arXiv subjects

F. Robicheaux

Publications and source records attributed to F. Robicheaux.

At least 37 records · Page 2Linked to original sources

arXiv2021

Probing ultracold gases using photoionization fine structure

Photoionization of atoms immersed in an environment such as an ultracold gas is investigated. We show that the interference of two ionization pathways, one passing directly to the continuum and one accounting for scattering processes between the photoelectron and a neighboring atom, produces a fine structure in the photoionization cross-section over an energy range less than 1 eV above threshold. This fine structure includes all the details of the corresponding three-body system, e.g. the interatomic distance or the scattering information of the electron-atom subsystem; therefore, photoelectrons produced in a multi-particle environment can be utilized as structural probes. As an illustration, for experimentally relevant parameters, we propose a scheme based on the photoionization of a Rydberg molecule where the low-energy electron-atom phase shifts are extracted from the fine structure spectra using neural networks.

physics.atom-ph↗

arXiv2021

Theoretical study of early time superradiance for atom clouds and arrays

We explore conditions for Dicke superradiance in a cloud of atoms by examining the Taylor series expansion of the photon emission rate at $t= 0$. By defining superradiance as an increasing photon emission rate for $t\sim 0$, we have calculated the conditions for superradiance for a variety of cases. We investigate superradiance as defined for photon emission into all angles as well as directional superradiance where the photon emission is only detected in a particular direction. Although all of the examples are for two level atoms that are fully inverted at $t=0$, we also give equations for partially inverted two level atoms and for fully inverted multilevel atoms. We give an algorithm for efficiently evaluating these equations for atom arrays and determine superradiance conditions for large atom number.

quant-ph↗

arXiv2021

Theory of the line shape of the 1S--2S transition for magnetically trapped antihydrogen

The physics that determines the line shape of the 1S--2S transition in magnetically trapped $\bar{\rm H}$ is explored. Besides obtaining an understanding of the line shape, one goal is to replace the dependence on large scale simulations of $\bar{\rm H}$ with a simpler integration over well defined functions. For limiting cases, analytic formulas are obtained. Example calculations are performed to illustrate the limits of simplifying assumptions. We also describe a $χ^2$ method for choosing experimental parameters that can lead to the most accurate determination of the transition frequency.

physics.atom-ph↗

arXiv2021

Laser driven superradiant ensembles of two-level atoms near Dicke's regime

We report the experimental observation of superradiant emission emanating from an elongated dense ensemble of laser cooled two-level atoms, with a radial extent smaller than the transition wavelength. In the presence of a strong driving laser, we observe that the system is superradiant along its symmmetry axis. This occurs even though the driving laser is orthogonal to the superradiance direction. This superradiance modifies the spontaneous emission, and, resultantly, the Rabi oscillations. We also investigate Dicke superradiance in the emission of an almost fully-inverted system as a function of atom numnber. The experimental results are in qualitative agreement with ab-initio, beyond-mean-field calculations.

physics.atom-ph↗

arXiv2021

Beyond lowest order mean field theory for light interacting with atom arrays

Results from higher order mean field calculations of light interacting with atom arrays are presented for calculations of one- and two-time expectation values. The atoms are approximated as two-levels and are fixed in space. Calculations were performed for mean field approximations that include the expectation value of one operator (mean field), two operators (mean field-2), and three operators (mean field-3). For the one-time expectation values, we examined three different situations to understand the convergence with increasing order of mean field and some limitations of higher order mean field approximations. As a representation of a two-time expectation value, we calculated the $g^{(2)}(τ)$ for a line of atoms illuminated by a perpendicular plane wave at several emission angles and two different intensities. For many cases, the mean field-2 will be sufficiently accurate to quantitatively predict the response of the atoms as measured by one-time expectation values. However, the mean field-3 approximation will often be needed for two-time expectation values.

