x = df.column_A.str.extract('(?P<ones>\\d+)/(?P<len>\\d+)', expand=True
    ).astype(int).assign(o='1', z='0')
x
x.o.str.repeat(x.ones) + x.z.str.repeat(x.len - x.ones)
Pandas Dataframe: How to parse integers into string of 0s and 1s?