Observation of a motional Stark effect to determine the second-order Doppler effect

Fingerprints of nonequilibrium stationary distributions in dispersion relations

Distributions different from those predicted by equilibrium statistical mechanics are commonplace in a number of physical situations, such as plasmas and self-gravitating systems. The best strategy for probing these distributions and unavailing their origins consists in combining theoretical knowledge with experiments, involving both direct and indirect measurements, as those associated with dispersion relations. This paper addresses, in a quite general context, the signature of nonequilibrium distributions in dispersion relations. We consider the very general scenario of distributions corresponding to a superposition of equilibrium distributions, that are well-suited for systems exhibiting only local equilibrium, and discuss the general context of systems obeying the combination of the Schrödinger and Poisson equations, while allowing the Planck's constant to smoothly go to zero, yielding the classical kinetic regime. Examples of media where this approach is applicable are plasmas, gravitational systems, and optical molasses. We analyse in more depth the case of classical dispersion relations for a pair plasma. We also discuss a possible experimental setup, based on spectroscopic methods, to directly observe these classes of distributions.

Two-photon frequency comb spectroscopy of atomic hydrogen

We have performed two-photon ultraviolet direct frequency comb spectroscopy on the 1S-3S transition in atomic hydrogen to illuminate the so-called proton radius puzzle and to demonstrate the potential of this method. The proton radius puzzle is a significant discrepancy between data obtained with muonic hydrogen and regular atomic hydrogen that could not be explained within the framework of quantum electrodynamics. By combining our result [f _1S-3S = 2,922,743,278,665.79(72) kilohertz] with a previous measurement of the 1S-2S transition frequency, we obtained new values for the Rydberg constant [R _∞ = 10,973,731.568226(38) per meter] and the proton charge radius [r _p = 0.8482(38) femtometers]. This result favors the muonic value over the world-average data as presented by the most recent published CODATA 2014 adjustment.

Cryogenic atomic hydrogen beam apparatus with velocity characterization

Precision spectroscopy of hydrogen often relies on effusive thermal atomic beams, and the uncertainty in the velocity distribution of these beams can introduce systematic errors and complicate lineshape models. Here, we present an apparatus capable of high signal-to-noise studies of these velocity distributions at cryogenic temperatures for both ground state (1S) and metastable (2S) hydrogen using a simple time-of-flight technique. We also investigate how the cryogenic nozzle geometry affects these results.

New Measurement of the 1S-3S Transition Frequency of Hydrogen: Contribution to the Proton Charge Radius Puzzle

We present a new measurement of the 1S-3S two-photon transition frequency of hydrogen, realized with a continuous-wave excitation laser at 205 nm on a room-temperature atomic beam, with a relative uncertainty of 9×10^{-13}. The proton charge radius deduced from this measurement, r_{p}=0.877(13)  fm, is in very good agreement with the current CODATA-recommended value. This result contributes to the ongoing search to solve the proton charge radius puzzle, which arose from a discrepancy between the CODATA value and a more precise determination of r_{p} from muonic hydrogen spectroscopy.

Controlling Positronium Annihilation with Electric Fields

We show that the annihilation dynamics of excited positronium (Ps) atoms can be controlled using parallel electric and magnetic fields. To achieve this, Ps atoms were optically excited to n=2 sublevels in fields that were adjusted to control the amount of short-lived and long-lived character of the resulting mixed states. Inclusion of the former offers a practical approach to detection via annihilation radiation, whereas the increased lifetimes due to the latter can be exploited to optimize resonance-enhanced two-photon excitation processes (e.g., 1^{3}S→2^{3}P→nS/nD), either by minimizing losses through intermediate state decay, or by making it possible to separate the excitation laser pulses in time. In addition, photoexcitation of mixed states with a 2^{3}S_{1} component represents an efficient route to producing long-lived pure 2^{3}S_{1} atoms via single-photon excitation.

Experimental considerations for testing antimatter antigravity using positronium 1S-2S spectroscopy

In this contribution to the WAG 2013 workshop we report on the status of our measurement of the 1S-2S transition frequency of positronium. The aim of this experiment is to reach a precision of 0.5 ppb in order to cross check the QED calculations. After reviewing the current available sources of Ps, we consider laser cooling as a route to push the precision in the measurement down to 0.1 ppb. If such an uncertainty could be achieved, this would be sensitive to the gravitational redshift and therefore be able to assess the sign of gravity for antimatter.