  Measurements of plasma electric fields are essential to the advancement of
plasma science and applications. Methods for non-invasive in situ measurements
of plasma fields on sub-millimeter length scales with high sensitivity over a
large field range remain an outstanding challenge. Here, we introduce and
demonstrate a new method for plasma electric field measurement that employs
electromagnetically induced transparency as a high-resolution quantum-optical
probe for the Stark energy level shifts of plasma-embedded Rydberg atoms, which
serve as highly-sensitive field sensors with a large dynamic range. The method
is applied in diagnostics of plasmas photo-excited out of a cesium vapor. The
plasma electric fields are extracted from spatially-resolved measurements of
field-induced shape changes and shifts of Rydberg resonances in rubidium tracer
atoms. Measurement capabilities over a range of plasma densities and
temperatures are exploited to characterize plasmas in applied magnetic fields
and to image electric-field distributions in cyclotron-heated plasmas.
