  The surprising recent observation of highly emissive triplet-states in lead
halide perovskites accounts for their orders-of-magnitude brighter optical
signals and high quantum efficiencies compared to other semiconductors. This
makes them attractive for future optoelectronic applications, especially in
bright low-threshold nano-lasers. Whilst non-resonantly pumped lasing from
all-inorganic lead-halide perovskites is now well-established as an attractive
pathway to scalable low-power laser sources for nano-optoelectronics, here we
showcase a resonant optical pumping scheme on a fast triplet-state in CsPbBr3
nanocrystals. The scheme allows us to realize a polarized triplet-laser source
that dramatically enhances the coherent signal by one order of magnitude whilst
suppressing non-coherent contributions. The result is a source with highly
attractive technological characteristics including a bright and polarized
signal, and a high stimulated-to-spontaneous emission signal contrast that can
be filtered to enhance spectral purity. The emission is generated by pumping
selectively on a weakly-confined excitonic state with a Bohr radius ~10 nm in
the nanocrystals. The exciton fine-structure is revealed by the
energy-splitting resulting from confinement in nanocrystals with tetragonal
symmetry. We use a linear polarizer to resolve two-fold non-degenerate
sub-levels in the triplet exciton and use photoluminescence excitation
spectroscopy to determine the energy of the state before pumping it resonantly.
