  We report on a quantitative measurement of the spatial coherence of electrons
emitted from a sharp metal needle tip. We investigate the coherence in
photoemission using near-ultraviolet laser triggering with a photon energy of
3.1 eV and compare it to DC-field emission. A carbon-nanotube is brought in
close proximity to the emitter tip to act as an electrostatic biprism. From the
resulting electron matter wave interference fringes we deduce an upper limit of
the effective source radius both in laser-triggered and DC-field emission mode,
which quantifies the spatial coherence of the emitted electron beam. We obtain
$(0.80\pm 0.05)\,$nm in laser-triggered and $(0.55\pm 0.02)\,$nm in DC-field
emission mode, revealing that the outstanding coherence properties of electron
beams from needle tip field emitters are largely maintained in laser-induced
emission. In addition, the relative coherence width of 0.36 of the photoemitted
electron beam is the largest observed so far. The preservation of electronic
coherence during emission as well as ramifications for time-resolved electron
imaging techniques are discussed.
