We reformulate the WKB wave function for a multidimensional tunneling system, which represents the quasi-ground-state of the metastable vacuum, in a covariant manner. Then we extend the formalism to the case of field theory and develop a systematic method to construct the mode functions which determine the quantum state after tunneling. A clear interpretation of the resulting quantum state is given in the language of the conventional second quantization picture. As a simple example, we apply our method to a scalar field on the background of spatially homogeneous false vacuum decay. The resulting quantum state is found to be highly nontrivial, having some similarity with a thermal state. Some implications of the results are discussed.
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