Wick rotation transforming a quantum field theory to Euclidean space via imaginary time substitution is a foundational definition in theoretical physics, requiring no citation.
The claim states a standard textbook definition and foundational mathematical procedure in theoretical physics (Wick rotation, connecting real-time quantum field theory to Euclidean statistical mechanics by substituting time with imaginary time). Since it is a definitional identity known by observation and consensus across physics literature, it represents common knowledge and does not require empirical paper citation.