  The exchange coupling between quantum dot spin qubits is isotropic, which
restricts the types of quantum gates that can be formed. Here, we propose a
method for controlling anisotropic interactions between spins arranged in a bus
geometry. The symmetry is broken by an external magnetic field, resulting in
XXZ-type interactions that can efficiently generate Greenberger-Horne-Zeilinger
(GHZ) states or universal gate sets for exchange-only quantum computing.
Analysis of the bus ground state provides a link between quantum critical
phenomena and the communication capacity.
