Browsing by Subject "Atomic physics"

Ion traps are a leading candidate for scaling quantum computers. The component technologies can be difficult to integrate and manufacture. Experimental systems are also subject to mechanical drift creating a large maintenance overhead. A full system redesign with stability and scalability in mind is presented. The center of our approach is a compact cryogenic ion trap package (trap cryopackage). A surface trap is mounted to a modified ceramic pin grid array (CPGA) this is enclosed using a copper lid. The differentially pumped trap cryopackage has all necessary optical feedthroughs and an ion source (ablation target). The lid pressure is held at ultra-high vacuum (UHV) by cryogenic sorption pumping using carbon getter. We install this cryopackage into a commercial low-vibration closed-cycle cryostat which sits inside a custom monolithic enclosure. The system is tested and trapped ions are found to have common mode heating rate on the order of 10 quanta/s. The modular optical setup provides for a couterpropagating single qubit coherence time of 527 ms. We survey a population of FM two-qubit gates (gate times 120 μs - 450 μs) and find an average gate fidelity of 98\%. We study the gate survey with quantum Monte Carlo simulation and find that our two-qubit gate fidelity is limited by low frequency (30 Hz - 3 kHz) coherent electrical noise on our motional modes.

Collective behavior in many-body systems, where the dynamics of an individual element depend on the state of the entire ensemble, play an important role in both basic science research and applied technologies. Over the last twenty years, studies of such effects in cold atomic vapors have lead to breakthroughs in areas such as quantum information science and atomic and condensed matter physics. Nevertheless, in order to generate photon-mediated atom-atom coupling strengths that are large enough to produce collective behavior, these studies employ techniques that intrinsically limit their applicability. In this thesis, I describe a novel nonlinear optical process that enables me to overcome these limitations and realize a new regime of collective light-matter interaction.

My experiment involves an anisotropic cloud of cold rubidium atoms illuminated by a pair of counterpropagating optical (pump) fields propagating at an angle to the trap's long axis. When the pump beam intensities exceed a threshold value, a collective instability occurs in which new beams of light are generated spontaneously and counterpropagate along the trap's long axis. In order to understand the physical mechanism responsible for this behavior, I study first the system's nonlinear optical response when driven below the instability threshold. I find that the incident optical fields produce an optical lattice that causes the atoms to become spatially organized on the sub-wavelength length scale. This organization corresponds to the formation of an atomic density grating, which effectively couples the involved fields to one another and enables the transfer of energy between them. The loading of atoms into this grating is enhanced by my choice of field polarizations, which simultaneously results in cooling of the atoms from T~30 μK to T~3 μK via the Sisyphus effect. As a result, I observe a fifth-order nonlinear susceptibility χ^{(5)}=1.9x10^-12 (m/V)^4 that is 7 orders of magnitude larger than previously observed. In addition, because of the unique scaling of the resulting nonlinear response with material parameters, the magnitude of the nonlinearity can be large for small pump intensities (\ie, below the resonant electronic saturation intensity 1.6 mW/cm^2) while simultaneously suffering little linear absorption. I confirm my interpretation of the nonlinearity by developing a theoretical model that agrees quantitatively with my experimental observations with no free parameters.

The collective instability therefore corresponds to the situation where the cold vapor transitions spontaneously from a spatially-homogeneous state to an ordered one. This emergent organization leads to the simultaneous emission of new optical fields in a process that one can interpret either in terms of mirrorless parametric self-oscillation or superradiance. By mapping out the phase diagram for this transition, I find that the instability can occur for pump intensities as low as 1 mW/cm^2, which is approximately 50 times smaller than previous observations of similar phenomena. The intensity of the emitted light can be up to 20% of the pump beam intensity and depends superlinearly on the number of atoms, which is a clear signature of collective behavior. In addition, the generated light demonstrates temporal correlations between the counterpropagating modes of up to 0.987 and is nearly coherent over several hundred μs. The most significant attributes of the light, though, are that it consists of multiple transverse spatial modes and persists in steady-state. This result represents the first observation of such dynamics, which have been shown theoretically to lead to a rich array of new phenomena and possible applications.

