arXiv subjects

Alan O. Jamison

Publications and source records attributed to Alan O. Jamison.

At least 19 recordsLinked to original sources

arXiv2025

The Varieties of Schelling Model Experience

The Schelling model is a prototype for agent-based modeling in social systems. We produce a comprehensive analysis of Schelling model rule variants by classifying the space of macroscopic outcomes using phase diagrams. Among 54 rule variants, only 3 phase diagram classes are found, characterized by the number of phase transitions. This classification scheme is found to be robust to the use of sociological and percolation-inspired measures of segregation. The statistical and dynamic drivers of these transitions are elucidated by analyzing the roles of vision, movement criteria, vacancies, the initial state, and rivalry. Schelling's original step function dictating satisfaction is found to be pathological at high thresholds, producing coordination failures as satisfactory sites become increasingly rare. This comprehensive classification gives new insight into the drivers of transitions in the Schelling model and creates a basis for studying more complex Schelling-like models.

cond-mat.stat-mech↗

arXiv2024

Collisional Cooling of Ultracold Molecules

Since the original work on Bose-Einstein condensation, quantum degenerate gases of atoms have allowed the quantum emulation of important systems from condensed matter and nuclear physics, as well as the study of novel many-body states with no analog in other fields of physics. Ultracold molecules in the micro- and nano-Kelvin regimes promise to bring powerful new capabilities to quantum emulation and quantum computing, thanks to their rich internal degrees of freedom compared to atoms. They also open new possibilities for precision measurement and the study of quantum chemistry. Quantum gases of atoms were made possible by collision-based cooling schemes, such as evaporative cooling. For ultracold molecules, thermalization and collisional cooling have not been realized. With other techniques such as supersonic jets and cryogenic buffer gases, studies have been limited to temperatures above 10 mK. Here we show cooling of NaLi molecules at micro- and nano-Kelvin temperatures through collisions with ultracold Na atoms, both prepared in their stretched hyperfine spin states. We find a lower bound on the elastic to inelastic collision ratio between molecules and atoms greater than 50 -- large enough to support sustained collisional cooling. By employing two stages of evaporation, we increase the phase-space density (PSD) of the molecules by a factor of 20, achieving temperatures as low as 220 nK. The favorable collisional properties of a Na and NaLi mixture show great promise for making deeply quantum degenerate dipolar molecules and suggest the potential for such cooling in other systems.

cond-mat.quant-gas↗

arXiv2023

Ab initio calculation of the spectrum of Feshbach resonances in NaLi + Na collisions

We present a combined experimental and theoretical study of the spectrum of magnetically tunable Feshbach resonances in NaLi $(a^3Σ^+)$ $+$ Na collisions. In the accompanying paper, we observe experimentally 8 and 17 resonances occur between $B=0$ and $1400$~G in upper and lower spin-stretched states, respectively. Here, we perform ab initio calculations of the NaLi $+$ Na interaction potential and describe in detail the coupled-channel scattering calculations of the Feshbach resonance spectrum. The positions of the resonances cannot be predicted with realistic uncertainty in the state-of-the-art ab initio potential, but our calculations yield a typical number of resonances that is in near-quantitative agreement with experiment. We show that the main coupling mechanism results from spin-rotation and spin-spin couplings in combination with the anisotropic atom-molecule interaction. The calculations furthermore explain the qualitative difference between the numbers of resonances in either spin state.

physics.atom-ph↗

arXiv2023

Can the dipolar interaction suppress dipolar relaxation?

Magnetic atoms in a thin layer have repulsive interactions when their magnetic moments are aligned perpendicular to the layer. We show experimentally and theoretically how this can suppress dipolar relaxation, the dominant loss process in spin mixtures of highly magnetic atoms. Using dysprosium, we observe an order of magnitude extension of the lifetime, and another factor of ten is within reach based on the models which we have validated with our experimental study. The loss suppression opens up many new possibilities for quantum simulations with spin mixtures of highly magnetic atoms.

cond-mat.quant-gas↗

arXiv2023

Spectrum of Feshbach resonances in NaLi $+$ Na collisions

Collisional resonances of molecules can offer a deeper understanding of interaction potentials and collision complexes, and allow control of chemical reactions. Here, we experimentally map out the spectrum of Feshbach resonances in collisions between ultracold triplet ro-vibrational ground-state NaLi molecules and Na atoms over a range of 1400 G. Preparation of the spin-stretched state puts the system initially into the non-reactive quartet potential. A total of 25 resonances are observed, in agreement with quantum-chemistry calculations using a coupled-channels approach. Although the theory cannot predict the positions of resonances, it can account for several experimental findings and provide unprecedented insight into the nature and couplings of ultracold, strongly interacting complexes. Previous work has addressed only weakly bound complexes. We show that the main coupling mechanism results from spin-rotation and spin-spin couplings in combination with the anisotropic atom-molecule interaction, and that the collisional complexes which support the resonances have a size of 30-40 $a_0$. This study illustrates the potential of a combined experimental and theoretical approach.

