  We present an exact rate analysis for a secret key that can be shared among
two parties employing a linear quantum repeater chain. One of our main
motivations is to address the question whether simply placing quantum memories
along a quantum communication channel can be beneficial in a realistic setting.
The underlying model assumes deterministic entanglement swapping of single-spin
quantum memories and it excludes probabilistic entanglement distillation, and
thus two-way classical communication, on higher nesting levels. Within this
framework, we identify the essential properties of any optimal repeater scheme:
entanglement distribution in parallel, entanglement swapping as soon and
parallel quantum storage as little as possible. While these features are
obvious or trivial for the simplest repeater with one middle station, for more
stations they cannot always be combined. We propose an optimal scheme including
channel loss and memory dephasing, proving its optimality for the case of two
stations and conjecturing it for the general case. In an even more realistic
setting, we consider additional tools and parameters such as memory cut-offs,
multiplexing, initial state and swapping gate fidelities, and finite link
coupling efficiencies in order to identify potential regimes in memory-assisted
quantum key distribution beyond one middle station that exceed the rates of the
smallest quantum repeaters as well as those obtainable in all-optical schemes
unassisted by stationary memory qubits and two-way classical communication.
