Searcharxiv⌕ Search

arXiv subjects

F. Robicheaux

Publications and source records attributed to F. Robicheaux.

At least 55 records · Page 3Linked to original sources
arXiv2019

Simulations of Majorana spin flips in an antihydrogen trap

The properties of antihydrogen ($\bar{\rm H}$) have, thus far, been probed at magnetic fields of $\sim 1$ T. It may be fruitful to perform some of these measurements at magnetic fields approaching 0 T. In this case, there could occur zeros in the magnitude of the $B$-field. The number and properties of the magnetic field zeros are investigated. For typical magnetic field geometries in $\bar{\rm H}$ traps, the zeros will occur as two groups of 5 closely spaced points instead of as a single point. Except in special cases, results from calculations show that these 10 zeros can be treated as independent sources of spin flip probability. Although the behavior of Majorana spin flip near higher order zeros should not be important in the $\bar{\rm H}$ traps, the probability for spin flip is calculated for the case of a quadratic zero. Finally, results are presented for a simple model of how magnetic field zeros would affect the trapped population of $\bar{\rm H}$.

physics.atom-ph↗
arXiv2018

Parametric Feedback Cooling of Rigid Body Nanodumbbells in Levitated Optomechanics

We theoretically investigate the rigid body dynamics of an optically levitated nanodumbbell under parametric feedback cooling and provide a simplified model for describing the motion. Differing from previous studies, the spin of the nanoparticle about its symmetry axis is considered non-negligible. Simulations reveal that standard parametric feedback cooling can extract energy from two of the five rotational degrees of freedom when the nanoparticle is levitated using a linearly polarized laser beam. The dynamics after feedback cooling are characterized by a normal mode describing precession about the laser polarization axis together with spin about the nanoparticle's symmetry axis. Cooling the remaining mode requires an asymmetry in the two librational frequencies associated with motion about the polarization axis as well as information about the two frequencies of rotation about the polarization axis. Introducing an asymmetric potential allows full cooling of the librational coordinates if the frequencies of both are used in the feedback modulation and is an avenue for entering the librational quantum regime. The asymmetry in the potential needs to be large enough for practical cooling times as the cooling rate of the system depends non-linearly on the degree of asymmetry, a condition that is easily achieved experimentally.

quant-ph↗
arXiv2018

Heating and cooling of electrons in an ultracold neutral plasma using Rydberg atoms

We have experimentally demonstrated both heating and cooling of electrons in an ultracold neutral plasma (UNP) by embedding Rydberg atoms into the plasma soon after its creation. We have determined the relationship between the initial electron temperature, $T_{e,i}$, and the binding energy of the added Rydberg atoms, $E_b$, at the crossover between heating and cooling behaviors (that is, the binding energy of the atoms, which, when they are added to the plasma, neither accelerate or slow down the plasma expansion). Specifically, this condition is $|E_b| \approx 2.7 \times k_B T_{e,i}$ when the diagnostic used is the effect of the Rydberg atoms on the plasma asymptotic expansion velocity. Additionally, we have obtained experimental estimates for the amount of heating or cooling which occurs when the Rydberg binding energy does not satisfy the crossover condition. The experimental results for the crossover condition, and the degree of heating or cooling away from the crossover, are in agreement with predictions obtained from numerical modeling of the interactions between Rydberg atoms and the plasma. We have also developed a simple intuitive picture of how the Rydberg atoms affect the plasma which supports the concept of a `bottleneck' in the Rydberg state distribution of atoms in equilibrium with a co-existing plasma.

physics.atom-ph↗
arXiv2017

Theory of long range interactions for Rydberg states attached to hyperfine split cores

