  We demonstrate a new method for measuring radio frequency (RF) electric
fields based on quantum interference in an atom. Using a bright resonance
prepared within an electromagnetically induced transparency window we are able
to achieve a sensitivity of ~ 30 {\mu}V cm^-1 sqrt(Hz)^-1. For this work, we
demonstrated detection of RF electric fields as small as ~ 8 {\mu}V cm^-1. The
sensitivity is currently limited by the spectral bandwidth of the lasers used
in the experiment and can be significantly improved in the future. The method
can serve as a new quantum based standard for RF electrometry. The
reproducibility, accuracy and stability of using a quantum system for measuring
RF electric fields promises to advance electrometry to the current levels of
magnetometry.
