arXiv subjects

F. Robicheaux

Publications and source records attributed to F. Robicheaux.

At least 73 records · Page 4Linked to original sources

arXiv2014

Simulations of prompt many-body ionization in a frozen Rydberg gas

The results of a theoretical investigation of prompt many-body ionization are reported. Our calculations address an experiment that reported ionization in Rydberg gases for densities two orders of magnitude less than expected from ionization between pairs of atoms. The authors argued that the results were due to the simultaneous interaction between many atoms. We performed classical calculations for many interacting Rydberg atoms with the ions fixed in space and have found that the many atom interaction does allow ionization at lower densities than estimates from two atom interactions. However, we found that the density fluctuations in a gas play a larger role. These two effects are an order of magnitude too small to account for the experimental results suggesting at least one other mechanism strongly affects ionization.

physics.atom-ph↗

arXiv2014

Using stochastic acceleration to place experimental limits on the charge of antihydrogen

Assuming hydrogen is charge neutral, CPT invariance demands that antihydrogen also be charge neutral. Quantum anomaly cancellation also demands that antihydrogen be charge neutral. Standard techniques based on measurements of macroscopic quantities of atoms cannot be used to measure the charge of antihydrogen. In this paper, we describe how the application of randomly oscillating electric fields to a sample of trapped antihydrogen atoms, a form of stochastic acceleration, can be used to place experimental limits on this charge.

physics.atom-ph↗

arXiv2014

In situ electromagnetic field diagnostics with an electron plasma in a Penning-Malmberg trap

We demonstrate a novel detection method for the cyclotron resonance frequency of an electron plasma in a Penning-Malmberg trap. With this technique, the electron plasma is used as an in situ diagnostic tool for measurement of the static magnetic field and the microwave electric field in the trap. The cyclotron motion of the electron plasma is excited by microwave radiation and the temperature change of the plasma is measured non-destructively by monitoring the plasma's quadrupole mode frequency. The spatially-resolved microwave electric field strength can be inferred from the plasma temperature change and the magnetic field is found through the cyclotron resonance frequency. These measurements were used extensively in the recently reported demonstration of resonant quantum interactions with antihydrogen.

physics.plasm-ph↗

arXiv2014

The Spectrum of Quasistable States in a Strong Microwave Field

When atoms are exposed to intense laser or microwave pulses ~10% of the atoms are found in Rydberg states subsequent to the pulse, even if it is far more intense than required for static field ionization. The optical spectra of the surviving Li atoms in the presence of a 38 GHz microwave field suggest how atoms survive an intense pulse. The spectra exhibit a periodic train of peaks 38 GHz apart. One peak is just below the limit, and with a 90 V/cm field amplitude the train extends from 300 GHz above the limit to 3000 GHz below it. The spectra and quantum mechanical calculations imply that the atoms survive in quasi stable states in which the Rydberg electron is in a weakly bound orbit infrequently returning to the ionic core during the intense pulse.

physics.atom-ph↗

arXiv2014

A proposal for sympathetically cooling neutral molecules using cold ions

We describe a method for cooling neutral molecules that have magnetic and electric dipole moments using collisions with cold ions. An external magnetic field is used to split the ground rovibrational energy levels of the molecule. The highest energy state within the ground rovibrational manifold increases in energy as the distance to the ion decreases leading to a repelling potential. At low energy, inelastic collisions are strongly suppressed due to the large distance of closest approach. Thus, a collision between a neutral molecule and a cold ion will lead to a decrease in the molecule's kinetic energy with no change in internal energy. We present results for the specific case of OH molecules cooled by Be$^+$, Mg$^+$, or Ca$^+$ ions.

physics.atom-ph↗

arXiv2014

Hole burning and higher order photon effects in attosecond light-atom interaction

We have performed calculations of attosecond laser-atom interactions for laser intensities where interesting two and three photon effects become relevant. In particular, we examine the case of "hole burning" in the initial orbital. Hole burning is present when the laser pulse duration is shorter than the classical radial period because the electron preferentially absorbs the photon near the nucleus. We also examine how 3 photon Raman process can lead to a time delay in the outgoing electron for the energy near one photon absorption. For excitation out of the hydrogen $2s$ state, an intensity of $2.2\times 10^{16}$ W/cm$^2$ leads to a 6 attosecond delay of the outgoing electron. We argue that this delay is due to the hole burning in the initial state.

