Browsing by Author "Killian, Thomas C"

Now showing 1 - 12 of 12
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    Design and Construction of an Apparatus for Optically Pumping 87Sr
    (2017-08-10) Hill, Joshua C; Killian, Thomas C
    The ability to control the population of ground-state magnetic sublevels in ultracold atomic gasses of fermionic87Sr is critical for experiments studying quantum magnetism in systems with many spin-components. This thesis describes the design and construction of hardware allowing the manipulation of spin populations via optical pumping. The apparatus operates using light scattering on the 1S0 to 3P1, F=9/2 to F'=9/2 transition in strontium, near 689nm.
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    Dynamics and Dissociation of Collisionally Formed Heavy-Rydberg Ion-Pair States
    (2015-02-27) Wang, Changhao; Dunning, F. Barry; Killian, Thomas C; Brooks, Philip R
    Heavy-Rydberg ion-pair states, molecular quantum systems that are bound by their long-range pure Coulomb attraction, are formed through Rydberg atom collisions with molecules that attach low energy electrons. Collisions of potassium Rydberg atoms K(np) with the attaching species CCl4 and SF6 lead, respectively, to dissociative and non-dissociative electron attachment with subsequent formation of bound ion-pair states K+..Cl- and K+..SF6-. The experimental apparatus used to produce and detect heavy-Rydberg ion-pair states consists of a reaction region and a separate analysis region. It separates a fraction of the product ion pairs from the parent Rydberg atoms, eliminating spurious background signals associated with blackbody-induced and electric-field ionization of Rydberg atoms. A position sensitive detector in the analysis region is employed to measure their velocity, angular, and binding energy distribution and lifetimes. Measurements of the spatial distribution of ion pairs provide information on their velocity and angular distributions, and their binding energy distribution is measured using electric field-induced dissociation. The lifetime of the ion-pair states is influenced by multiple processes including internal-to-translational energy transfer, autodetachment of the electron, and neutralization through charge transfer. The experimental results are compared with the results of a Monte Carlo collision code that models both the initial Rydberg electron attachment and the subsequent evolution and molecular dynamics of the ion pairs. This model highlights the factors such as the kinematics of the Rydberg atom and the attaching particle and energy released in dissociation (in the case of dissociative attachment) that are important in governing ion pair formation. The model calculations are in good agreement with the experimental data.
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    High Resolution Measurement and Modeling of Ion Dynamics in an Ultracold Neutral Plasma
    (2015-04-22) McQuillen, Patrick Clark; Killian, Thomas C; Dunning, Barry F.; Link, Stephan
    Using high-resolution laser-induced fluorescence spectroscopy, ion dynamics in an expanding ultracold neutral plasma (UNP) have been studied in unprecedented detail. The evolution of the ion temperature is of great interest because the ions in an ultracold neutral plasma are strongly coupled, meaning their Coulomb interaction energy exceeds the thermal energy. This leads to novel plasma properties that are hard to describe theoretically. Understanding all the factors that contribute to ion temperature evolution is critical for designing schemes to cool the ions and achieve even stronger Coulomb coupling. This work has also observed several phenomena that have not previously been studied in ultracold plasmas. Ion adiabatic cooling has been observed in a UNP for the first time, resulting in ion temperatures as low as 100mK and up to a tripling of the Coulomb coupling parameter. The importance of electron-ion energy transfer is demonstrated with the first observations of electron-ion collisional heating. Inclusion of this effect into existing numerical models provides better agreement with experimental results. Results of molecular dynamics simulations of equilibrating plasmas have been compared to early time experimental data, which provides a new method of density determination that is much less sensitive to experimental systematics and is accurate to within 10%. This improved certainty in the density allows accurate comparison between data and a theoretical model of the plasma dynamics. The agreement is excellent, but on a hydrodynamic time scale a small amount of extra heat (on order of 100mK), not ascribable to electron-ion collisions or other sources in the model, is detected in the ion component. Potential sources are discussed within as well as proposals for further studies and improvements.
