where σ σ i x / y / z are Pauli spin-1/2 operators in a chain of length L , all the couplings { J i , h i } are positive random numbers drawn from distributions with means J ¯ ¯ , h ¯ ¯ , and the system has Ising symmetry P = ∏ ∏ i σ σ i x corresponding to flipping all spins in the z basis. The J i n t term introduces interactions, and the system is mappable to a free fermion model (the Kitaev chain) when J i n t = 0 .