arXiv subjects

Brian C. Sawyer

Publications and source records attributed to Brian C. Sawyer.

36 records · Page 2Linked to original sources

arXiv2016

Quantum spin dynamics and entanglement generation with hundreds of trapped ions

Quantum simulation of spin models can provide insight into complex problems that are difficult or impossible to study with classical computers. Trapped ions are an established platform for quantum simulation, but only systems with fewer than 20 ions have demonstrated quantum correlations. Here we study non-equilibrium, quantum spin dynamics arising from an engineered, homogeneous Ising interaction in a two-dimensional array of $^9$Be$^+$ ions in a Penning trap. We verify entanglement in the form of spin-squeezed states for up to 219 ions, directly observing 4.0$\pm$0.9 dB of spectroscopic enhancement. We also observe evidence of non-Gaussian, over-squeezed states in the full counting statistics. We find good agreement with ab-initio theory that includes competition between entanglement and decoherence, laying the groundwork for simulations of the transverse-field Ising model with variable-range interactions, for which numerical solutions are, in general, classically intractable.

quant-ph↗

arXiv2014

Reversing Hydride Ion Formation in Quantum Information Experiments with Be$^+$

We demonstrate photodissociation of BeH$^+$ ions within a Coulomb crystal of thousands of $^9$Be$^+$ ions confined in a Penning trap. Because BeH$^+$ ions are created via exothermic reactions between trapped, laser-cooled Be$^+$($^2\text{P}_{3/2}$) and background H$_2$ within the vacuum chamber, they represent a major contaminant species responsible for infidelities in large-scale trapped-ion quantum information experiments. The rotational-state-insensitive dissociation scheme described here makes use of 157 nm photons to produce Be$^+$ and H as products, thereby restoring Be$^+$ ions without the need for reloading. This technique facilitates longer experiment runtimes at a given background H$_2$ pressure, and may be adapted for removal of MgH$^+$ and AlH$^+$ impurities.

physics.atom-ph↗

arXiv2014

Spin Dephasing as a Probe of Mode Temperature, Motional State Distributions, and Heating Rates in a 2D Ion Crystal

We employ spin-dependent optical dipole forces to characterize the transverse center-of-mass (COM) motional mode of a two-dimensional Wigner crystal of hundreds of $^9$Be$^+$. By comparing the measured spin dephasing produced by the spin-dependent force with the predictions of a semiclassical dephasing model, we obtain absolute mode temperatures in excellent agreement with both the Doppler laser cooling limit and measurements obtained from a previously published technique (B. C. Sawyer et al. Phys. Rev. Lett. \textbf{108}, 213003 (2012)). Furthermore, the structure of the dephasing histograms allows for discrimination between initial thermal and coherent states of motion. We also apply the techniques discussed here to measure, for the first time, the ambient heating rate of the COM mode of a 2D Coulomb crystal in a Penning trap. This measurement places an upper limit on the anomalous single-ion heating rate due to electric field noise from the trap electrode surfaces of $\frac{d\bar{n}}{dt}\sim 5$ s$^{-1}$ for our trap at a frequency of 795 kHz, where $\bar{n}$ is the mean occupation of quantized COM motion in the axial harmonic well.

quant-ph↗

arXiv2013

Relaxation timescales and decay of correlations in a long-range interacting quantum simulator

We study the time evolution of correlation functions in long-range interacting quantum Ising models. For a large class of initial conditions, exact analytic results are obtained in arbitrary lattice dimension, both for ferromagnetic and antiferromagnetic coupling, and hence also in the presence of geometric frustration. In contrast to the nearest-neighbour case, we find that correlations decay like stretched or compressed exponentials in time. Provided the long-range character of the interactions is sufficiently strong, pronounced prethermalization plateaus are observed and relaxation timescales are widely separated. Specializing to a triangular lattice in two spatial dimensions, we propose to utilize these results for benchmarking of a recently developed ion-trap based quantum simulator.