Using a single atom, we have observed the flip of a single molecule. The dipole-phonon interaction between the permanent dipole of a diatomic molecular ion and the secular oscillation of the ion chain manifests as a Jaynes-Cummings-type interaction. When combined with quantum logic using a co-trapped atomic ion, this interaction enables state preparation and measurement of quantum information encoded within a molecular ion. Here, we present the first experimental implementation of dipole- phonon quantum logic (DPQL) along with the preliminary research leading up to the demonstration. After an investigation of suitable molecular ions for experiments with DPQL, the ground rotational state of calcium oxide (CaO+) was identified as a prime candidate for the molecular ion qubit. We demonstrate sympathetic motional ground-state cooling with a co-trapped calcium ion (Ca+), and we showcase the ability to adiabatically ramp the trap secular frequency without perturbing the ions from the ground-state: two crucial steps toward implementing DPQL. However, due to the low population of the ground rotational state of CaO+ at room temperature, the molecular ion qubit space must be prepared via projective measurement. Because the rate of measurement is largely limited by ground-state cooling, we investigate via numerical optimization and experiments the comparative cooling performance of two commonly used methods of ground-state cooling, pulsed and continuous sideband cooling. Finally, we present the first observations of the dipole-phonon interaction via quantum logic readout, which have statistical significance as high as 7.4σ.

Experimental systems that trap single, neutral atoms have recently emerged as a promising platform for experiments in a range of disciplines such as quantum information science, quantum simulation and fundamental light-atom interaction. In this thesis, I build such a system and use it to trap and study a single, neutral atom of 87Rb. I confront and overcome several experimental challenges while designing and building the system. For example, I develop a MOT of unusual geometry with which to load the single-atom trap and also a detection scheme that robustly detects the trapped atom nondestructively, that is, without pushing it out of the trap. The result of this design and construction process is a system that stably traps a single atom in an optical dipole trap. I achieve trap lifetimes of over 1 minute in the absence of near-resonant laser light.

In addition to the experimental apparatus, I develop a thorough rate-equation model to predict the population dynamics of the trapped atom's internal quantum state when probed by near-resonant light. This model gives unique insight into the influence of the atom's internal dynamics on the detected scattering rate. I use this model to predict several important experimental parameters and compare it to the experimental data. This allows me to characterize the parameters that govern how the atom interacts with near-resonant laser light and how that interaction affects the experimental data. For example, I perform an absolute calibration of the collection efficiency of the experimental system, a first for a single, neutral-atom trap.

Using these experimental and modeling tools, I investigate the scattering rate of an atom in the presence of near-resonant linearly-polarized laser light. This is of great interest to the field because it is used to measure the atom's internal quantum state, in a process known as quantum-state readout. Fast and accurate quantum-state readout is crucial to the success of many protocols in quantum information science and quantum simulation. Using the tools described here, I achieve quantum-state readout with an average fidelity of 97.6±0.2% using a linearly-polarized probe beam. The readout requires a measurement time of 160±20 μs, and the atom remains in the trap after the readout in 97.1±0.1% of the trials. I use linearly-polarized light instead of circularly-polarized light because it makes the readout less sensitive to the atom's occupation of a specific magnetic sublevel, and hence does not require sublevel-specific state preparation. It also allows for a more flexible experimental geometry. This is the fastest and highest-fidelity nondestructive readout of a single neutral atom performed with a linearly-polarized probe beam reported to date.

In addition, I identify a decay in the atom's scattering rate over the course of the readout time that limits the quantum-state readout fidelity. I investigate possible sources of this decay using the rate-equation model and a model of the readout protocol, and I conclude that it is likely caused by a combination of Raman transitions and heating. The heating is related to the near-resonant probe light and also to the optical dipole trap that holds the atom. I discuss ways that this decay can be avoided, but point out that these possible solutions result in longer readout times. This investigation has applications across a wide variety of experiments that require fast quantum-state readout.