quant-ph↗

arXiv2021

Limit on the Electric Charge of Antihydrogen

The ALPHA collaboration has successfully demonstrated the production and the confinement of cold antihydrogen, $\overline{\mathrm{H}}$. An analysis of trapping data allowed a stringent limit to be placed on the electric charge of the simplest antiatom. Charge neutrality of matter is known to a very high precision, hence a neutrality limit of $\overline{\mathrm{H}}$ provides a test of CPT invariance. The experimental technique is based on the measurement of the deflection of putatively charged $\overline{\mathrm{H}}$ in an electric field. The tendency for trapped $\overline{\mathrm{H}}$ atoms to be displaced by electrostatic fields is measured and compared to the results of a detailed simulation of $\overline{\mathrm{H}}$ dynamics in the trap. An extensive survey of the systematic errors is performed, with particular attention to those due to the silicon vertex detector, which is the device used to determine the $\overline{\mathrm{H}}$ annihilation position. The limit obtained on the charge of the $\overline{\mathrm{H}}$ atom is \mbox{$ Q = (-1.3\pm1.8\pm0.4)\times10^{-8}$}, representing the first precision measurement with $\overline{\mathrm{H}}$.

physics.atom-ph↗

arXiv2021

Spectroscopy of Rydberg States in Erbium using Electromagnetically Induced Transparency

We present a study of the Rydberg spectrum in \ts{166}Er for series connected to the $4f^{12} (^3H_6) 6s$, $J_c=13/2 $ and $J_c=11/2 $ ionic core states using an all-optical detection based on electromagnetically induced transparency in an effusive atomic beam. Identifying approximately 550 individual states, we find good agreement with a multi-channel quantum defect theory (MQDT) which allows assignment of most states to $ns$ or $nd$ Rydberg series. We provide an improved accuracy for the lowest two ionization thresholds to $E_{\textrm{IP}, J_c = 13/2 } = 49260.750(1)\,$cm$^{-1}$ and $E_{\textrm{IP}, J_c = 11/2 } = 49701.184(1)\,$cm$^{-1}$ as well as the corresponding quantum defects for all observed series. We identify Rydberg states in five different isotopes, and states between the two lowest ionization thresholds. Our results open the way for future applications of Rydberg states for quantum simulation using erbium and exploiting its special open-shell structure.

physics.atom-ph↗

arXiv2021

Photon induced atom recoil in collectively interacting planar arrays

The recoil of atoms in arrays due to the emission or absorption of photons is studied for sub-wavelength interatomic spacing. The atoms in the array interact with each other through collective dipole-dipole interactions and with the incident laser field in the low intensity limit. Shining uniform light on the array gives rise to patterns of excitation and recoil in the array. These arise due to the interference of different eigenmodes of excitation. The relation between the recoil and the decay dynamics is studied when the array is in its excitation eigenstates. The recoil experienced by a subradiant collective decay is substantially larger than from independent atom decay. A method to calculate the rate of recoil when steady state has been achieved with a constant influx of photons is also described.

quant-ph↗

arXiv2020

Interference in nonlinear Compton scattering using a Schrödinger's equation approach

The interference between Compton scattering and nonlinear Compton scattering from a two-color field in the X-ray regime is theoretically examined for bound electrons. The underlying phase shifts are analysed using a perturbative approach in the incoming classical field. The perturbative approach is bench marked with a non-perturbative approach in the classical field. The interference for different combinations of linear polarization of the two fields is examined when the Compton and the nonlinear Compton scattered waves have the same wave vector and polarization. Only two cases exhibit interference. When there is interference, the calculations reveal an intrinsic phase difference between the Compton scattered wave function and the nonlinear Compton scattered wave function of either 0 or $π$ depending on the scattering angle.

