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.