  In this work we demonstrate an approach for the measurement of
radio-frequency (RF) power using electromagnetically induced transparency (EIT)
in a Rydberg atomic vapor. This is accomplished by placing alkali atomic vapor
in a rectangular waveguide and measuring the electric (E) field strength
(utilizing EIT and Autler-Townes splitting) for a wave propagating down the
waveguide. The RF power carried by the wave is then related to this measured
E-field, which leads to a new direct International System of Units (SI)
measurement of RF power. To demonstrate this approach, we first measure the
field distribution of the fundamental mode in the waveguide and then measure
the power carried by the wave at both 19.629 GHz and 26.526 GHz. We obtain good
agreement between the power measurements obtained with this new technique and
those obtained with a conventional power meter.
