 Hepp found that, because of the superselection structure of this system, this measurement device admits mixed states that can indicate the original state of the single fermion, even when the latter begins in a pure superposition. Suppose we denote the \(z\)-spin observable for the nth fermion in the measurement device as, \(s_n = I \otimes I \otimes \cdots (n\,times) \cdots \otimes \sigma_z \otimes I \cdots.\) We now construct a new observable, given by,