  Unveiling the structure of the Broad Line Region (BLR) of AGNs is critical to
understand the quasar phenomenon. Resolving a few BLRs by optical
interferometry will bring decisive information to confront, complement and
calibrate the reverberation mapping technique, basis of the mass-luminosity
relation in quasars. BLRs are much smaller than the angular resolution of the
VLT and Keck interferometers and they can be resolved only by differential
interferometry very accurate measurements of differential visibility and phase
as a function of wavelength. The latter yields the photocenter variation with
wavelength, and constrains the size, position and velocity law of various
regions of the BLR. AGNs are below the magnitude limit for spectrally resolved
interferometry set by currently available fringe trackers. A new "blind"
observation method and a data processing based on the accumulation of 2D
Fourier power and cross spectra permitted us to obtain the first spectrally
resolved interferometric observation of a BLR, on the K=10 quasar 3C273. A
careful bias analysis is still in progress, but we report strong evidence that,
as the baseline increases, the differential visibility decreases in the
Pa-alpha line. Combined with a differential phase smaller than 3 degree, this
yields an angular equivalent radius of the BLR larger than 0.4 milliarcseconds,
or 1000 light days at the distance of 3C273, much larger than the reverberation
mapping radius of 300 light days. Explaining the coexistence of these two
different sizes, and possibly structures and mechanisms, implies very new
insights into the BLR of 3C273
