  In this Letter, we derive an entropic Einstein-Podolsky-Rosen (EPR) steering
inequality for continuous variable (CV) systems using only experimentally
measured discrete probability distributions and details of the measurement
apparatus. We use this inequality to witness EPR steering between the positions
and momenta of photon pairs generated in spontaneous parametric downconversion
(SPDC). We examine the asymmetry between parties in this inequality, and show
that this asymmetry can be used to reduce the technical requirements of
experimental setups intended to demonstrate the EPR paradox. Furthermore, we
develop a more stringent steering inequality that is symmetric between parties,
and use it to show that the downconverted photon pairs also exhibit symmetric
EPR steering.
