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Spectral properties of reduced fermionic density operators and parity superselection rule

We consider pure fermionic states with a varying number of quasiparticles and analyze two types of reduced density operators: one is obtained via tracing out modes, the other is obtained via tracing out particles. We demonstrate that spectra of mode-reduced states are not identical in general and fully characterize pure states with equispectral mode-reduced states. Such states are related via local unitary operations with states satisfying the parity superselection rule. Thus, valid purifications for fermionic density operators are found. To get particle-reduced operators for a general system, we introduce the operation $Φ(\varrho) = \sum_i a_i \varrho a_i^†$. We conjecture that spectra of $Φ^p(\varrho)$ and conventional $p$-particle reduced density matrix $\varrho_p$ coincide. Nontrivial generalized Pauli constraints are derived for states satisfying the parity superselection rule.