2002ApJ...581L..67E

We describe a novel method to determine direct geometrical distances to quasars that can measure the cosmological constant, Λ, with minimal assumptions. This method is equivalent to geometric parallax, with the ``standard length'' being the size of the quasar broad emission line region (BELR) as determined from the light-travel time measurements of reverberation mapping. The effect of nonzero Λ on angular diameter is large, 40% at z=2, so mapping angular diameter distances versus redshift will give Λ with (relative) ease. In principle, these measurements could be made in the UV, optical, near-infrared, or even X-ray bands. Interferometers with a resolution of 0.01 mas are needed to measure the size of the BELR in z=2 quasars, which appear plausible given reasonable short-term extrapolations of current technology.