physics.atom-ph↗

arXiv2014

The effect of random positions for dipole hopping through a Rydberg gas

We calculate the effect of two kinds of randomness on the hopping of an excitation through a nearly regular Rydberg gas. We present calculations for how fast the excitation can hop away from its starting position for different dimensional lattices and for different levels of randomness. We also examine the asymptotic in time final position of the excitation to determine whether or not the excitation can be localized. The one dimensional system is an example of Anderson localization where the randomness is in the off-diagonal elements although the long-range nature of the interaction leads to non-exponential decay with distance. The two dimensional square lattice shows a mixture of extended and localized states for large randomness while there is no visible sign of localized states for weak randomness. The three dimensional cubic lattice has few localized states even for strong randomness.

physics.atom-ph↗

arXiv2012

A proposal for laser cooling antihydrogen atoms

We present a scheme for laser cooling applicable for an extremely dilute sample of magnetically trapped antihydrogen atoms($\bar{H}$). Exploiting and controlling the dynamical coupling between the $\bar{H}$'s motional degrees of freedom in a magnetic trap, three-dimensional cooling can be achieved from Doppler cooling on one dimension using the $1s_{1/2}-2p_{3/2}$ transition. The lack of three-dimensional access to the trapped $\bar{H}$ and the nearly separable nature of the trapping potential leads to difficulties in cooling. Using realistic models for the spatial variation of the magnetic fields, we find that it should be possible to cool the $\bar{H}$'s to $\sim 20$ mK even with these constraints.

physics.atom-ph↗

arXiv2012

Antihydrogen and mirror-trapped antiproton discrimination: Discriminating between antihydrogen and mirror-trapped antiprotons in a minimum-B trap

Recently, antihydrogen atoms were trapped at CERN in a magnetic minimum (minimum-B) trap formed by superconducting octupole and mirror magnet coils. The trapped antiatoms were detected by rapidly turning off these magnets, thereby eliminating the magnetic minimum and releasing any antiatoms contained in the trap. Once released, these antiatoms quickly hit the trap wall, whereupon the positrons and antiprotons in the antiatoms annihilated. The antiproton annihilations produce easily detected signals; we used these signals to prove that we trapped antihydrogen. However, our technique could be confounded by mirror-trapped antiprotons, which would produce seemingly-identical annihilation signals upon hitting the trap wall. In this paper, we discuss possible sources of mirror-trapped antiprotons and show that antihydrogen and antiprotons can be readily distinguished, often with the aid of applied electric fields, by analyzing the annihilation locations and times. We further discuss the general properties of antiproton and antihydrogen trajectories in this magnetic geometry, and reconstruct the antihydrogen energy distribution from the measured annihilation time history.

physics.atom-ph↗

arXiv2011

Magic-wavelength optical traps for Rydberg atoms

We propose blue-detuned optical traps that are suitable for trapping of both ground state and Rydberg excited atoms. Addition of a background compensation field or suitable choice of the trap geometry provides a magic trapping condition for ground and Rydberg atoms at the trap center. Deviations from the magic condition at finite temperature are calculated. Designs that achieve less than 200 kHz differential trap shift between Cs ground and 125s Rydberg states for 10 μK Cs atoms are presented. Consideration of the trapping potential and photoionization rates

physics.atom-ph↗

arXiv2011

Confinement of antihydrogen for 1000 seconds

Atoms made of a particle and an antiparticle are unstable, usually surviving less than a microsecond. Antihydrogen, made entirely of antiparticles, is believed to be stable, and it is this longevity that holds the promise of precision studies of matter-antimatter symmetry. We have recently demonstrated trapping of antihydrogen atoms by releasing them after a confinement time of 172 ms. A critical question for future studies is: how long can anti-atoms be trapped? Here we report the observation of anti-atom confinement for 1000 s, extending our earlier results by nearly four orders of magnitude. Our calculations indicate that most of the trapped anti-atoms reach the ground state. Further, we report the first measurement of the energy distribution of trapped antihydrogen which, coupled with detailed comparisons with simulations, provides a key tool for the systematic investigation of trapping dynamics. These advances open up a range of experimental possibilities, including precision studies of CPT symmetry and cooling to temperatures where gravitational effects could become apparent.

