Quantum XX chain
= Quantum XX chain
{title2=$H=-J\sum_n(S_n^xS_{n+1}^x+S_n^yS_{n+1}^y)-h\sum_nS_n^z$}
= Isotropic quantum XY chain
{synonym}
The spin-one-half <quantum XX chain> has equal nearest-neighbour transverse exchange. Its continuous <spin rotation> <symmetry> conserves total longitudinal <magnetization>. The <Jordan–Wigner transformation> maps it to number-conserving free <fermions>. This quantum chain is distinct from the classical <XY model>.