The requirement of performing both single-qubit and two-qubit operations in
the implementation of universal quantum logic often leads to very demanding
constraints on quantum computer design. We show here how to eliminate the need
for single-qubit operations in a large subset of quantum computer proposals:
those governed by isotropic and XXZ,XY-type anisotropic exchange interactions.
Our method employs an encoding of one logical qubit into two physical qubits,
while logic operations are performed using an analogue of the NMR selective
recoupling method.Comment: 5 pages, 1 table, no figures. Published versio
