'How to correctly convert Year-month to Year-quarter

I am trying to convert a pandas dataframe containing date in YYYYMM format to YYYYQ format as below

import pandas as pd
dat = pd.DataFrame({'date' : ['200612']})
pd.PeriodIndex(pd.to_datetime(dat.date), freq='Q')

However this generates output as 2012Q2, whereas correct output should be 2006Q4

What is the right way to get correct Quarter?

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