  The time evolution of entanglement for excitons in two quantum dots embedded
in a single mode cavity is studied in a ``spin-boson'' regime. It is found that
although with the dissipation from the boson mode, the excitons in the two
quantum dots can be entangled by only modulating their energy bias $\epsilon$
under the influence of external driving magnetic field. Initially, the two
excitons are prepared in a pure separate state. When the time-dependent
magnetic field is switched on, a highly entangled state is produced and
maintained even in a very long time interval. The mechanism may be used to
control the quantum devices in practical applications.
