  Novel schemes for generating ultra-low emittance electron beams have been
developed in past years and promise compact particle sources with excellent
beam quality suitable for future high-energy physics experiments and
free-electron lasers. Recent theoretical work has proposed a laser-based method
capable of resolving emittances in the sub 0.1 mm mrad regime, by modulating
the electron phase-space ponderomotively. Here we present a first experimental
demonstration of this scheme using a laser wakefield accelerator. The observed
emittance and source size is consistent with published values. We also show
calculations demonstrating that tight bounds on the upper limit for emittance
and source size can be derived from the 'laser-grating' method even in the
presence of low signal to noise and uncertainty in laser-grating parameters.
