  Efficiently scaling quantum networks to long ranges requires local processing
nodes to perform basic computation and communication tasks. Trapped ions have
demonstrated all the properties required for the construction of such a node,
storing quantum information for up to 12 minutes, implementing deterministic
high fidelity logic operations on one and two qubits, and ion-photon coupling.
While most ions suitable for quantum computing emit photons in visible to near
ultraviolet (UV) frequency ranges poorly suited to long-distance fibre optical
based networking, recent experiments in frequency conversion provide a
technological solution by shifting the photons to frequencies in the telecom
band with lower attenuation for fused silica fibres. Encoding qubits in
frequency rather than polarization makes them more robust against decoherence
from thermal or mechanical noise due to the conservation of energy. To date,
ion-photonic frequency qubit entanglement has not been directly shown. Here we
demonstrate a frequency encoding ion-photon entanglement protocol in
$^{171}$Yb$^+$ with correlations equivalent to 92.4(8)% fidelity using a
purpose-built UV hyperfine spectrometer. The same robustness against
decoherence precludes our passive optical setup from rotating photonic qubits
to unconditionally demonstrate entanglement, however it is sufficient to allow
us to benchmark the quality of ion-UV photon correlations prior to frequency
conversion to the telecom band.
