Publications

Previous work suggested that a passing gravitational wave would produce individual quanta of sound,called phonons, in a BEC or imprint a measureable phase profile on phonons already in the BEC. We consider a way to amplify this effect and then look at practical questions about how such an experiment could really be done. We find that intensity noise in the lasers trapping the BEC presents both technical and fundamental challenges for such measurements.

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.

We describe experiments producing a magneto-optical trap (MOT) for dysprosium (Dy) atoms. Dy has a weak transition that allows for very low MOT temperatures. However, it also reduces the capture velocity of the MOT, making initial loading more difficult than for a MOT using the strong transition. We add a second stage of slowing after the Zeeman slower to solve this low capture velocity issue and demonstrate enhanced loading rates.

Magneto-optical traps (MOTs) based on the 626-nm, 136-kHz-wide intercombination line of Dy, which has an attractively low Doppler temperature of 3.3μK, 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 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-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.

In 2019 the kilogram was officially redefined by fixing the value of Planck’s constant. This follows the precedent set when the speed of light was given a fixed value, thus defining the meter, when combined with the existing definition of the second. We recount a bit of this history and then explore the new kilogram in terms familiar to atomic physicists: Namely, we see how the kilogram is a just a frequency, once you fix the speed of light and Planck’s constant. We discuss a variety of ways one might measure this frequency, from the conceptual to the practical.

We achieve, for the first time, collisional cooling of molecules below 100mK. We cool triplet NaLi molecules using a bath of Na atoms down to 240nK, showing a 20-fold increase in phase-space density. This behavior is surprising since Na + NaLi is a chemically reactive system not expected to show collisional stability of any kind, much less show stability sufficient to achieve strong cooling. This work sets the stage for achieving deep quantum degeneracy in molecular systems.

Since the original work on Bose–Einstein condensation1,2, the use of quantum degenerate gases of atoms has enabled the quantum emulation of important systems in condensed matter and nuclear physics, as well as the study of many-body states that have no analogue in other fields of physics3. Ultracold molecules in the micro- and nanokelvin regimes are expected to bring powerful capabilities to quantum emulation4 and quantum computing5, owing to their rich internal degrees of freedom compared to atoms, and to facilitate precision measurement and the study of quantum chemistry6. Quantum gases of ultracold atoms can be created using collision-based cooling schemes such as evaporative cooling, but thermalization and collisional cooling have not yet been realized for ultracold molecules. Other techniques, such as the use of supersonic jets and cryogenic buffer gases, have reached temperatures limited to above 10 millikelvin7,8. Here we show cooling of NaLi molecules to micro- and nanokelvin temperatures through collisions with ultracold Na atoms, with both molecules and atoms prepared in their stretched hyperfine spin states. We find a lower bound on the ratio of elastic to inelastic molecule–atom collisions that is greater than 50—large enough to support sustained collisional cooling. By employing two stages of evaporation, we increase the phase-space density of the molecules by a factor of 20, achieving temperatures as low as 220 nanokelvin. The favourable collisional properties of the Na–NaLi system could enable the creation of deeply quantum degenerate dipolar molecules and raises the possibility of using stretched spin states in the cooling of other molecules.

Summary coming soon!

We demonstrate how the combination of oscillating magnetic forces and radio-frequency (rf) pulses endows rf photons with tunable momentum. We observe velocity-selective spin-flip transitions and the associated Doppler shift. Recoil-dressed photons are a promising tool for measurements and quantum simulations, including the realization of gauge potentials and spin-orbit coupling schemes which do not involve optical transitions.

Summary coming soon!

We apply a Bloch-band 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.

Summary coming soon!

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 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 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 7×107  rad/s. We discuss the applicability of our method towards a new measurement of the fine-structure constant and a test of QED.

Summary coming soon!

We employ two-photon spectroscopy to study the vibrational states of the triplet ground state potential (a3Σ+) of the 23Na6Li 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 a3Σ+ potential are reported. We also observe rotational splitting in the lowest vibrational state.

Summary coming soon!

We perform photoassociation spectroscopy in an ultracold 23Na–6Li mixture to study the c3Σ+ 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 c3Σ+ potential, as well as photoassociation in the NaLi system.

Summary coming soon!

We create fermionic dipolar 23Na6Li molecules in their triplet ground state from an ultracold mixture of 23Na and 6Li. Using magnetoassociation across a narrow Feshbach resonance followed by a two-photon stimulated Raman adiabatic passage to the triplet ground state, we produce 3×104 ground state molecules in a spin-polarized state. We observe a lifetime of 4.6 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.

Summary coming soon!

