  In liquid-state NMR quantum computation, a selective entanglement operator
between qubits 2 and 3 of a three-qubit molecule is conventionally realized by
applying a pair of short $\pi$-pulses to qubit 1. This method, called
refocusing, is well suited for heteronuclear molecules. When the molecule is
homonuclear, however, the $\pi$-pulses applied to qubit 1 often induce unwanted
$z$-rotations on qubits 2 and 3, even if the $z$-components of qubits 2 and 3
are left unchanged. This phenomenon is known as the transient Bloch-Siegert
effect, and compensation thereof is required for precise gate operation. We
propose an alternative refocusing method, in which a weak square pulse is
applied to qubit 1. This technique has the advantage of curbing the
Bloch-Siegert effect, making it suitable for both hetero- and homonuclear
molecules.
