Single Photons for a Modular Ion Trap Quantum Network*

Trapped atomic ions of different species are an ideal candidate for a modular quantum computing network partially due to low crosstalk between memory and communication qubits. A dual species Yb⁺/Ba⁺ ion trap is used to create entanglement between memory spin qubits and photonic qubits. The flying qubits are emitted via the 6S_1/2 ↔ 6P_1/2 transition in ¹³⁸Ba⁺. It is advantageous to excite the atom on the 5D_3/2 ↔ 6P_1/2 transition at 650 nm, while still collecting 493 nm photons. This removes the excitation light as a source of noise and reduces double excitation errors. Most significantly, the D ↔ P transition allows us to use a slower excitation, such that instead of requiring a picosecond pulsed laser, we can gate a CW laser using AO modulators. We demonstrate a single photon source for quantum networking based on a trapped barium ion subject to pulsed excitation with a second-order coherence of g⁽²⁾(0) = (8.1 ± 2.3) × 10^-5 without background subtraction, a state-of-the-art result for replicable single photon sources.