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    Magnetic Confinement of an Ultracold Neutral Plasma
    (2022-04-12) Gorman, Grant M; Killian, Thomas C
    Ultracold neutral plasmas (UCNPs), created by the photoionization of a cold gas, have proven to be an excellent platform for studying plasmas in far more complex environments. Through their ultracold temperatures and dilute densities, UCNPs occupy an exotic regime of plasma physics where the Coulomb energy between neighboring ions exceeds the average thermal energy. Under such conditions of strong coupling, they display a rich assortment of physical phenomena in regimes that are challenging to model theoretically. The application of modern atomic physics techniques provides powerful diagnostics and a high level of control over the ion density, velocity, and internal-state distributions. This allows UCNPs to be sculpted in ways that induce and isolate a wide variety of phenomena, and makes these systems ideal for studies of fundamental plasma physics. Over the last two decades, UCNPs have been on the forefront of experimental study of strongly coupled plasmas, however, recently there has been emerging interest in plasmas that are both magnetized and strongly coupled because the combined effects challenge our understanding of plasma equilibration and transport. Strong coupling introduces short-range spatial correlations between particles that invalidate closure schemes used to derive plasma kinetic equations, while strong magnetization frustrates the use of conventional collision operators because particle gyromotion occurs on length scales relevant for collisions. In recent years, great progress has been made to develop kinetic theories that accurately describe plasmas in asymptotic regimes of either strong coupling or magnetization, but how these regimes merge is still an open question. UCNPs occupy an interesting regime of density and temperature that makes them uniquely suited for the study of plasmas at the intersection of Coulomb coupling and magnetization. Despite this fact, rather few experiments have been conducted on UCNPs in external magnetic fields. This thesis describes the magnetic confinement of an ultracold neutral plasma created at the null of a biconic cusp (or quadrupole) magnetic field. This work utilized Doppler-sensitive laser-induced-fluorescence (LIF) images of the ions. LIF has long been used to probe the ions in UCNPs, but non-uniform magnetic fields complicate LIF due to the spatially varying Zeeman shifts and quantization axis of the ions. This thesis describes the development of a quantitative model of LIF imaging in a non-uniform magnetic field, which uses velocity-resolved rate equations to describe the transfer of ion population due to photon scatter and spontaneous emission. This probe also offers the new ability to measure the electron-spin polarization of the ions, which is inherited from the precursor atoms during photoionization, and should open new possibilities for studying plasma diffusion. The magnetization and confinement of ultracold plasmas promises an open frontier for the UCNP research field. UCNPs confined within biconic cusp fields offer a platform for studying plasmas with changing length scales and dominant physical processes. Beyond fundamental interest, magnetic confinement should circumvent the limitations that the rapid plasma expansion imposes on laser cooling of the ions in a UCNP and magneto-optical forces should enhance plasma confinement, helping stretch the boundaries of Coulomb coupling strength available in UCNP experiments.
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    Narrow Line Cooling of 84Sr
    (2016-12-12) Ding, Roger; Dunning, F Barry; Killian, Thomas C
    Laser cooling has become a powerful tool for producing quantum gases and has enabled the study of a wide range of phenomena only accessible at ultracold temperatures. This thesis will describe the implementation and characterization of a magneto-optical trap (MOT) for laser cooling strontium to microkelvin temperatures operating on the narrow 689nm transition (the “red” MOT). The red MOT is the second cooling stage required to create quantum degenerate gases of strontium, bridging the temperatures and densities produced by the first cooling stage (the 461 nm “blue” MOT) and the shallow conservative traps for evaporation to quantum degeneracy.
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    Observation of antiferromagnetic correlations in the Fermi-Hubbard model
    (2014-11-26) Duarte-Gelvez, Pedro M; Hulet , Randall G; Killian, Thomas C; Wolynes, Peter G
    The Hubbard model contains only the essential ingredients to describe the behavior of strongly interacting electrons moving in a periodic lattice. It describes particles that can tunnel between sites in the lattice and that acquire an on-site interaction energy when two of them occupy the same lattice site. This simple model is a prominent example of how strongly correlated phases emerge from simple Hamiltonians. It gives rise to a Mott-metal insulator transition, and at a density of one particle per site shows an antiferromagnetic ground state. It is also considered to contain the essence of high-temperature superconductivity as observed in the cuprates, a question that remains open due to the difficulty in numerically accessing the solutions of the model at densities different than one particle per site. In this work we have realized the Hubbard model with a spin mixture of ultracold atoms in a simple cubic optical lattice. Atoms in lattices have emerged in the last decade as promising systems in which to perform quantum simulations of condensed matter Hamiltonians. In the laboratory we can create defect-free optical lattice potentials with laser light, and we can control the interactions between the atoms using a magnetic Feshbach resonance. For this work we implemented a novel compensated optical lattice setup, which allows us to control the density of the sample and mitigate the non-adiabaticity in the lattice loading process which often leads to heating of the sample or to out of equilibrium distributions. Using the compensated optical lattice we are able to get closer to the ground state of the Hubbard model than anybody before us has been able to do so with ultracold atoms. To demonstrate this achievement we use spin-sensitive Bragg scattering of light to measure the spin-structure factor, a measure of the antiferromagnetic correlations in the collection of spins. Measurements of the spin-structure factor are compared to theoretical calculations to establish precise thermometry of the atoms in the lattice. We have also studied the in-situ density distribution of the system, which confirms that the temperature of our sample is in a regime where most of the remaining entropy in the system resides in the spin degree of freedom. The results presented here represent an important step in the field of quantum simulation with ultracold atoms. In the future, we expect to further explore and exploit the experimental possibilities opened up by the compensated lattice potential and by light scattering thermometry, with the ultimate goal of addressing the existence of d-wave superfluidity in the Hubbard model.
