  Trapped ions are one of the leading candidates for scalable and long-distance
quantum networks because of their long qubit coherence time, high fidelity
single- and two-qubit gates, and their ability to generate photons entangled
with the qubit state of the ion. One method for creating ion-photon
entanglement is to exploit optically transitions from the P_(1/2) to S_(1/2)
levels, which naturally emit spin-photon entangled states. But these optical
transitions typically lie in the ultra-violet and visible wavelength regimes.
These wavelengths exhibit significant fiber-optic propagation loss, thereby
limiting the transfer of quantum information to tens of meters. Quantum
frequency conversion is essential to convert these photons to telecom
wavelengths so that they can propagate over long distances in fiber-based
networks, as well as for compatibility with the vast number of telecom-based
opto-electronic components. Here, we generate O-band telecom photons via a low
noise quantum frequency conversion scheme from photons emitted from the P_(1/2)
to S_(1/2) dipole transition of a trapped barium ion. We use a two-stage
quantum frequency conversion scheme to achieve a frequency shift of 375.4 THz
between the input visible photon and the output telecom photon achieving a
conversion efficiency of 11%. We attain a signal-to-background ratio of over
100 for the converted O-band telecom photon with background noise less than 15
counts/sec. These results are an important step toward achieving trapped ion
quantum networks over long distances for distributed quantum computing and
quantum communication.