Trapped ions provide a promising platform to build a practical quantum computer. Scaling the high performance of small systems to longer ion chains is a technical endeavor that benefits from both better hardware system design and gate-level control techniques. In this thesis, I discuss our work on building a small-scale trapped-ion quantum computing system that features stable laser beam control, low-crosstalk individual addressing and capability to implement high-fidelity multi-qubit gates.

We develop control techniques to extend the pack-leading fidelity of entangling gates in two-ion systems to longer chains. A major error source limiting entangling gate fidelities in ion chains is crosstalk between target and neighboring spectator qubits. We propose and demonstrate a crosstalk suppression scheme that eliminates all first-order crosstalk utilizing only local control of target qubits, as opposed to an existing scheme which requires control over all neighboring qubits. Using the scheme, we achieve a $99.5\%$ gate fidelity in a 5-ion chain. Complex quantum circuits can benefit from native multi-qubit gates such as the $N$-Toffoli gate, which substantially reduce the overhead cost from performing universal decomposition into single- and two-qubit gates. We take advantage of novel performance benefits of long ion chains to realize scalable Cirac-Zoller gates, which uses a simple pulse sequence to efficiently implement $N$-Toffoli gates. We demonstrate the Cirac-Zoller 3- and 4-Toffoli gates in a five-ion chain with higher fidelities than previous results using trapped ions. We also present the first experimental realization of a 5-Toffoli gate.

The nonlinear interaction between light and atoms is an extensive field of study with a broad range of applications in quantum information science and condensed matter physics. Nonlinear optical phenomena occurring in cold atoms are particularly interesting because such slowly moving atoms can spatially organize into density gratings, which allows for studies involving optical interactions with structured materials. In this thesis, I describe a novel nonlinear optical effect that arises when cold atoms spatially bunch in an optical lattice. I show that employing this spatial atomic bunching provides access to a unique physical regime with reduced thresholds for nonlinear optical processes and enhanced material properties. Using this method, I observe the nonlinear optical phenomenon of transverse optical pattern formation at record-low powers. These transverse optical patterns are generated by a wave- mixing process that is mediated by the cold atomic vapor. The optical patterns are highly multimode and induce rich non-equilibrium atomic dynamics. In particular, I find that there exists a synergistic interplay between the generated optical pat- terns and the atoms, wherein the scattered fields help the atoms to self-organize into new, multimode structures that are not externally imposed on the atomic sample. These self-organized structures in turn enhance the power in the optical patterns. I provide the first detailed investigation of the motional dynamics of atoms that have self-organized in a multimode geometry. I also show that the transverse optical patterns induce Sisyphus cooling in all three spatial dimensions, which is the first observation of spontaneous three-dimensional cooling. My experiment represents a unique means by which to study nonlinear optics and non-equilibrium dynamics at ultra-low required powers.

Optical control of interactions in ultracold gases opens new fields of research by creating ``designer" interactions with high spatial and temporal resolution. However, previous optical methods using single optical fields generally suffer from atom loss due to spontaneous scattering. This thesis reports new optical methods, employing two optical fields to control interactions in ultracold gases, while suppressing spontaneous scattering by quantum interference. In this dissertation, I will discuss the experimental demonstration of two optical field methods to control narrow and broad magnetic Feshbach resonances in an ultracold gas of $^6$Li atoms. The narrow Feshbach resonance is shifted by $30$ times its width and atom loss suppressed by destructive quantum interference. Near the broad Feshbach resonance, the spontaneous lifetime of the atoms is increased from $0.5$ ms for single field methods to $400$ ms using our two optical field method. Furthermore, I report on a new theoretical model, the continuum-dressed state model, that calculates the optically induced scattering phase shift for both the broad and narrow Feshbach resonances by treating them in a unified manner. The continuum-dressed state model fits the experimental data both in shape and magnitude using only one free parameter. Using the continuum-dressed state model, I illustrate the advantages of our two optical field method over single-field optical methods.