quant-ph↗

arXiv2013

Simulating Quantum Magnetism with Correlated Non-Neutral Ion Plasmas

By employing forces that depend on the internal electronic state (or spin) of an atomic ion, the Coulomb potential energy of a strongly coupled array of ions can be modified in a spin-dependent way to mimic effective quantum spin Hamiltonians. Both ferromagnetic and antiferromagnetic interactions can be implemented. We use simple models to explain how the effective spin interactions are engineered with trapped-ion crystals. We summarize the type of effective spin interactions that can be readily generated, and discuss an experimental implementation using single-plane ion crystals in a Penning trap.

quant-ph↗

arXiv2012

Spectroscopy and Thermometry of Drumhead Modes in a Mesoscopic Trapped-Ion Crystal using Entanglement

We demonstrate spectroscopy and thermometry of individual motional modes in a mesoscopic 2D ion array using entanglement-induced decoherence as a method of transduction. Our system is a $\sim$400 $μ$m-diameter planar crystal of several hundred $^9$Be$^+$ ions exhibiting complex drumhead modes in the confining potential of a Penning trap. Exploiting precise control over the $^9$Be$^+$ valence electron spins, we apply a homogeneous spin-dependent optical dipole force to excite arbitrary transverse modes with an effective wavelength approaching the interparticle spacing ($\sim$20 \nolinebreak$μ$m). Center-of-mass displacements below 1 nm are detected via entanglement of spin and motional degrees of freedom.

quant-ph↗

arXiv2012

Engineered 2D Ising interactions on a trapped-ion quantum simulator with hundreds of spins

The presence of long-range quantum spin correlations underlies a variety of physical phenomena in condensed matter systems, potentially including high-temperature superconductivity. However, many properties of exotic strongly correlated spin systems (e.g., spin liquids) have proved difficult to study, in part because calculations involving N-body entanglement become intractable for as few as N~30 particles. Feynman divined that a quantum simulator - a special-purpose "analog" processor built using quantum particles (qubits) - would be inherently adept at such problems. In the context of quantum magnetism, a number of experiments have demonstrated the feasibility of this approach. However, simulations of quantum magnetism allowing controlled, tunable interactions between spins localized on 2D and 3D lattices of more than a few 10's of qubits have yet to be demonstrated, owing in part to the technical challenge of realizing large-scale qubit arrays. Here we demonstrate a variable-range Ising-type spin-spin interaction J_ij on a naturally occurring 2D triangular crystal lattice of hundreds of spin-1/2 particles (9Be+ ions stored in a Penning trap), a computationally relevant scale more than an order of magnitude larger than existing experiments. We show that a spin-dependent optical dipole force can produce an antiferromagnetic interaction J_ij ~ 1/d_ij^a, where a is tunable over 0<a<3; d_ij is the distance between spin pairs. These power-laws correspond physically to infinite-range (a=0), Coulomb-like (a=1), monopole-dipole (a=2) and dipole-dipole (a=3) couplings. Experimentally, we demonstrate excellent agreement with theory for 0.05<a<1.4. This demonstration coupled with the high spin-count, excellent quantum control and low technical complexity of the Penning trap brings within reach simulation of interesting and otherwise computationally intractable problems in quantum magnetism.

quant-ph↗

arXiv2011

Microwave state transfer and adiabatic dynamics of magnetically trapped polar molecules

Cold and ultracold polar molecules with nonzero electronic angular momentum are of great interest for studies in quantum chemistry and control, investigations of novel quantum systems, and precision measurement. However, in mixed electric and magnetic fields, these molecules are generically subject to a large set of avoided crossings among their Zeeman sublevels; in magnetic traps, these crossings lead to distorted potentials and trap loss from electric bias fields. We have characterized these crossings in OH by microwave-transferring trapped OH molecules from the upper |f; M = +3/2> parity state to the lower |e; +3/2> state and observing their trap dynamics under an applied electric bias field. Our observations are very well described by a simple Landau-Zener model, yielding insight to the rich spectra and dynamics of polar radicals in mixed external fields.

