  Excellent energy resolution is one of the primary advantages of
electroluminescent high pressure xenon TPCs, and searches for rare physics
events such as neutrinoless double-beta decay ($\beta\beta0\nu$) require
precise energy measurements. Using the NEXT-White detector, developed by the
NEXT (Neutrino Experiment with a Xenon TPC) collaboration, we show for the
first time that an energy resolution of 1% FWHM can be achieved at 2.6 MeV,
establishing the present technology as the one with the best energy resolution
of all xenon detectors for $\beta\beta0\nu$ searches.
