Previously it was shown that the presence of negative-norm states or negative-frequency solutions is indispensable for a fully covariant quantization (Krein space quantization) of the minimally coupled free scalar field in de Sitter spacetime. This new method of quantization has been extended to free boson and spinor fields in Minkowski spacetime. The presence of unphysical negative-frequency states plays the role of an automatic renormalization tool for the theory.
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