The theory is developed for one and two atom interactions when the atom has a Rydberg electron attached to a hyperfine split core state. This situation is relevant for some of the rare earth and alkaline earth atoms that have been proposed for experiments on Rydberg-Rydberg interactions. For the rare earth atoms, the core electrons can have a very substantial total angular momentum, $J$, and a non-zero nuclear spin, $I$. In the alkaline earth atoms there is a single, $s$, core electron whose spin can couple to a non-zero nuclear spin for odd isotopes. The resulting hyperfine splitting of the core state can lead to substantial mixing between the Rydberg series attached to different thresholds. Compared to the unperturbed Rydberg series of the alkali atoms, the series perturbations and near degeneracies from the different parity states could lead to qualitatively different behavior for single atom Rydberg properties (polarizability, Zeeman mixing and splitting, etc) as well as Rydberg-Rydberg interactions ($C_5$ and $C_6$ matrices).

physics.atom-ph↗
arXiv2017

Degenerate Zeeman Ground States in the Single-Excitation Regime

In this work, we demonstrate the importance of considering correlations between degenerate Zeeman sublevels that develop in dense atomic ensembles. In order to do this, we develop a set of equations capable of simulating large numbers of atoms while still incorporating correlations between degenerate Zeeman sublevels. This set of equations is exact in the single-photon limit, and may be interpreted as a generalization of the coupled harmonic oscillator equations typically used the literature. Using these equations, we demonstrate that in sufficiently dense systems, correlations between Zeeman sublevels can cause non-trivial differences in the photon scattering lineshape in arrays and clouds of atoms.

physics.atom-ph↗
arXiv2017

Quantum calculation of feedback cooling a laser levitated nanoparticle in the shot-noise-dominant regime

In this paper, results of quantum calculations are presented for feedback cooling of an optically trapped nanoparticle in the laser-shot-noise-dominant regime. We numerically investigate the system using both parametric and force feedback cooling schemes. For the same measurement efficiency, the cooling limit from the force feedback is lower than that from the parametric feedback. We also develop a set of semi-classical equations for feedback cooling that accurately match the quantum results. It is demonstrated, by rescaling the semi-classical equations, that the cooling dynamics is uniquely determined by the parameter set: the feedback strength, the measurement efficiency and the change of occupation number over one oscillation period due to the shot noise. The minimum occupation number is determined by the measurement efficiency and the change of occupation number over one period.

quant-ph↗
arXiv2017

Shot noise dominant regime for ellipsoidal nanoparticles in a linearly polarized beam

Results on the heating and the parametric feedback cooling of an optically trapped anisotropic nanoparticle in the laser shot noise dominant regime are presented. The related dynamical parameters, such as the oscillating frequency and shot noise heating rate, depend on the shape of the trapped particle. For an ellipsoidal particle, the ratio of the axis lengths and the overall size controls the shot noise heating rate relative to the frequency. For a particle with smaller ellipticity or bigger size, the relative heating rate for rotation tends to be smaller than that for translation indicating a better rotational cooling. For one feedback scheme, we also present results on the lowest occupation number that can be achieved as a function of the heating rate and the amount of classical uncertainty in the position measurement.

quant-ph↗
arXiv2016

Electron Plasmas Cooled by Cyclotron-Cavity Resonance

We observe that high-Q electromagnetic cavity resonances increase the cyclotron cooling rate of pure electron plasmas held in a Penning-Malmberg trap when the electron cyclotron frequency, controlled by tuning the magnetic field, matches the frequency of standing wave modes in the cavity. For certain modes and trapping configurations, this can increase the cooling rate by factors of ten or more. In this paper, we investigate the variation of the cooling rate and equilibrium plasma temperatures over a wide range of parameters, including the plasma density, plasma position, electron number, and magnetic field.

physics.plasm-ph↗
arXiv2016

Torsional optomechanics of a levitated nonspherical nanoparticle

An optically levitated nanoparticle in vacuum is a paradigm optomechanical system for sensing and studying macroscopic quantum mechanics. While its center-of-mass motion has been investigated intensively, its torsional vibration has only been studied theoretically in limited cases. Here we report the first experimental observation of the torsional vibration of an optically levitated nonspherical nanoparticle in vacuum. We achieve this by utilizing the coupling between the spin angular momentum of photons and the torsional vibration of a nonspherical nanoparticle whose polarizability is a tensor. The torsional vibration frequency can be one order of magnitude higher than its center-of-mass motion frequency, which is promising for ground state cooling. We propose a simple yet novel scheme to achieve ground state cooling of its torsional vibration with a linearly-polarized Gaussian cavity mode. A levitated nonspherical nanoparticle in vacuum will also be an ultrasensitive nanoscale torsion balance with a torque detection sensitivity on the order of $10^{-29} ~\mathrm{N}\cdot \mathrm{m}/\sqrt{\mathrm{ Hz}}$ under realistic conditions.

