Quantum ToolkitOperatorsNumberNumberOn this pageNumberNumber NumberNumber(integer i, integer j) -> FermionOperator Returns the two-site fermionic number-number interaction operator ninj=fi†fifj†fjn_{i} n_{j} = f_{i}^\dagger f_{i} f_{j}^\dagger f_{j}ninj=fi†fifj†fj with support on the specified sites. Parameters i: The first site index. j: The second site index. Example NumberNumber(1,2)*Fbit("0110") // Returns (1 + 0i)|0110>