  We analyze and evaluate perfect controlled teleportation via three-qubit
entangled channels from the point of view of the controller. The key idea in
controlled teleportation is that the teleportation is performed only with the
participation of the controller. We calculate a quantitative measure of the
controller's power and establish a lower bound on the control power required
for controlled teleportation. We show that the maximally entangled GHZ state is
a suitable channel for controlled teleportation of arbitrary single qubits -
the controller's power meets the bound and the teleportation fidelity without
the controller's permission is no better than the fidelity of a classical
channel. We also construct partially entangled channels that exceed the bound
for controlled teleportation of a restricted set of states called the
equatorial states. We calculate the minimum entanglement required in these
channels to exceed the bound. Moreover, we find that in these restricted
controlled teleportation schemes, the partially entangled channels can
outperform maximally entangled channels with respect to the controller's power.
Our results provide a new perspective on controlled teleportation schemes and
are of practical interest since we propose useful partially entangled channels.
