  In this article we describe the basic principles of Rydberg atom-based RF
sensing and present the development of atomic pulsed RF detection and RF phase
sensing establishing capabilities pertinent to applications in communications
and sensing. To date advances in Rydberg atom-based RF field sensors have been
rooted in a method in which the fundamental physical quantity being detected
and measured is the electric field amplitude, $E$, of the incident RF
electromagnetic wave. The first part of this paper is focused on using
atom-based $E$-field measurement for RF field-sensing and communications
applications. With established phase-sensitive technologies, such as synthetic
aperture radar (SAR) as well as emerging trends in phased-array antennas in 5G,
a method is desired that allows robust, optical retrieval of the RF phase using
an enhanced atom-based field sensor. In the second part of this paper we
describe our fundamentally new atomic RF sensor and measurement method for the
phase of the RF electromagnetic wave that affords all the performance
advantages exhibited by the atomic sensor. The presented phase-sensitive RF
field detection capability opens atomic RF sensor technology to a wide array of
application areas including phase-modulated signal communication systems,
radar, and field amplitude and phase mapping for near-field/far-field antenna
characterizations.
