  In this paper we present an optimal protocol by which an unknown state on a
Hilbert space of dimension $N$ can be approximately stored in an
$M$-dimensional quantum system or be approximately teleported via an
$M$-dimensional quantum channel. The fidelity of our procedure is determined
for pure states as well as for mixed states and states which are entangled with
auxiliary quantum systems of varying Hilbert space dimension, and it is
compared with theoretical results for the maximally achievable fidelity.

Quantum Physics