quant-ph↗
arXiv2016

Generalized local frame transformation theory for excited species in external fields

A rigorous theoretical framework is developed for a generalized local frame transformation theory (GLFT). The GLFT is applicable to the following systems: to Rydberg atoms or molecules in an electric field, or to negative ions in any combination of electric and/or magnetic fields. A first test application to the photoionization spectra of Rydberg atoms in an external electric field demonstrates dramatic improvement over the first version of the local frame transformation theory developed initially by Fano and Harmin. This revised GLFT theory yields non-trivial corrections because it now includes the full on-shell Hilbert space without adopting the truncations in the original theory. Comparisons of the semi-analytical GLFT Stark spectra with {\it ab initio} numerical simulations yields errors in the range of a few tens of MHz, an improvement over the original Fano-Harmin theory whose errors are 10-100 times larger. Our analysis provides a systematic pathway to precisely describe the corresponding photoabsorption spectra that should be accurate enough to meet most modern experimental standards.

physics.atom-ph↗
arXiv2016

Collective Dipole-Dipole Interactions in an Atomic Array

The coherent dipole-dipole interactions of atoms in an atomic array are studied. It is found that the excitation probability of an atom in an array parallel to the direction of laser propagation ($\boldsymbol{\hat{k}}$) will either grow or decay logarithmically along $\boldsymbol{\hat{k}}$, depending on the detuning of the laser. The symmetry of the system for atomic separations of $δr = jλ/2$, where $j$ is an integer, causes the excitation distribution and scattered radiation to abruptly become symmetric about the center of the array. For atomic separations of $δr < λ/2$, the appearance of a collection of extremely subradiant states ($Γ\sim 0$), disrupts the described trend. In order to interpret the results from a finite array of atoms, a band structure calculation in the $N\rightarrow \infty$ limit is conducted where the decay rates and the Collective Lamb Shifts of the eigenmodes along the Brillouin zone are shown. Finally, the band structure of an array strongly affects its scattered radiation, allowing one to manipulate the Collective Lamb Shift as well as the decay rate (from superradiant to subradiant) by changing the angle of the driving laser.

physics.atom-ph↗
arXiv2016

Decoherence of rotational degrees of freedom

The mechanism of decoherence for a quantum system with rotational degrees of freedom is studied. From a simple model of elastic scattering, we show that the non-diagonal density matrix elements of the system exponentially decay. The decay rate depends on the difference of scattering amplitudes for different rotational configurations, leading to the gradual loss of quantum coherence between the pointer states in the system orientational space. For a dielectric ellipsoid immersed in a photon-gas environment (assuming no absorption), the decay rate is found to be proportional to the seventh power of the temperature. For an ellipsoidal object interacting with mass particles, the decay rate is proportional to the 5/2 power of the temperature. Both are different from the case of translational decoherence induced by the same environment scattering. For photon scattering, the coherence time in the rotational degrees of freedom is shown to be much shorter than that in the translational degrees of freedom.

quant-ph↗
arXiv2016

Closed-orbit theory for photodetachment in a time-dependent electric field

The standard closed-orbit theory is extended for the photodetachment of negative ions in a time-dependent electric field. The time-dependent photodetachment rate is specifically studied in the presence of a single-cycle terahertz pulse, based on exact quantum simulations and semiclassical analysis. We find that the photodetachment rate is unaffected by a weak terahertz field, but oscillates complicatedly when the terahertz pulse gets strong enough. Three types of closed classical orbits are identified for the photoelectron motion in a strong single-cycle terahertz pulse, and their connections with the oscillatory photodetachment rate are established quantitatively by generalizing the standard closed-orbit theory to a time-dependent form. By comparing the negative hydrogen and fluorine ions, both the in-phase and antiphase oscillations can be observed, depending on a simple geometry of the contributed closed classical orbits. On account of its generality, the presented theory provides an intuitive understanding from a time-dependent viewpoint for the photodetachment dynamics driven by an external electric field oscillating at low frequency.

