Performance of the ATLAS Liquid Argon Calorimeter after three years of LHC operation and plans for a future upgrade

The ATLAS experiment is designed to study the proton-proton collisions produced at the Large Hadron Collider (LHC) at CERN. Liquid argon sampling calorimeters are used for all electromagnetic calorimetry as well as hadronic calorimetry in the endcaps. After installation in 2004--2006, the calorimeters were extensively commissioned over the three--year period prior to first collisions in 2009, using cosmic rays and single LHC beams. Since then, approximately 27~fb$\mathbf{^{-1}}$ of data have been collected at an unprecedented center of mass energy. During all these stages, the calorimeter and its electronics have been operating almost optimally, with a performance very close to specifications. This paper covers all aspects of these first years of operation. The excellent performance achieved is especially presented in the context of the discovery of the elusive Higgs boson. The future plans to preserve this performance until the end of the LHC program are also presented.