  An exactly solvable model for the decoherence of one and two-qubit states
interacting with a spin-bath, in the presence of a time-dependent magnetic
field is studied. The magnetic field is static along $\hat{z}$ direction and
oscillatory in the transverse plane. The transition probability and Rabi
oscillations between the spin-states of a single qubit is shown to depend on
the size of bath, the distribution of qubit-bath couplings and the initial bath
polarization. In contrast to the fast Gaussian decay for short times, the
polarization of the qubit shows an oscillatory power-law decay for long times.
The loss of entanglement for the maximally entangled two-qubit states, can be
controlled by tuning the frequency of the rotating field. The decay rates of
entanglement and purity for all the Bell-states are same when the qubits are
non-interacting, and different when they are interacting.

Quantum Physics;Mesoscale and Nanoscale Physics