physics.atom-ph↗

arXiv2020

Photon recoil and laser focusing limits to Rydberg gate fidelity

Limits to Rydberg gate fidelity that arise from the entanglement of internal states of neutral atoms with the motional degrees of freedom due to the momentum kick from photon absorption and re-emission is quantified. This occurs when the atom is in a superposition of internal states but only one of these states is manipulated by visible or UV photons. The Schrödinger equation that describes this situation is presented and two cases are explored. In the first case, the entanglement arises because the spatial wave function shifts due to the separation in time between excitation and stimulated emission. For neutral atoms in a harmonic trap, the decoherence can be expressed within a sudden approximation when the duration of the laser pulses are shorter than the harmonic oscillator period. In this limit, the decoherence is given by simple analytic formulas that account for the momentum of the photon, the temperature of the atoms, the harmonic oscillator frequency, and atomic mass. In the second case, there is a reduction in gate fidelity because the photons causing absorption and stimulated emission are in focused beam modes. This leads to a dependence of the optically induced changes in the internal states on the center of mass atomic position. In the limit where the time between pulses is short, the decoherence can be expressed as a simple analytic formula involving the laser waist, temperature of the atoms, the trap frequency and the atomic mass. These limits on gate fidelity are studied for the standard $π-2π-π$ Rydberg gate and a new protocol based on a single adiabatic pulse with Gaussian envelope.

quant-ph↗

arXiv2020

Simulation of sympathetic cooling an optically levitated magnetic nanoparticle via coupling to a cold atomic gas

A proposal for cooling the translational motion of optically levitated magnetic nanoparticles is presented. The theoretical cooling scheme involves the sympathetic cooling of a ferromagnetic YIG nanosphere with a spin-polarized atomic gas. Particle-atom cloud coupling is mediated through the magnetic dipole-dipole interaction. When the particle and atom oscillations are small compared to their separation, the interaction potential becomes dominantly linear which allows the particle to exchange energy with the $N$ atoms. While the atoms are continuously Doppler cooled, energy is able to be removed from the nanoparticle's motion as it exchanges energy with the atoms. The rate at which energy is removed from the nanoparticle's motion was studied for three species of atoms (Dy, Cr, Rb) by simulating the full $N+1$ equations of motion and was found to depend on system parameters with scalings that are consistent with a simplified model. The nanoparticle's damping rate due to sympathetic cooling is competitive with and has the potential to exceed commonly employed cooling methods.

physics.atom-ph↗

arXiv2020

Stability and dynamics of optically levitated dielectric disks in a Gaussian standing wave beyond the harmonic approximation

Forces and torques exerted on dielectric disks trapped in a Gaussian standing wave are analyzed theoretically for disks of radius $2~μ\text{m}$ with index of refraction $n=1.45$ and $n=2.0$ as well as disks of radius 200 nm with $n=1.45$. Calculations of the forces and torques were conducted both analytically and numerically using a discrete-dipole approximation method. Besides harmonic terms, third order ro-translational coupling terms in the potential energy can be significant and a necessary consideration when describing the dynamics of disks outside of the Rayleigh limit. The coupling terms are a result of the finite extension of the disk coupling to both the Gaussian and standing wave geometry of the beam. The resulting dynamics of the degrees of freedom most affected by the coupling terms exhibit several sidebands as evidenced in the power spectral densities. Simulations show that for Gaussian beam waists of $1-4~μ\text{m}$ the disk remains stably trapped.

physics.optics↗

arXiv2020

Dressed ion-pair states of an ultralong-range Rydberg molecule

We predict the existence of a universal class of ultralong-range Rydberg molecular states whose vibrational spectra form trimmed Rydberg series. A dressed ion-pair model captures the physical origin of these exotic molecules, accurately predicts their properties, and reveals features of ultralong-range Rydberg molecules and heavy Rydberg states with a surprisingly small Rydberg constant. The latter is determined by the small effective charge of the dressed anion, which outweighs the contribution of the molecule's large reduced mass. This renders these molecules the only known few-body systems to have a Rydberg constant smaller than $R_\infty/2$.

