Observation of geometric phase for unpolarized and partially polarized light fields

Geometric phase, owing to its topological nature and properties of fault tolerance, plays an important role in devising real world applications in both classical and quantum domain. For classical systems, geometric phase has been observed and studied so far for fully polarized light only. Using an interferometric experiment we demonstrate, for the first time, the existence of Pancharatnam-Berry phase for states covering all empty space inside the Poincar'e sphere namely the unpolarized and partially polarized light fields. The observed geometric phase is found identical to its fully polarized counterpart in excellent agreement with the theoretical predictions.