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    Probing nonlocal correlations with ultralong-range Rydberg molecules
    (2021-06-14) Whalen, Joseph D; Killian, Thomas C
    Ultracold atomic systems provide pristine environments for creating and manipulating strongly-interacting quantum systems. The flexibility and utility of ultracold atomic systems comes at the cost of isolation of the quantum system away from environmental perturbations inside of an ultra-high vacuum chamber. This isolation reduces the number of ways that these systems can be probed, in particular, there are relatively few ways to probe correlations in these systems at mesoscopic length scales. In this thesis we present a new technique for probing correlations in ultracold atomic gases using ultralong-range Rydberg molecules. We will present a description of the relevant experimental apparatus and techniques developed for laser cooling and trapping ultracold mixtures of Sr, and a brief theoretical discussion on the relationship between the pair correlation function $g^{(2)}(R)$ and the excitation rates of ultralong-range Rydberg molecules. We present two publications: the first on using ultralong-range Rydberg molecules to probe $g^{(2)}(R)$ in thermal gases, and the second on the creation of heteronuclear ultralong-range Rydberg molecules from a strongly interacting Bose mixture of $^{88}$Sr and $^{84}$Sr. Finally, we will present conclusions and a brief outlook for the future experiments utilizing this method.
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    Rydberg Molecules and Polarons in Ultracold Strontium Gases
    (2017-08-07) Camargo, Francisco; Killian, Thomas C
    In this work I describe the excitation spectra of Rydberg atoms in ultracold gases of strontium and their decay mechanisms. In a few-body regime, we observe a highly structured spectrum re ecting excitation of ultralong-range molecules consisting of one or more ground-state atoms bound to the Rydberg core in potential wells formed by the Rydbergelectron wave function. In a many-body regime, with hundreds of ground-state atoms within the Rydberg orbital, the Rydberg atoms can be viewed as an impurity in a quantum gas, connecting to important concepts in condensed matter physics. The spectrum for impurity excitation displays signatures of polaronic states, in which the Rydberg atom signi cantly perturbs the density of the background gas. Additionally, decay channels of Rydberg excitations are examined by monitoring the time evolution of Rydberg populations. The measured lifetimes reveal new information on the decay processes of Rydberg molecules and place limits on the time scales over which studies involving Rydberg species in cold, dense atomic gases can be undertaken and limit the coherence times for such measurements.
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    Spectroscopy of ⁸⁷Sr Rydberg Atoms and Molecules
    (2019-09-16) Ding, Roger; Dunning, F B; Killian, Thomas C
    Spectroscopy of ⁸⁷Sr Rydberg Atoms and Molecules This dissertation describes the development of an apparatus to undertake spectroscopic studies of ⁸⁷Sr Rydberg states and their application in the production of ultra-long-range Rydberg molecules (ULRRMs). Most previous spectroscopic studies of strontium have been performed with the bosonic isotope ⁸⁸Sr which has no nuclear spin (I=0), resulting in a relatively simple and well-understood Rydberg excitation spectrum. In contrast, fermionic ⁸⁷Sr has a large nuclear spin (I=9/2) that leads to strong hyperfine interactions which greatly complicate the Rydberg excitation spectrum. In order to understand the Rydberg states in ⁸⁷Sr, two-photon spectroscopy was performed to measure and identify the (5sns)³S and (5snd)³D hyperfine Rydberg states for 30≲n≲99. Working with theory collaborators, a detailed understanding of how the hyperfine interaction affects the Rydberg levels is developed and used to extract revised quantum defects. The detailed understanding of the ⁸⁷Sr hyperfine Rydberg structure was exploited to produce the first ULRRMs in a fermionic gas. Unlike traditional molecular binding mechanisms, ULRRMs comprise of one or more ground-state atoms embedded in the electron cloud of a Rydberg atom with the entire system bound together through the weak Rydberg electron-neutral atom scattering. Therefore, production of ULRRMs is dependent on both the principal quantum number (n) of the parent Rydberg atom and the initial spatial distribution of atoms. At low temperatures, the effects of quantum statistics becomes important and result in bunching (bosons) and antibunching (fermions). These differences in spatial distributions can influence the excitation rates of ULRRMs. Current progress in exploring the role of quantum statistics in the excitation of ULRRMs using cold, dense strontium gases is described, with an emphasis on ⁸⁷Sr, and how such measurements can be used to extract the pair correlation function g⁽²⁾(R).