physics.atom-ph↗

arXiv2022

Magnetic trapping of ultracold molecules at high density

Trapping ultracold molecules in conservative traps is essential for applications -- such as quantum state-controlled chemistry, quantum simulations, and quantum information processing. These applications require high densities or phase-space densities. We report magnetic trapping of NaLi molecules in the triplet ground state at high density ($\approx 10^{11} \; \rm{cm}^{-3}$) and ultralow temperature ($\approx 1\;{\rm μK}$). Magnetic trapping at these densities allows studies on both atom-molecule and molecule-molecule collisions in the ultracold regime in the absence of trapping light, which has often lead to undesired photo-chemistry. We measure the inelastic loss rates in a single spin sample and spin-mixtures of fermionic NaLi as well as spin-stretched NaLi$+$Na mixtures. We demonstrate sympathetic cooling of NaLi molecules in the magnetic trap by radio frequency evaporation of co-trapped Na atoms and observe an increase in the molecules' phase-space density by a factor of $\approx 16$.

physics.atom-ph↗

arXiv2022

A Feshbach resonance in collisions between ultracold ground state molecules

Collisional resonances are an important tool which has been used to modify interactions in ultracold gases, for realizing novel Hamiltonians in quantum simulations, for creating molecules from atomic gases and for controlling chemical reactions. So far, such resonances have been observed for atom-atom collisions, atom-molecule collisions and collisions between Feshbach molecules which are very weakly bound. Whether such resonances exist for ultracold ground state molecules has been debated due to the possibly high density of states and/or rapid decay of the resonant complex. Here we report a very pronounced and narrow (25 mG) Feshbach resonance in collisions between two ground state NaLi molecules. This molecular Feshbach resonance has two special characteristics. First, the collisional loss rate is enhanced by more than two orders of magnitude above the background loss rate which is saturated at the $p$-wave universal value, due to strong chemical reactivity. Second, the resonance is located at a magnetic field where two open channels become nearly degenerate. This implies the intermediate complex predominantly decays to the second open channel. We describe the resonant loss feature using a model with coupled modes which is analogous to a Fabry-Pérot cavity. Our observations prove the existence of long-lived coherent intermediate complexes even in systems without reaction barriers and open up the possibility of coherent control of chemical reactions.

cond-mat.quant-gas↗

arXiv2022

Control of reactive collisions by quantum interference

In this study, we achieved magnetic control of reactive scattering in an ultracold mixture of $^{23}$Na atoms and $^{23}$Na$^{6}$Li molecules. In most molecular collisions, particles react or are lost near short range with unity probability, leading to the so-called universal rate. By contrast, the Na{+}NaLi system was shown to have only $\sim4\%$ loss probability in a fully spin-polarized state. By controlling the phase of the scattering wave function via a Feshbach resonance, we modified the loss rate by more than a factor of $100$, from far below to far above the universal limit. The results are explained in analogy with an optical Fabry-Perot resonator by interference of reflections at short and long range. Our work demonstrates quantum control of chemistry by magnetic fields with the full dynamic range predicted by our models.

physics.atom-ph↗

arXiv2021

Detection of Gravitational Waves using Parametric Resonance in Bose-Einstein Condensates

An interesting proposal for detecting gravitational waves involves quantum metrology of Bose-Einstein condensates (BECs). We consider a forced modulation of the BEC trap, whose frequency matches that of an incoming continuous gravitational wave. The trap modulation induces parametric resonance in the BEC, which in turn enhances sensitivity of the BEC to gravitational waves. We find that such a BEC detector could potentially be used to detect gravitational waves across several orders of magnitude in frequency, with the sensitivity depending on the speed of sound, size of the condensate, and frequency of the phonons. We outline a possible BEC experiment and discuss the current technological limitations. We also comment on the potential noise sources as well as what is necessary for such a detector to become feasible.

gr-qc↗

arXiv2019

Bloch-bands Picture for Light Pulse Atom Diffraction and Interferometry

We apply a Bloch-bands approach to the analysis of pulsed optical standing wave diffractive elements in optics and interferometry with ultracold atoms. We verify our method by comparison to a series of experiments with Bose-Einstein condensates. The approach provides accurate Rabi frequencies for diffraction pulses and is particularly useful for the analysis and control of diffraction phases, an important systematic effect in precision atom interferometry. Utilizing this picture, we also demonstrate a method to determine atomic band structure in an optical lattice through a measurement of phase shifts in an atomic contrast interferometer.

physics.atom-ph↗

arXiv2019

Enhancing the capture velocity of a Dy magneto-optical trap with two-stage slowing

Magneto-optical traps (MOTs) based on the $626\;{\rm nm}$, $136\;{\rm kHz}$-wide intercombination line of Dy, which has an attractively low Doppler temperature of $3.3\;μ{\rm K}$, have been implemented in a growing number of experiments over the last several years. A challenge in loading these MOTs comes from their low capture velocities. Slowed atomic beams can spread out significantly during free-flight from the Zeeman slower to the MOT position, reducing the fraction of the beam captured by the MOT. Here we apply, for the first time in a Dy experiment, a scheme for enhancing the loading rate of the MOT wherein atoms are Zeeman-slowed to a final velocity larger than the MOT's capture velocity, and then undergo a final stage of slowing by a pair of near-detuned beams addressing the $421\;{\rm nm}$ transition directly in front of the MOT. By reducing the free-flight time of the Zeeman-slowed atomic beam, we greatly enhance the slowed flux delivered to the MOT, leading to more than an order of magnitude enhancement in the final MOT population.

