Fred L. Walls

Bio: Fred L. Walls is an academic researcher from National Institute of Standards and Technology. The author has contributed to research in topics: Phase noise & Noise spectral density. The author has an hindex of 31, co-authored 121 publications receiving 3673 citations. Previous affiliations of Fred L. Walls include United States Department of the Army & University of Washington.

Radiation-Pressure Cooling of Bound Resonant Absorbers

Abstract: We report the first observation of radiation-pressure cooling on a system of resonant absorbers which are elastically bound to a laboratory fixed apparatus. Mg ii ions confined in a Penning electromagnetic trap are cooled to 40 K by irradiating them with the 8-\ensuremath{\mu}W output of a frequency doubled, single- mode dye laser tuned to the low- frequency side of the Doppler profile on the $^{2}S_{\frac{1}{2}}\ensuremath{\leftrightarrow}^{2}P_{\frac{3}{2}}$ (${M}_{J}=+\frac{1}{2}\ensuremath{\leftrightarrow}{M}_{J}=+\frac{3}{2}$ or ${M}_{J}=\ensuremath{-}\frac{1}{2}\ensuremath{\leftrightarrow}{M}_{J}=\ensuremath{-}\frac{3}{2}$) transitions. Cooling to approximately ${10}^{\ensuremath{-}3}$ K should be possible.

Measurement of total cross sections for electron recombination with NO+ and O2 + using ion storage techniques

Abstract: The total cross section as a function of electron energy for recombination of electrons with room temperature NO+ has been measured with a trapped ion technique. Measurements were made in the electron energy range 0.045–4 eV with an energy resolution between 0.045 and 0.120 eV, and the cross sections, which showed some structure, ranged from 1.25 × 10−14 cm² at the lowest energy to 1.7 × 10−16cm² at the highest energy. Similar measurements were made on O2+, the species used to calibrate the apparatus geometry. A Maxwellian distribution of electron velocities was used with the measured cross sections to calculate rate constants, giving values extending to electron temperatures as high as 40,000°K. Comparison with previously measured rate coefficients at lower temperatures is quite satisfactory.

Characterization of frequency stability in precision frequency sources

Abstract: The authors present a short review of the progress that has occurred during years 1955-91 in both the theoretical and practical characterization of frequency stability of precision frequency sources. The emphasis is on the evolution of ideas and concepts for the characterization of random noise processes in such standards in the time domain and the Fourier frequency domain, rather than a rigorous mathematical treatment of the problem. Numerous references to the mathematical treatments are made. >

Accuracy evaluation of NIST-F1

Abstract: The evaluation procedure of a new laser-cooled caesium fountain primary frequency standard developed at the National Institute of Standards and Technology (NIST) is described. The new standard, NIST-F1, is described in some detail and typical operational parameters are discussed. Systematic frequency biases for which corrections are made - second-order Zeeman shift, black-body radiation shift, gravitational red shift and spin-exchange shift - are discussed in detail. Numerous other frequency shifts are evaluated, but are so small in this type of standard that corrections are not made for their effects. We also discuss comparisons of this standard both with local frequency standards and with standards at other national laboratories.

"Bolometric" Technique for the rf Spectroscopy of Stored Ions

Abstract: A simple technique is proposed in which rf transitions between suitable energy levels of an atomic or molecular ion, a member of a stored, radiatively thermalized ion gas, may be detected by monitoring the translational temperature of the ion gas. An experimental study of the cyclotron resonance of an electron gas confined in a Penning trap demonstrates its usefulness, and possible applicability to spin resonance.

Observation of Bose-Einstein Condensation in a Dilute Atomic Vapor

Abstract: A Bose-Einstein condensate was produced in a vapor of rubidium-87 atoms that was confined by magnetic fields and evaporatively cooled. The condensate fraction first appeared near a temperature of 170 nanokelvin and a number density of 2.5 x 10 12 per cubic centimeter and could be preserved for more than 15 seconds. Three primary signatures of Bose-Einstein condensation were seen. (i) On top of a broad thermal velocity distribution, a narrow peak appeared that was centered at zero velocity. (ii) The fraction of the atoms that were in this low-velocity peak increased abruptly as the sample temperature was lowered. (iii) The peak exhibited a nonthermal, anisotropic velocity distribution expected of the minimum-energy quantum state of the magnetic trap in contrast to the isotropic, thermal velocity distribution observed in the broad uncondensed fraction.

Quantum dynamics of single trapped ions

Abstract: Single trapped ions represent elementary quantum systems that are well isolated from the environment. They can be brought nearly to rest by laser cooling, and both their internal electronic states and external motion can be coupled to and manipulated by light fields. This makes them ideally suited for quantum-optical and quantum-dynamical studies under well-controlled conditions. Theoretical and experimental work on these topics is reviewed in the paper, with a focus on ions trapped in radio-frequency (Paul) traps.

Optics and interferometry with atoms and molecules

Abstract: The development of wave optics for light brought many new insights into our understanding of physics, driven by fundamental experiments like the ones by Young, Fizeau, Michelson-Morley and others. Quantum mechanics, and especially the de Broglie’s postulate relating the momentum p of a particle to the wave vector k of an matter wave: k = 2 λ = p/ℏ, suggested that wave optical experiments should be also possible with massive particles (see table 1), and over the last 40 years electron and neutron interferometers have demonstrated many fundamental aspects of quantum mechanics [1].

Geonium theory: Physics of a single electron or ion in a Penning trap

Abstract: A single charged particle in a Penning trap is a bound system that rivals the hydrogen atom in its simplicity and provides similar opportunities to calculate and measure physical quantities at very high precision. We review the theory of this bound system, beginning with the simple first-order orbits and progressively dealing with small corrections which must be considered owing to the experimental precision that is being achieved. Much of the discussion will also be useful for experiments with more particles in the trap, and several of the mathematical techniques have a wider applicability.