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The design of low-delay multibit adders in quantum dot cellular automata is considered in this brief. We present a general approach for delay reduction based on two new theorems called decomposition theorems. We consider the carry-lookahead adder (CLA) and the carry-flow adder (CFA) as specific applications of the theorems. For 16-bit CLA and 16-bit CFA, the decomposition theorems yield reductions in delay for the leading carry of approximately 60% and 25%, respectively, when compared to the best existing designs. In addition, the decomposition theorems lead to designs with low area-delay product. Simulations in QCADesigner are also presented.