A trapped Fermi gas near a collisional resonance provides a unique laboratory for testing many-body theories in a variety of fields. The ultracold Fermi gas produced in our lab is comprised of the lowest two spin states of $^6$Li. At 834 G there is a collisional or Feshbach resonance between the two spin states. The scattering length between trapped atoms of opposing spins far exceeds the interparticle spacing of the gas. On resonance, a strongly interacting, unitary, Fermi gas is created which exhibits universal behavior. The unitary Fermi gas is a prototype for other exotic systems in nature from nuclear matter to neutron stars and high temperature superconductors.

For magnetic fields less than 834 G the scattering length is positive, and pairs Fermi atoms can form molecular dimers. These dimers, comprised of two fermions, are bosons. At ultracold temperatures the molecular bosons populate the lowest energy level and form a Bose Einstein Condensate (BEC). For magnetic fields greater than 834G the scattering length between fermions in opposing spin states is negative, like Cooper pairs formed between electrons in a superconductor. The Bardeen, Cooper, and Shriefer (BCS) theory was developed to describe the pairing effect in the context of superconductors. In our experiment we produce an ultracold unitary gas. By tuning the magnetic field to either side of the Feshbach resonance we can transform the gas into a weakly interacting BEC or BCS superfluid. Therefore, the region near a Feshbach resonance is called the BEC-BCS crossover.

This dissertation presents a precision measurement of the hydrodynamic sound velocity in an ultracold Fermi gas near a Feshbach resonance. The sound velocity is measured at various magnetic fields both above and below resonance. Moreover, we are able compare our measurements to theoretical descriptions of hydrodynamic sound propagation. Further, our measurement of sound velocity exactly reproduces the non-perturbative case, eliminating the need to consider nonlinear effects. At resonance the sound velocity exhibits universal scaling with the Fermi velocity to within 1.8\% over a factor of 30 in density. In a near zero temperature unitary gas the average sound velocity at the axial center was measured, $c(0)/v_F$ = 0.364(0.005), as well as the universal constant, $\beta$ = -0.565(0.015). The measurement of sound velocity in an ultracold gas throughout the BEC-BCS crossover provides further evidence of the continuous connection between the physics of the BEC, unitary, and BCS systems.

Quantum simulation is pivotal in understanding and modeling complex quantum phenomena that are challenging to study using classical computational methods. This thesis investigates the potential of trapped ion systems for advancing the field of quantum simulation. By leveraging the unique properties of trapped ions, particularly Ytterbium (Yb) ions, this research aims to enhance the precision and scalability of quantum simulations.

This thesis reports our progress on developing and optimizing the experimental setup and operational techniques required for effective manipulation of trapped ions. Key advancements include refining the processes for ion trapping, cooling, state manipulation, and phase tracking, as well as addressing technical challenges to maintain high coherence and low error rates. Significant applications of the trapped ion system are demonstrated through simulations that provide deeper insights into quantum dynamics and interactions. These applications showcase the ability of trapped ion systems to model complex environments and phenomena, such as energy transfer processes and the effects of structured environments on quantum dynamics.

Overall, this thesis underscores the versatility and power of trapped ion systems as a platform for quantum simulation. The findings pave the way for future research and practical applications in quantum computing and information processing, highlighting the promising role of trapped ion technology in the advancement of quantum science.

The advent of quantum computation foretells a new era in science and technology, but the fragility of quantum bits (qubits) and the unreliability of gates hinder the realization of functioning quantum computers. For ion trap quantum computers in particular, 2-qubit operations relying on the M\o lmer-S\o rensen interaction have the greatest error rates. This dissertation introduces frequency-modulated (FM) pulses as a measure to maximize 2-qubit gate fidelity and a way to calibrate gate errors through the measurement of circuit performance.