physics.atom-ph↗
arXiv2015

Schwinger variational principle theory of collisions in the presence of multiple potentials

A theoretical method for treating collisions in the presence of multiple potentials is developed by employing the Schwinger variational principle. The current treatment agrees with the local (regularized) frame transformation theory and extends its capabilities. Specifically, the Schwinger variational approach gives results without the divergences that need to be regularized in other methods. Furthermore, it provides a framework to identify the origin of these singularities and possibly improve the local frame transformation. We have used the method to obtain the scattering parameters for different confining potentials symmetric in $x,y$. The method is also used to treat photodetachment processes in the presence of various confining potentials, thereby highlighting effects of the infinitely many closed channels. Two general features predicted are the vanishing of the total photoabsorption probability at {\it every} channel threshold and the occurrence of resonances below the channel thresholds for negative scattering lengths. In addition, the case of negative ion photodetachment in the presence of uniform magnetic fields is also considered where unique features emerge at large scattering lengths.

physics.atom-ph↗
arXiv2014

Field-ionization threshold and its induced ionization-window phenomenon for Rydberg atoms in a short single-cycle pulse

We study the field-ionization threshold behavior when a Rydberg atom is ionized by a short single-cycle pulse field. Both hydrogen and sodium atoms are considered. The required threshold field amplitude is found to scale \emph{inversely} with the binding energy when the pulse duration becomes shorter than the classical Rydberg period, and, thus, more weakly bound electrons require larger fields for ionization. This threshold scaling behavior is confirmed by both 3D classical trajectory Monte Carlo simulations and numerically solving the time-dependent Schrödinger equation. More surprisingly, the same scaling behavior in the short pulse limit is also followed by the ionization thresholds for much lower bound states, including the hydrogen ground state. An empirical formula is obtained from a simple model, and the dominant ionization mechanism is identified as a nonzero spatial displacement of the electron. This displacement ionization should be another important mechanism beyond the tunneling ionization and the multiphoton ionization. In addition, an "ionization window" is shown to exist for the ionization of Rydberg states, which may have potential applications to selectively modify and control the Rydberg-state population of atoms and molecules.

physics.atom-ph↗
arXiv2014

Photoionization microscopy in terms of local frame transformation theory

Two-photon ionization of an alkali-metal atom in the presence of a uniform electric field is investigated using a standardized form of local frame transformation and generalized quantum defect theory. The relevant long-range quantum defect parameters in the combined Coulombic plus Stark potential is calculated with eigenchannel R-matrix theory applied in the downstream parabolic coordinate $η$. The present formulation permits us to express the corresponding microscopy observables in terms of the local frame transformation, and it gives a critical test of the accuracy of the Harmin-Fano theory permitting a scholastic investigation of the claims presented in Zhao {\it et al.} [Phys. Rev. A 86, 053413 (2012)].

quant-ph↗
arXiv2014

Simulations of an ultracold, neutral plasma with equal masses

The results of a theoretical investigation of an ultracold, neutral plasma composed of equal mass positive and negative charges are reported. In our simulations, the plasma is created by the fast dissociation of a neutral particle. The temperature of the plasma is controlled by the relative energy of the dissociation. We studied the early time evolution of this system where the initial energy was tuned so that the plasma is formed in the strongly coupled regime. In particular, we present results on the temperature evolution and three body recombination. In the weakly coupled regime, we studied how an expanding plasma thermalizes and how the scattering between ions affects the expansion. Because the expansion causes the density to drop, the velocity distribution only evolves for a finite time with the final distribution depending on the number of particles and initial temperature of the plasma.

physics.plasm-ph↗