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    Two-photon photoassociative spectroscopy of strontium-86
    (2019-09-06) Aman, James Allen; Killian, Thomas C
    This dissertation describes two-photon photoassociation to the least-bound vibrational level of the X$^1\Sigma_g^+$ electronic ground state of the $^{86}$Sr$_2$ dimer, which represents the first observation of this naturally occurring halo molecule. We measure the binding energy of this state to be $E_b=83.00(7)(20)$\,kHz. Using the precise determination of the halo state binding energy and the universal theory for a very weakly-bound state on a potential that asymptotes to a van der Waals form, we determine $s$-wave scattering lengths for all strontium isotopes. Our results are consistent with, but substantially more accurate than the previously determined values found from spectroscopic determination of long-range coefficients using other strontium isotopes. With a radial expectation value of $\langle r \rangle \approx 21$\,nm the halo state extends well into the classically forbidden region, which results in a large sensitivity of the dimer binding energy to light near-resonant with a bound-bound transition on the metastable $^1S_0-{^3P_1}$ potential. This suggests that $^{86}$Sr may be a promising candidate for manipulating atomic interactions via optical coupling of the molecular bound states and for probing naturally occurring Efimov states. Furthermore, we observe novel multi-photon spectral loss features due to strong coupling and small detuning of the photoassociation lasers. Numerical simulations of a simple three-level model undergoing a simultaneous two-frequency drive, shows that solutions of the time-dependent Schr\"{o}dinger equation accurately reproduce these additional loss features. A Floquet analysis of this model, yields analytic formulas for predicting the AC Stark shift of the halo molecule. Additionally, we report on the current state and recent modifications of our ultracold neutral strontium apparatus with a particular emphasis on the installation of a $532$\,nm optical lattice. Characterization of this laser system and applications are discussed for further investigation of the strontium-$86$ halo molecule and additional photoassociation experiments.
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    Ultralong-Range Molecules and Rydberg Blockade in Ultracold 84Sr
    (2015-07-30) DeSalvo, Brian J.; Killian, Thomas C; Hulet, Randall G; Brooks, Philip R
    My dissertation describes experiments on two-photon excitation of ultracold Sr to the 3S1 Rydberg series and represents the first experiments exciting Rydberg atoms via an intermediate triplet excited state. Due to the narrow linewidth (7.5 kHz) of the 1S0 – 3P1 transition in Sr, this excitation scheme yields longer coherence times and less loss from the intermediate state compared to methods using the usual dipole allowed transitions. This is advantageous for realizing the possibility of Rydberg dressing, where a small amount of Rydberg character is admixed to ground state atoms allowing for continuously tunable long-range interactions. With this goal in mind, we explore the interplay of Rydberg blockade, Rydberg-Rydberg interactions, and ground-Rydberg interactions in high density, ultracold gases through Autler-Townes spectroscopy and photoassociation of ultralong-range Rydberg molecules.
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    Universality in the Equilibration of Quenched Yukawa One Component Plasmas
    (2016-02-15) Langin, Thomas Karl; Killian, Thomas C
    A Yukawa one-component plasma (OCP) is an idealized model in which particles interact through a 1/r potential with an additional exponential falloff with length scale 1/κ expressed in units of the average interparticle spacing. This model is used to approximately describe a wide range of physical systems, especially strongly coupled plasmas; i.e., those which have Γ≥1, where Γ is the ratio of the Coulomb interaction energy between neighboring particles to the kinetic energy per particle. All dynamics and physical properties in Yukawa OCPs are expected to be universal in κ when expressed in appropriate scaled units. We study a particularly clean realization of the Yukawa OCP, an ultracold neutral plasma (UNP) created by photoionization of an ultracold atomic gas. The rapid quench to a UNP results in an equilibration process known as Disorder Induced Heating (DIH). Even after DIH the plasma is strongly coupled, with Γ~3. During DIH, oscillations in 1/Γ (i.e. scaled Kinetic Energy) occurring at twice the plasma frequency, ω_{pi}, are observed, possibly indicating coupling to collective modes. Universality is demonstrated by showing that 1/Γ(t) curves taken at similar κ collapse onto the same curve when plotted vs. ω_{pi}t. We also compare our results to molecular dynamics (MD) simulations. The utility of the universality is shown by using the MD simulations to determine the density from experimental DIH measurements.