quant-ph↗

arXiv2020

Distribution of laser shot noise energy delivered to a levitated nanoparticle

This paper quantifies the rate at which laser shot noise energy is delivered to a nanoparticle for the various scenarios commonly encountered in levitated optomechanics. While previous articles have the same form and dependencies, the proportionality constants often differ in the literature. This paper resolves these discrepancies. The rate at which energy is delivered to an optically trapped particle's respective degrees of freedom depends on the radiation pattern of scattered light as well as the direction of laser propagation. For a traveling plane wave with linearly polarized light, in the Rayleigh regime this leads the translational shot noise heating rate to be proportional to $1/10$ of the total rate in the laser polarization direction, $7/10$ in the laser propagation direction, and $2/10$ in the direction perpendicular to both. Analytical expressions for the shot noise heating rate are provided in the Rayleigh limit as well as numerical calculations for particles in the Mie regime for silica and diamond. For completeness, numerical calculations of the shot noise heating for silica Mie particles at the focus of a strongly focused laser beam are calculated for varying numerical aperture and common laser wavelengths. Both numerical calculations show that the Rayleigh expression generally gives an overestimate of the shot noise heating especially for larger radii, but is still a good approximation even for incident focal fields. The exception to the relative decrease is when a Mie resonance is reached which was found for diamond. Lastly, Rayleigh expressions for the rotational shot noise heating for a symmetric top-like particle for linear, elliptically, and unpolarized light are also provided.

physics.atom-ph↗

arXiv2020

Generalized local frame transformation theory for ultralong-range Rydberg molecules

A detailed theoretical framework for highly excited Rydberg molecules is developed based on the generalized local frame transformation. Our approach avoids the use of pseudopotentials and yields analytical expressions for the body-frame reaction matrix. The latter is used to obtain the molecular potential energy curves, but equally it can be employed for photodissociation, photoionization, or other processes. To illustrate the reliability and accuracy of our treatment we consider the Rb$^*-$Rb Rydberg molecule and compare our treatment with state-of-the-art alternative approaches. As a second application, the present formalism is used to re-analyze the vibrational spectra of Sr$^*-$Sr molecules, providing additional physical insight into their properties and a comparison of our results with corresponding measurements.

physics.atom-ph↗

arXiv2019

Simulations of sawtooth-wave adiabatic passage with losses

The results of simulations of cooling based on Sawtooth-Wave Adiabatic Passage (SWAP) are presented including the possibility of population leaking to states outside of the cycling transition. The amount of population leaking can be substantially suppressed compared to Doppler cooling, which could be useful for systems that are difficult to repump back to the cycling transition. The suppression of the leaked population was more effective when simulating the slowing of a beam than in cooling a thermal distribution. As expected, calculations of the leaked population versus branching ratio of spontaneous emission show that the suppression is more effective for narrow linewidth transitions. In this limit, using SWAP to slow a beam may be worth pursuing even when the branching ratio out of the cycling transition is greater than 10%.

physics.atom-ph↗

arXiv2019

Photon scattering from a cold, Gaussian atom cloud

We study the effect of a weakly driven atomic cloud's polarization distribution on its photon scattering lineshape. In doing this, we find three distinct polarization regimes. First, for dilute clouds, the polarization magnitude is relatively constant. Second, for denser clouds, polarization builds at the front of the cloud for near-resonant light. Third, when the cloud condenses to the point where its dimensions become comparable to the wavelength, light refocuses towards the back of the cloud for red detuning. For these regimes, we show which `mean-field' frameworks accurately describe the differing photon scattering lineshapes. Finally, for even denser clouds, mean field models become inaccurate and necessitate the full point dipole model that includes atom-atom correlations.

physics.atom-ph↗

arXiv2019

Scaling behaviour of the ground-state antihydrogen yield from CTMC simulation as a function of positron density and temperature

Antihydrogen production has reached such a level that precision spectroscopic measurements of its properties are within reach. In particular, the ground-state level population is of central interest for experiments aiming at antihydrogen spectroscopy. The positron density and temperature dependence of the ground-state yield is a result of the interplay between recombination, collisional, and radiative processes. Considering the fact that antihydrogen atoms with the principal quantum number n=15 or lower quickly cascade down to the ground state within 1ms, the number of such states are adopted as a measure of useful antihydrogen atoms. It has been found that the scaling behaviour of the useful antihydrogen yield is different depending on the positron density and positron temperature.

physics.atom-ph↗