physics.atom-ph↗

arXiv2011

Centrifugal separation and equilibration dynamics in an electron-antiproton plasma

Charges in cold, multiple-species, non-neutral plasmas separate radially by mass, forming centrifugally-separated states. Here, we report the first detailed measurements of such states in an electron-antiproton plasma, and the first observations of the separation dynamics in any centrifugally-separated system. While the observed equilibrium states are expected and in agreement with theory, the equilibration time is approximately constant over a wide range of parameters, a surprising and as yet unexplained result. Electron-antiproton plasmas play a crucial role in antihydrogen trapping experiments.

physics.plasm-ph↗

arXiv2011

Alpha Antihydrogen Experiment

ALPHA is an experiment at CERN, whose ultimate goal is to perform a precise test of CPT symmetry with trapped antihydrogen atoms. After reviewing the motivations, we discuss our recent progress toward the initial goal of stable trapping of antihydrogen, with some emphasis on particle detection techniques.

nucl-ex↗

arXiv2010

Evaporative Cooling of Antiprotons to Cryogenic Temperatures

We report the application of evaporative cooling to clouds of trapped antiprotons, resulting in plasmas with measured temperature as low as 9~K. We have modeled the evaporation process for charged particles using appropriate rate equations. Good agreement between experiment and theory is observed, permitting prediction of cooling efficiency in future experiments. The technique opens up new possibilities for cooling of trapped ions and is of particular interest in antiproton physics, where a precise \emph{CPT} test on trapped antihydrogen is a long-standing goal.

physics.plasm-ph↗

arXiv2010

Using Cold Atoms to Measure Neutrino Mass

We propose a beta decay experiment based on a sample of ultracold atomic tritium. These initial conditions enable detection of the helium ion in coincidence with the beta. We construct a two-dimensional fit incorporating both the shape of the beta-spectrum and the direct reconstruction of the neutrino mass peak. We present simulation results of the feasible limits on the neutrino mass achievable in this new type of tritium beta-decay experiment.

nucl-ex↗

arXiv2010

Antihydrogen formation dynamics in a multipolar neutral anti-atom trap

Antihydrogen production in a neutral atom trap formed by an octupole-based magnetic field minimum is demonstrated using field-ionization of weakly bound anti-atoms. Using our unique annihilation imaging detector, we correlate antihydrogen detection by imaging and by field-ionization for the first time. We further establish how field-ionization causes radial redistribution of the antiprotons during antihydrogen formation and use this effect for the first simultaneous measurements of strongly and weakly bound antihydrogen atoms. Distinguishing between these provides critical information needed in the process of optimizing for trappable antihydrogen. These observations are of crucial importance to the ultimate goal of performing CPT tests involving antihydrogen, which likely depends upon trapping the anti-atom.

hep-ex↗

arXiv2009

Spectral Line Width Broadening from Pair Fluctuation in a Frozen Rydberg Gas

Spectral line width broadening in Rydberg gases, a phenomenon previously attributed to the many-body effect, was observed experimentally almost a decade ago. The observed line width was typically 80-100 times larger than the average interaction strength predicted from a binary interaction. The interpretation of such a phenomenon is usually based on the so-called diffusion model, where the line width broadening mostly originates from the diffusion of excitations. In this paper, we present a model calculation to show that diffusion is not the main mechanism to the line width broadening. We find that the rare pair fluctuation at small separation is the dominant factor contributing to this broadening. Our results give a width of about 20-30 times larger than the average interaction strength. More importantly, by turning off the diffusion process, we do not observe order of magnitude change in the spectral line width.

quant-ph↗

arXiv2009

Measurement and simulation of laser-induced fluorescence from non-equilibrium ultracold neutral plasmas

We report new measurements and simulations of laser-induced fluorescence in ultracold neutral plasmas. We focus on the earliest times, when the plasma equilibrium is evolving and before the plasma expands. In the simulation, the ions interact via the Yukawa potential in a small cell with wrapped boundary conditions. We solve the optical Bloch equation for each ion in the simulation as a function of time. Both the simulation and experiment show the initial Bloch vector rotation, disorder-induced heating, and coherent oscillation of the rms ion velocity. Detailed modeling of the fluorescence signal makes it possible to use fluorescence spectroscopy to probe ion dynamics in ultracold and strongly coupled plasmas.

physics.atom-ph↗