Supersolidity combines superfluid flow with long-range spatial periodicity of solids1, two properties that are often mutually exclusive. The original discussion of quantum crystals2 and supersolidity focused on solid 4He and triggered extensive experimental efforts3,4 that, instead of supersolidity, revealed exotic phenomena including quantum plasticity and mass supertransport4. The concept of supersolidity was then generalized from quantum crystals to other superfluid systems that break continuous translational symmetry. Bose–Einstein condensates with spin–orbit coupling are predicted to possess a stripe phase5,6,7 with supersolid properties8,9. Despite several recent studies of the miscibility of the spin components of such a condensate10,11,12, the presence of stripes has not been detected. Here we observe the predicted density modulation of this stripe phase using Bragg reflection (which provides evidence for spontaneous long-range order in one direction) while maintaining a sharp momentum distribution (the hallmark of superfluid Bose–Einstein condensates). Our work thus establishes a system with continuous symmetry-breaking properties, associated collective excitations and superfluid behaviour.

Summary coming soon!

We propose and demonstrate 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.

Summary coming soon!

We have developed a dual-axis ytterbium (Yb) vapor cell and used it to simultaneously address the two laser cooling transitions in Yb at wavelengths 399 nm and 556 nm, featuring the disparate linewidths of 2π × 29 MHz and 2π × 182 KHz, respectively. By utilizing different optical paths for the two wavelengths, we simultaneously obtain comparable optical densities suitable for saturated absorption spectroscopy for both the transitions and keep both the lasers frequency stabilized over several hours. We demonstrate that by appropriate control of the cell temperature profile, two atomic transitions differing in relative strength across a large range of over three orders of magnitude can be simultaneously addressed, making the device adaptable to a variety of spectroscopic needs. We also show that our observations can be understood with a simple theoretical model of the Yb vapor.

Summary coming soon!

Using a three-path contrast interferometer (CI) geometry and laser-pulse diffraction gratings, we create a matter-wave interferometer with ytterbium (Yb) atoms. We present advances in contrast interferometry relevant to high-precision measurements. By comparing to a traditional atom interferometer, we demonstrate the immunity of the CI to vibrations for long interaction times (>20ms). We characterize and demonstrate control over the two largest systematic effects for a high-precision measurement of the fine-structure constant via photon recoil with our interferometer: diffraction phases and atomic interactions. Diffraction phases are an important systematic for most interferometers using large-momentum transfer beam splitters; atomic interactions are a key concern for any Bose-Einstein condensate (BEC) interferometer. Finally, we consider the prospects for a future subpart per billion photon recoil measurement using a Yb CI.

Summary coming soon!

This dissertation describes the creation of the first matter-wave interferometer using ytterbium (Yb) atoms. Most of the experiments focus on a contrast interferometer geometry with a Bose-Einstein condensate (BEC) as source. The recoil frequency of the 174-Yb atom is measured with this interferometer. The recoil frequency of an atom is part of a set of precision measurements that together give a value for the fine structure constant. The experimental results of this dissertation lay the groundwork for a future sub part-per-billion (ppb) precision measurement of the Yb recoil frequency. The contrast interferometry technique is extended to substantially longer times scales than those achieved in previous experiments. A measurement at the ~10 parts-per-million level is made. Systematic effects and statistical scaling are studied and found to be compatible with the desired sub-ppb precision for a future measurement. Such a measurement requires a detailed theoretical study of possible systematic shifts to the measured value. A substantial portion of this dissertation consists of this analysis, carried out in sufficient generality as to guide future sub-ppb level measurements. In addition to a large number of possible systematic shifts due to well-understood physics, two more complex effects are identified and studied: Diffraction phases and atom-atom interactions.

Summary coming soon!

Considering the existence of old neutron stars puts strong limits on the dark matter-nucleon cross section for bosonic asymmetric dark matter. Key to these bounds is formation of a Bose-Einstein condensate (BEC) of the asymmetric dark matter particles. We consider the effects of the host neutron star’s gravitational field on the BEC transition. We find this substantially shifts the transition temperature and so strengthens the bounds on cross section. In particular, for the well-motivated mass range of 5–15 GeV, we improve previous bounds by an order of magnitude.

Summary coming soon!

Quantum-degenerate mixtures of one-electron and two-electron atoms form the starting point for studying few- and many-body physics of mass-imbalanced pairs as well as the production of paramagnetic polar molecules. We recently reported the achievement of dual-species quantum degeneracy of a mixture of lithium and ytterbium atoms. Here we present details of the key experimental steps for the all-optical preparation of these mixtures. Further, we demonstrate the use of the magnetic field gradient tool to compensate for the differential gravitational sag of the two species and control their spatial overlap.

Summary coming soon!

We present investigations of the formation rate and collisional stability of lithium Feshbach molecules in an ultracold three-component mixture composed of two resonantly interacting fermionic 6Li spin states and bosonic 174Yb. We observe long molecule lifetimes (>100 ms) even in the presence of a large ytterbium bath and extract reaction rate coefficients of the system. We find good collisional stability of the mixture in the unitary regime, opening new possibilities for studies and probes of strongly interacting quantum gases in contact with a bath species.

