  Optically linked ion traps are promising as components of network-based
quantum technologies, including communication systems and modular computers.
Experimental results achieved to date indicate that the fidelity of operations
within each ion trap module will be far higher than the fidelity of operations
involving the links; fortunately internal storage and processing can
effectively upgrade the links through the process of purification. Here we
perform the most detailed analysis to date on this purification task, using a
protocol which is balanced to maximise fidelity while minimising the device
complexity and the time cost of the process. Moreover we 'compile down' the
quantum circuit to device-level operations including cooling and shutting
events. We find that a linear trap with only five ions (two of one species,
three of another) can support our protocol while incorporating desirable
features such as 'global control', i.e. laser control pulses need only target
an entire zone rather than differentiating one ion from its neighbour. To
evaluate the capabilities of such a module we consider its use both as a
universal communications node for quantum key distribution, and as the basic
repeating unit of a quantum computer. For the latter case we evaluate the
threshold for fault tolerant quantum computing using the surface code, finding
acceptable fidelities for the 'raw' entangling link as low as 83% (or under 75%
if an additional ion is available).