A key challenge of two-qubit gates in ion chains is unwanted residual entanglement between the ion spin and its motion. Frequency-modulated pulses are developed to achieve such goal. This theoretical advance has led to high-fidelity 2-qubit gates that are robust against small frequency drifts in a 5-ion experiment. Combining frequency and amplitude modulation, numerical calculations suggest that entanglement between an arbitrary pair of qubits are possible in a lattice with up to 50 ions. More recently, long-distance 2-qubit gates have been realized within a 17-ion chain.

Quantum circuit calibration is proposed to improve quantum circuits using feedback from measurement results. A relationship between the error parameters and measured observables can be established to identify systematic circuit errors. The calibration of a 6-qubit parity check circuit targeting 2-qubit overrotations has been implemented using measurement results from an experimental 15-ion trap. This improvement is conducive to quantum error correction protocols which involve high-weight stabilizers. A 4-bit Toffoli circuit with an error vector of length 6 is calibrated using a custom circuit simulator, reducing the average error size by a factor of 4. Using linear and quadratic approximation, a 6-bit Toffoli circuit with 12 error parameters is calibrated in the presence of 3 ancilla qubits.

Spin-imbalanced Fermi gases serve as a testbed for fundamental notions and are efficient table-top emulators of a variety of quantum matter ranging from neutron stars, the quark-gluon plasma, to high critical temperature superconductors. A macroscopic quantum phenomenon which occurs in spin-imbalanced Fermi gases is that of phase separation; in three dimensions, a spin-balanced, fully-paired superfluid core is surrounded by an imbalanced normal-fluid shell, followed by a fully polarized shell. In one-dimension, the behavior is reversed; a balanced phase appears outside a spin-imbalanced core. This thesis details the first density profile measurements and studies on spin-imbalanced quasi-2D Fermi gases, accomplished with high-resolution, rapid sequential spin-imaging. The measured cloud radii and central densities are in disagreement with mean-field Bardeen-Cooper-Schrieffer theory for a 2D system. Data for normal-fluid mixtures are well fit by a simple 2D polaron model of the free energy. Not predicted by the model is an observed phase transition to a spin-balanced central core above a critical polarization.

Since the inception of hyperpolarized 129Xe MRI, the field has yearned for more efficient production of more highly polarized 129Xe. For nearly all polarizers built to date, both peak 129Xe polarization and production rate fall far below theoretical predictions. This thesis sought to develop a fundamental understanding of why the observed performance of large-scale 129Xe hyperpolarization lagged so badly behind theoretical predictions.

This is done by thoroughly characterizing a high-volume, continuous-flow polarizer using optical cells having three different internal volumes, and employing two different laser sources. For each of these 6 combinations, 129Xe polarization was carefully measured as a function of production rate across a range of laser absorption levels. The resultant peak polarizations were consistently a factor of 2-3 lower than predicted across a range of absorption levels, and scaling of production rates deviated badly from predictions based on spin exchange efficiency.

To bridge this gap, we propose that paramagnetic, activated Rb clusters form during spin exchange optical pumping (SEOP), and depolarize Rb and 129Xe, while unproductively scattering optical pumping light. When a model was built that incorporated the effects of clusters, its predictions matched observations for both polarization and production rate for all 6 systems studied. This permits us to place a limit on cluster number density of <2 × 109 cm-3.

The work culminates with deploying this framework to identify methods to improve polarization to above 50%, leaving the SEOP cell. Combined with additional methods of preserving polarization, the polarization of a 300-mL batch of 129Xe increased from an average of 9%, before this work began, to a recent value of 34%.

We anticipate that these developments will lay the groundwork for continued advancement and scaling up of SEOP-based hyperpolarization methods that may one day permit real-time, on-demand 129Xe MRI to become a reality.