Summary coming soon!

We present theoretical tools for predicting and reducing the effects of atomic interactions in Bose-Einstein condensate (BEC) interferometry experiments. To address mean-field shifts during free propagation, we derive a robust scaling solution that reduces the three-dimensional Gross-Pitaevskii equation to a set of three simple differential equations valid for any interaction strength. To model the other common components of a BEC interferometer—condensate splitting, manipulation, and recombination—we generalize the slowly varying envelope reduction, providing both analytic handles and dramatically improved simulations. Applying these tools to a BEC interferometer to measure the fine structure constant, α [S. Gupta, K. Dieckmann, Z. Hadzibabic, and D. E. Pritchard, Phys. Rev. Lett. 89, 140401 (2002)], we find agreement with the results of the original experiment and demonstrate that atomic interactions do not preclude measurement to better than part-per-billion accuracy, even for atomic species with relatively large scattering lengths. These tools help make BEC interferometry a viable choice for a broad class of precision measurements.

Summary coming soon!

We have produced a quantum degenerate mixture of fermionic alkali-metal 6Li and bosonic spin-singlet 174Yb gases. This was achieved using sympathetic cooling of lithium atoms by evaporatively cooled ytterbium atoms in a far-off-resonant optical dipole trap. We observe the coexistence of Bose-condensed (T/Tc≃0.8) 174Yb with 2.3×104 atoms and Fermi degenerate (T/TF≃0.3) 6Li with 1.2×104 atoms. Quasipure Bose-Einstein condensates of up to 3×104 174Yb atoms can be produced in single-species experiments. Our results mark a significant step toward studies of few- and many-body physics with mixtures of alkali-metal and alkaline-earth-metal-like atoms, and for the production of paramagnetic polar molecules in the quantum regime. Our methods also establish a convenient scheme for producing quantum degenerate ytterbium atoms in a 1064 nm optical dipole trap.

Summary coming soon!

We report on the realization of a stable mixture of ultracold lithium and ytterbium atoms confined in a far-off-resonance optical dipole trap. We observe sympathetic cooling of 6Li by 174Yb and extract the s-wave scattering length magnitude |a6Li−174Yb|=(13±3)a0 from the rate of interspecies thermalization. Using forced evaporative cooling of 174Yb, we achieve reduction of the 6Li temperature to below the Fermi temperature, purely through interspecies sympathetic cooling.

Summary coming soon!

Using optical and electron microscopy, we analyze the energy and focusing angle dependence of structural changes induced in bulk glass by tightly focused femtosecond laser pulses. We observe a transition from small density variations in the material to void formation with increasing laser energy. At energies close to the threshold for producing a structural change, the shape of the structurally changed region is determined by the focal volume of the objective used to focus the femtosecond pulse, while at higher energies, the structural change takes on a conical shape. From these morphological observations, we infer the role of various mechanisms for structural change.

Summary coming soon!

In recent years, femtosecond lasers have been used for a multitude of micromachining tasks [1]. Several groups have shown that femtosecond laser pulses cleanly ablate virtually any material with a precision that consistently meets or exceeds that of other laser-based techniques, making the femtosecond laser an extremely versatile surface micromachining tool [2-7]. For large-band-gap materials, where laser machining relies on nonlinear absorption of high-intensity pulses for energy deposition, femtosecond lasers offer even greater benefit. Because the absorption in a transparent material is nonlinear, it can be confined to a very small volume by tight focusing, and the absorbing volume can be located in the bulk of the material, allowing three-dimensional micromachining [8, 9]. The extent of the structural change produced by femtosecond laser pulses can be as small as or even smaller than the focal volume. Recent demonstrations of three-dimensional micromachining of glass using femtosecond lasers include three-dimensional binary data storage [8, 10] and the direct writing of optical waveguides [9-12] and waveguide splitters [13]. The growing interest in femtosecond laser micromachining of bulk transparent materials makes it more important than ever to uncover the mechanisms responsible for producing permanent structural changes.

Summary coming soon!

Summary form only given. When an intense femtosecond laser pulse is tightly focused into a transparent material, energy is deposited in the focal volume by nonlinear absorption. If enough energy is deposited, permanent structural change results. Characterization of the morphology of structural changes and identification of the mechanisms producing these changes represent essential steps in refining the femtosecond laser as a micromachining tool. We present a systematic study of structural changes produced in bulk glass by tightly-focused femtosecond pulses. Using optical microscopy and scanning electron microscopy (SEM), we identify a transition from a structural change mechanism dominated by localized melting or bond-break to one dominated by an explosive expansion.