cond-mat.quant-gas↗

arXiv2018

Three-path atom interferometry with large momentum separation

We demonstrate the scale up of a symmetric three-path contrast interferometer to large momentum separation. The observed phase stability at separation of 112 photon recoil momenta ($112\hbar k$) exceeds the performance of earlier free-space interferometers. In addition to the symmetric interferometer geometry and Bose-Einstein condensate source, the robust scalability of our approach relies crucially on the suppression of undesired diffraction phases through a careful choice of atom optics parameters. The interferometer phase evolution is quadratic with number of recoils, reaching a rate as high as $7\times10^7$ radians/s. We discuss the applicability of our method towards a new measurement of the fine-structure constant and a test of QED.

physics.atom-ph↗

arXiv2018

How to dress radio-frequency photons with tunable momentum

We demonstrate how the combination of oscillating magnetic forces and radio-frequency (RF) pulses endows RF photons with tunable momentum. We observe velocity-selective spinflip transitions and the associated Doppler shift. This realizes the key component of purely magnetic spin-orbit coupling schemes for ultracold atoms, which does not involve optical transitions and therefore avoids the problem of heating due to spontaneous emission.

physics.atom-ph↗

arXiv2018

Photoassociation of Ultracold NaLi

We perform photoassociation spectroscopy in an ultracold $^{23}$Na-$^6$Li mixture to study the $c^3Σ^+$ excited triplet molecular potential. We observe 50 vibrational states and their substructure to an accuracy of 20 MHz, and provide line strength data from photoassociation loss measurements. An analysis of the vibrational line positions using near-dissociation expansions and a full potential fit is presented. This is the first observation of the $c^3Σ^+$ potential, as well as photoassociation in the NaLi system.

physics.atom-ph↗

arXiv2018

Two-Photon Spectroscopy of the NaLi Triplet Ground State

We employ two-photon spectroscopy to study the vibrational states of the triplet ground state potential ($a^3Σ^+$) of the $^{23}$Na$^{6}$Li molecule. Pairs of Na and Li atoms in an ultracold mixture are photoassociated into an excited triplet molecular state, which in turn is coupled to vibrational states of the triplet ground potential. Vibrational state binding energies, line strengths, and potential fitting parameters for the triplet ground $a^3Σ^+$ potential are reported. We also observe rotational splitting in the lowest vibrational state.

physics.atom-ph↗

arXiv2018

Long-Lived Ultracold Molecules with Electric and Magnetic Dipole Moments

We create fermionic dipolar $^{23}$Na$^6$Li molecules in their triplet ground state from an ultracold mixture of $^{23}$Na and $^6$Li. Using magneto-association across a narrow Feshbach resonance followed by a two-photon STIRAP transfer to the triplet ground state, we produce $3\,{\times}\,10^4$ ground state molecules in a spin-polarized state. We observe a lifetime of $4.6\,\text{s}$ in an isolated molecular sample, approaching the $p$-wave universal rate limit. Electron spin resonance spectroscopy of the triplet state was used to determine the hyperfine structure of this previously unobserved molecular state.

cond-mat.quant-gas↗

arXiv2017

Observation of the supersolid stripe phase in spin-orbit coupled Bose-Einstein condensates

Supersolidity is an intriguing concept. It combines the property of superfluid flow with the long-range spatial periodicity of solids, two properties which are often mutually exclusive. The original discussion of quantum crystals and supersolidity focuses on solid Helium-4 where it was predicted that vacancies could form dilute weakly interacting Bose-Einstein condensates. In this system, direct observation of supersolidity has been elusive. The concept of supersolidity was then generalized to include other superfluid systems which break the translational symmetry of space. One of such systems is a Bose-Einstein condensate with spin-orbit coupling which has a supersolid stripe phase. Despite several recent studies of this system, the stripe phase has not been observed. Here we report the direct observation of the predicted density modulation of the stripe phase using Bragg reflection. Our work establishes a system with unique symmetry breaking properties. Of future interest is further spatial symmetry breaking through the introduction of vortices, solitons, impurities or disorder.

cond-mat.quant-gas↗

arXiv2016

Spin-Orbit Coupling and Spin Textures in Optical Superlattices

We proposed and demonstrated a new approach for realizing spin orbit coupling with ultracold atoms. We use orbital levels in a double well potential as pseudospin states. Two-photon Raman transitions between left and right wells induce spin-orbit coupling. This scheme does not require near resonant light, features adjustable interactions by shaping the double well potential, and does not depend on special properties of the atoms. A pseudospinor Bose-Einstein condensate spontaneously acquires an antiferromagnetic pseudospin texture which breaks the lattice symmetry similar to a supersolid.

cond-mat.quant-gas↗