physics.atom-ph↗

arXiv2010

Cold heteromolecular dipolar collisions

We present the first experimental observation of cold collisions between two different species of neutral polar molecules, each prepared in a single internal quantum state. Combining for the first time the techniques of Stark deceleration, magnetic trapping, and cryogenic buffer gas cooling allows the enhancement of molecular interaction time by 10$^5$. This has enabled an absolute measurement of the total trap loss cross sections between OH and ND$_3$ at a mean collision energy of 3.6 cm$^{-1}$ (5 K). Due to the dipolar interaction, the total cross section increases upon application of an external polarizing electric field. Cross sections computed from \emph{ab initio} potential energy surfaces are in excellent agreement with the measured value at zero external electric field. The theory presented here represents the first such analysis of collisions between a $^2Π$ radical and a closed-shell polyatomic molecule.

physics.chem-ph↗

arXiv2008

Magneto-Optical Trap for Polar Molecules

We propose a method for laser cooling and trapping a substantial class of polar molecules, and in particular titanium (II) oxide (TiO). This method uses pulsed electric fields to nonadiabatically remix the ground-state magnetic sublevels of the molecule, allowing us to build a magneto-optical trap (MOT) based on a quasi-cycling $J'=J"-1$ transition. Monte-Carlo simulations of this electrostatically remixed MOT (ER-MOT) demonstrate the feasibility of cooling TiO to a temperature of 10 $\mathrmμK$ and trapping it with a radiation-pumping-limited lifetime on the order of 80 ms.

physics.atom-ph↗

arXiv2008

Low-energy molecular collisions in a permanent magnetic trap

Cold, neutral hydroxyl radicals are Stark decelerated and confined within a magnetic trap consisting of two permanent ring magnets. The OH molecules are trapped in the ro-vibrational ground state at a density of $\sim10^{6}$ cm$^{-3}$ and temperature of 70 mK. Collisions between the trapped OH sample and supersonic beams of atomic He and molecular D$_{2}$ are observed and absolute collision cross sections measured. The He--OH and D$_{2}$--OH center-of-mass collision energies are tuned from 60 cm$^{-1}$ to 230 cm$^{-1}$ and 145 cm$^{-1}$ to 510 cm$^{-1}$, respectively, yielding evidence of reduced He--OH inelastic cross sections at energies below 84 cm$^{-1}$, the OH ground rotational level spacing.

physics.atom-ph↗

arXiv2007

Prospects for the cavity-assisted laser cooling of molecules

Cooling of molecules via free-space dissipative scattering of photons is thought not to be practicable due to the inherently large number of Raman loss channels available to molecules and the prohibitive expense of building multiple repumping laser systems. The use of an optical cavity to enhance coherent Rayleigh scattering into a decaying cavity mode has been suggested as a potential method to mitigate Raman loss, thereby enabling the laser cooling of molecules to ultracold temperatures. We discuss the possibility of cavity-assisted laser cooling particles without closed transitions, identify conditions necessary to achieve efficient cooling, and suggest solutions given experimental constraints. Specifically, it is shown that cooperativities much greater than unity are required for cooling without loss, and that this could be achieved via the superradiant scattering associated with intracavity self-localization of the molecules. Particular emphasis is given to the polar hydroxyl radical (OH), cold samples of which are readily obtained from Stark deceleration.

quant-ph↗

arXiv2007

Mitigation of loss within a molecular Stark decelerator

The transverse motion inside a Stark decelerator plays a large role in the total efficiency of deceleration. We differentiate between two separate regimes of molecule loss during the slowing process. The first mechanism involves distributed loss due to coupling of transverse and longitudinal motion, while the second is a result of the rapid decrease of the molecular velocity within the final few stages. In this work, we describe these effects and present means for overcoming them. Solutions based on modified switching time sequences with the existing decelerator geometry lead to a large gain of stable molecules in the intermediate velocity regime, but fail to address the loss at very low final velocities. We propose a new decelerator design, the quadrupole-guiding decelerator, which eliminates distributed loss due to transverse/longitudinal couplings throughout the slowing process and also exhibits gain over normal deceleration to the lowest velocities.

