An integrated Bell-state analyzer for high fidelity entanglement between trapped ion quantum computers

Trapped ions are excellent candidates for quantum computing and quantum networks because of their long qubit coherence times, ability to generate photons entangled with the ion’s qubit states, and high-fidelity single- and two-qubit gates. To connect multiple trapped ion quantum computers in a scalable way, we need an integrated Bell-state analyzer that can herald high fidelity entanglement between remote ions. Thin-film lithium niobate is an attractive platform to implement integrated Bell-state analyzer because its large transparency window and high electro-optic coefficient. However, trapped ions emit polarization-encoded photonic qubits, while thin-film lithium niobate devices are polarization-sensitive due to the large mode anisotropy created during the fabrication process. In this work, we design a photonic Bell-state analyzer on a thin film lithium niobate platform for polarization-encoded qubits. We optimize the device dimensions and input coupler to achieve polarization-insensitive operation. We achieve high fidelity entanglement between two trapped ions and determine > 99.99% fidelity in the final optimized device. The proposed Bell-state analyzer can scale up trapped ion quantum computing as well as other optically active spin qubits, such as color centers in diamond, quantum dots, and rare-earth ions.