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International Journal of Modern Physics Conference Series

Atom control and gravity measurements using Rydberg positronium

D. B. Cassidy, S. D. Hogan

Published Jan 1, 2014• 77 citations
Publisher

Abstract

We consider some of the obstacles that will have to be overcome in order to perform a direct measurement of the gravitational free-fall of positronium atoms. Foremost among these are the production of positronium atoms in a cryogenic environment, efficient excitation of these atoms to suitably long-lived Rydberg states, and their subsequent control via the interaction of their large electric dipole moments with inhomogeneous electric fields. Recent developments in all of these areas can be directly applied to a positronium free-fall gravity measurement, making such an endeavour both timely and feasible.

DOI
10.1142/s2010194514602592

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We consider some of the obstacles that will have to be overcome in order to perform a direct measurement of the gravitational free-fall of positronium atoms. Foremost among these are the production of positronium atoms in a cryogenic environment, efficient excitation of these atoms to suitably long-lived Rydberg states, and their subsequent control via the interaction of their large electric dipole moments with inhomogeneous electric fields. Recent developments in all of these areas can be directly applied to a positronium free-fall gravity measurement, making such an endeavour both timely and feasible.
Abstract We describe an experimental scheme wherein phase shifts induced by the gravitational field of the Earth on atoms in superpositions of Rydberg states with large static electric dipole moments can be observed by matter-wave interferometry. Such measurements, performed using positronium (Ps) atoms would constitute a test of the weak equivalence principle for a purely leptonic matter-antimatter system. Using realistic experimental parameters, obtained from measured data, we estimate that this approach could make it possible to measure the gravitational acceleration of Ps with a precision on the order of 10%.
The production of long-lived Rydberg positronium (Ps) and correction for Doppler shifts in the excitation laser frequencies are crucial elements of proposed measurements of the gravitational freefall of antimatter and for precision measurements of the optical spectrum of Ps. Using a two-step optical transition via 2P2P2P levels, we have prepared Ps atoms in Rydberg states up to the term limit. The spectra are corrected for the first-order Doppler shift using measured velocities, and the Balmer transitions are resolved for $15\ensuremath{\le}n\ensuremath{\le}31$. The excitation signal amplitude begins to decrease for n>50n>50n>50, consistent with the onset of motional electric field ionization in the 3.5-mT magnetic field at the Ps formation target.
We demonstrate experimentally the production of Rydberg positronium (Ps) atoms in a two-step process, comprising incoherent laser excitation, first to the 2(3)P state and then to states with principal quantum numbers ranging from 10 to 25. We find that excitation of 2(3)P atoms to Rydberg levels occurs very efficiently (~90%) and that the ~25% overall efficiency of the production of Rydberg atoms is determined almost entirely by the spectral overlap of the primary excitation laser and the Doppler broadened width of the 1 (3)S-2(3)P transition. The observed efficiency of Rydberg Ps production can be explained if stimulated emission back to the 2P states is suppressed, for example, by intermixing of the Rydberg state Stark sublevels. The efficient production of long-lived Rydberg Ps in a high magnetic field may make it possible to perform direct measurements of the gravitational free fall of Ps.
In this paper we consider the feasibility of observing the gravitational quantum states of positronium. The proposed scheme employs the flow-throw technique used for the first observation of this effect with neutrons. Collimation and Stark deceleration of Rydberg positronium atoms allow to select the required velocity class. If this experiment could be realized with positronium it would lead to a determination of g for this matter-antimatter system at the few % level. As discussed in this contribution, most of the required techniques are currently available but important milestones have to be demonstrated experimentally before such an experiment could become reality. Those are: the efficient focusing of a bunched positron beam, Stark deceleration of Rydberg positronium and its subsequent excitation into states with large angular momentum. We provide an estimate of the efficiencies we expect for these steps and assuming those could be confirmed we calculate the signal rate.