physics.atom-ph↗

arXiv2007

Magneto-electrostatic trapping of ground state OH molecules

We report the magnetic confinement of neutral, ground state hydroxyl radicals (OH) at a density of $\sim3\times10^{3}$ cm$^{-3}$ and temperature of $\sim$30 mK. An adjustable electric field of sufficient magnitude to polarize the OH is superimposed on the trap in either a quadrupole or homogenous field geometry. The OH is confined by an overall potential established via molecular state mixing induced by the combined electric and magnetic fields acting on the molecule's electric dipole and magnetic dipole moments, respectively. An effective molecular Hamiltonian including Stark and Zeeman terms has been constructed to describe single molecule dynamics inside the trap. Monte Carlo simulation using this Hamiltonian accurately models the observed trap dynamics in various trap configurations. Confinement of cold polar molecules in a magnetic trap, leaving large, adjustable electric fields for control, is an important step towards the study of low energy dipole-dipole collisions.

physics.atom-ph↗

arXiv2006

OH hyperfine ground state: from precision measurement to molecular qubits

We perform precision microwave spectroscopy--aided by Stark deceleration--to reveal the low magnetic field behavior of OH in its ^2Π_{3/2} ro-vibronic ground state, identifying two field-insensitive hyperfine transitions suitable as qubits and determining a differential Lande g-factor of 1.267(5)\times10^{-3} between opposite parity components of the Λ-doublet. The data are successfully modeled with an effective hyperfine Zeeman Hamiltonian, which we use to make a tenfold improvement of the magnetically sensitive, astrophysically important ΔF=\pm1 satellite-line frequencies, yielding 1720529887(10) Hz and 1612230825(15) Hz.

physics.atom-ph↗

arXiv2006

Cold Molecule Spectroscopy for Constraining the Evolution of the Fine Structure Constant

We report precise measurements of ground-state, $λ$-doublet microwave transitions in the hydroxyl radical molecule (OH). Utilizing slow, cold molecules produced by a Stark decelerator we have improved over the precision of the previous best measurement by twenty-five-fold for the F' = 2 $\to$ F = 2 transition, yielding (1 667 358 996 $\pm$ 4) Hz, and by ten-fold for the F' = 1 $\to$ F = 1 transition, yielding (1 665 401 803 $\pm$ 12) Hz. Comparing these laboratory frequencies to those from OH megamasers in interstellar space will allow a sensitivity of 1 ppm for $Δα/α$ over $\sim$$10^{10}$ years.

physics.atom-ph↗

arXiv2005

Production of cold formaldehyde molecules for study and control of chemical reaction dynamics with hydroxyl radicals

We propose a method for controlling a class of low temperature chemical reactions. Specifically, we show the hydrogen abstraction channel in the reaction of formaldehyde (H$_{2}$CO) and the hydroxyl radical (OH) can be controlled through either the molecular state or an external electric field. We also outline experiments for investigating and demonstrating control over this important reaction. To this end, we report the first Stark deceleration of the H$_{2}$CO molecule. We have decelerated a molecular beam of H$_{2}$CO essentially to rest, producing cold molecule packets at a temperature of 100 mK with a few million molecules in the packet at a density of $\sim10^{6}$ cm$^{-3}$.

physics.atom-ph↗

arXiv2004

Efficient Stark deceleration of cold polar molecules

Stark deceleration has been utilized for slowing and trapping several species of neutral, ground-state polar molecules generated in a supersonic beam expansion. Due to the finite physical dimension of the electrode array and practical limitations of the applicable electric fields, only molecules within a specific range of velocities and positions can be efficiently slowed and trapped. These constraints result in a restricted phase space acceptance of the decelerator in directions both transverse and parallel to the molecular beam axis; hence, careful modeling is required for understanding and achieving efficient Stark decelerator operation. We present work on slowing of the hydroxyl radical (OH) elucidating the physics controlling the evolution of the molecular phase space packets both with experimental results and model calculations. From these results we deduce experimental conditions necessary for efficient operation of a Stark decelerator.

physics.atom-ph↗