  We study the problem of an impurity in fully polarized (spin-up) low density
neutron matter with the help of an accurate quantum Monte Carlo method in
conjunction with a realistic nucleon-nucleon interaction derived from chiral
effective field theory at next-to-next-to-leading-order. Our calculations show
that the behavior of the proton spin-down impurity is very similar to that of a
polaron in a fully polarized unitary Fermi gas. We show that our results can be
used to put tight constraints on the time-odd parts of the energy density
functional, independent of the time-even parts, in the density regime relevant
to neutron-rich nuclei and compact astrophysical objects such as neutron stars
and supernovae.
