  We demonstrate polarisation-preserving frequency conversion of
single-photon-level light at 854 nm, resonant with a trapped-ion transition and
qubit, to the 1550-nm telecom C band. A total photon in / fiber-coupled photon
out efficiency of $\sim$ 30 % is achieved, for a free-running photon noise rate
of $\sim$ 60 Hz. This performance would enable telecom conversion of 854-nm
polarisation qubits, produced in existing trapped-ion systems, with a
signal-to-noise ratio greater than 1. In combination with near-future
trapped-ion systems, our converter would enable the observation of entanglement
between an ion and a photon that has travelled more than 100 km in optical
fiber: three orders of magnitude further than the state-of-the-art.
