  We study the diffusivity of a tagged particle in a binary mixture of Brownian
particles with non-reciprocal interactions. Numerical simulations reveal that,
for a broad class of interaction potentials, non-reciprocity can significantly
increase the effective diffusion coefficient of tracer particles, and that this
diffusion enhancement is associated with a breakdown of the Einstein relation.
These observations are quantified and confirmed via two different and
complementary analytical approaches: (i) a linearized stochastic density field
theory, which is particularly accurate in the limit of soft interactions; (ii)
a reduced two-body description, which is exact at leading order in the density
of particles. The latter reveals that diffusion enhancement can be attributed
to the formation of transiently propelled dimers of particles, whose cohesion
and speed are controlled by the non-reciprocal interactions.
