  We consider the ``transport'' of the state of a spin across a
Heisenberg-coupled spin chain via the use of repeated SWAP gates, starting with
one of two states---one in which the leftmost spin is down and the others up,
and one in which the leftmost two spins are in a singlet state (i.e., they are
entangled), and the others are again all up. More specifically, we
``transport'' the state of the leftmost spin in the first case and the
next-to-leftmost spin in the second to the other end of the chain, and then
back again. We accomplish our SWAP operations here by increasing the exchange
coupling between the two spins that we operate on from a base value $J$ to a
larger value $J_\text{SWAP}$ for a time $t=\pi\hbar/4J_\text{SWAP}$. We
determine the fidelity of this sequence of operations in a number of
situations---one in which only nearest-neighbor coupling exists between spins
and there is no magnetic dipole-dipole coupling or noise (the most ideal case),
one in which we introduce next-nearest-neighbor coupling, but none of the other
effects, and one in which all of these effects are present. In the last case,
the noise is assumed to be quasistatic, i.e., the exchange couplings are each
drawn from a Gaussian distribution, truncated to only nonnegative values. We
plot the fidelity as a function of $J_\text{SWAP}$ to illustrate various
effects, namely crosstalk due to coupling to other spins, as well as noise,
that are detrimental to our ability to perform a SWAP operation. Our theory
should be useful to the ongoing experimental efforts in building
semiconductor-based spin quantum computer architectures.
