  We report a record-size, two-dimensional polariton condensate of a fraction
of a millimeter radius free from the presence of an exciton reservoir. This
macroscopically occupied state is formed by the ballistically expanding
polariton flow that relaxes and condenses over a large area outside of the
excitation spot. The density of this trap-free condensate is < 1
polariton/{\mu}m^2, reducing the phase noise induced by the interaction energy.
Moreover, the backflow effect, recently predicted for the nonparabolic
polariton dispersion, is observed here for the first time in the fast-expanding
wave packet.
