidium atoms subject to periodic optical potentials in standing waves of light known as optical lattices offer solutions to these problems. Rubidium atoms contained in miniaturized, glass vapor cells can be probed with a
778-nm laser in order to obtain the
ket5Srightarrowket5D transition frequency as a portable optical frequency standard. Optical traps can induce shifts that compensate those incurred by the probing lasers and confine the atoms to prevent residual Doppler effects, both of which would otherwise hamper the clock stability. Addressing the other two problems mentioned, circular-state Rydberg atoms that are able to be manipulated with optical lattices introduce a platform for high-resolution spectroscopy of their electronic transitions using very weak microwave fields. The long lifetimes of circular states, which approach 1~s, extend the probing times for a measurement of the Rydberg constant (
10−12 uncertainty) insensitive to the proton charge radius, and the existence of microwaves generated by axion couplings to the electromagnetic field. This dissertation explores the functionality of optical lattices as a tool that can contribute to the advancement of these aforementioned endeavors. In the first implementation of optical lattices for these goals, a measurement of the Rb
ket5D3/2 AC polarizability and photoionization cross section is performed using a cavity-enhanced optical lattice with an ultra-deep depth on the order of
sim105 single-photon recoils. The lattice wavelength is
lambda=1.064 mum, which induces shifts on
ket5D3/2 that can be characterized by the measured scalar polarizability of
alpha5D3/2S=−524(17)~atomic units. I choose this wavelength because it is commonly available as a narrow-linewidth, high-powered laser. At this wavelength, photoionization induces decay of this energy level and broadening of spectral lines as evidence of the significant measured cross section
sigmaPI=44(1)~Mb. The secondary use of lattices as a tool for fundamental physics concerns periodically driven ponderomotive potentials for the effort of initializing the circular states required for a Rydberg constant measurement and dark-matter detection. Experimentally, the principles are discussed in a newly developed lattice phase-modulation technique, where the
ket46S1/2rightarrowket46P1/2 and
ket48S1/2rightarrowket49S1/2 transitions were spectrocscopically measured with this driving mechanism. Additionally, the importance of cancelling stray electric and magnetic fields for this mechanism to be efficient is also demonstrated. The applicability of ponderomotive light-matter interactions for obtaining circular states is discussed in a theoretical proposal of all-optical circularization techniques.
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, 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 5
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.
We develop and demonstrate a spectroscopic method for Rydberg-Rydberg transitions using a phase-controlled and -modulated, standing-wave laser field focused on a cloud of cold ^{85}Rb Rydberg atoms. The method is based on the ponderomotive (A^{2}) interaction of the Rydberg electron, which has less-restrictive selection rules than electric-dipole couplings, allowing us to probe both nS_{1/2}→nP_{1/2} and nS_{1/2}→(n+1)S_{1/2} transitions in first order. Without increase in laser power, third- and fourth-order subharmonic drives are employed to access Rydberg transitions in the 40 to 70 GHz frequency range using widely available optical phase modulators in the Ku band (12 to 18 GHz). Measurements agree well with simulations based on the model we develop. The spectra have prominent Doppler-free components with linewidths ≲200 kHz. The method paves the way for optical Doppler-free high-precision spectroscopy of Rydberg-Rydberg transitions and for spatially selective qubit manipulation with μm-scale resolution in Rydberg-based simulators and quantum computers, provided that magic states are chosen and that the atoms are sufficiently cold.
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.