Under what condition, an ideal Carnot engine has 100% efficiency?

Efficiency of a Carnot engine is given by

where, { T }_{ 2 } = temperature of sink and
and { T }_{ 1 } = temperature of sink source.
So n = 1 or 100%, { T }_{ 2 } = 0 K or heat is rejected into a sink at 0 K temperature.