  The effect of the control on bipartite entanglement is discussed from a
geometric viewpoint for a nuclear magnetic resonance (NMR) system as a model of
the n-qubit control system. The Hamiltonian of the model is the sum of the
drift and control Hamiltonians, each of which describes the interaction between
pairs of qubits and between one of qubits and an external magnetic field,
respectively. According to a bipartite partition, the Schroedinger equation for
the NMR system is put in the matrix form. This paper gives a solution to the
Schroedinger equation with the assumptions of small coupling among qubits and
of constant controls. The solution is put in the form of power series in small
parameters. In particular, in the case of the two-qubit NMR system, the drift
and control Hamiltonians are shown to be coupled to work for entanglement
promotion, by examining solutions to the Schroedinger equation in detail. The
concurrence, a measure of entanglement, is evaluated along the solution for a
small time interval in order to observe that the control effect appears, not at
the first-order terms in t, but at the higher-order terms in t. The evaluated
concurrence also suggests which control makes the two-qubit more entangled or
less.
