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Challenge is simple, make the Chinese Empire last well into the 21st century with a POD no earlier than the Qin Dynasty and no later than the Ming Dynasty. This should be relatively easy considering you have a window that ranges from 220 BC to 1644 BC.
Caveats:
An Emperor must be in charge (even if it's just a figurehead)
Imperial Chinese culture must remain intact! If this is deviated, you FAIL!