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