  Two dimensional (2D) perovskites are promising materials for photonic
applications, given their outstanding nonlinear optical properties, ease of
fabrication and versatility. In particular, exploiting their high oscillator
strength, the crystalline form of 2D perovskites can be used as excitonic
medium in optical microcavities, allowing for the study of their optical
properties in the strong light-matter coupling regime. While polariton
condensation has been observed in different materials at room temperature, here
we observe for the first time two distinct threshold processes in a 2D
perovskite, a material that has never shown spontaneous phase transition up to
now. In particular, we demonstrate lasing from the bi-exciton state which
contributes to populate the lower polariton branch and, at higher excitation
powers, eventually leads to the formation of a polariton condensate. The
emission linewidth narrowing and a spatial coherence over 50 x 50 um2 area are
the smoking gun, the formation of a quantum coherent state in 2D hybrid
perovskite. Our results not only show the formation of a polariton condensate
in 2D perovskites but they are also crucial for the understanding of the
physical mechanisms that leads to coherent phase transition in perovskite-based
polariton microcavities.
