2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/212037We present an efficient scheme which couples any designated pair of spins in heteronuclear spin systems. The scheme is based on the existence of Hadamard matrices. For a system of $n$ spins with pairwise coupling, the scheme concatenates $cn$ intervals of system evolution and uses at most $c n^2$ pulses where $c \approx 1$. Our results demonstrate that, in many systems, selective recoupling is possible with linear overhead, contrary to common speculation that exponential effort is always required.7 pages, 4 figures, mypsfig2, revtex, submitted April 27, 1999Quantum Physicstext