An Integrated Bell-State Analyzer on a Thin Film Lithium Niobate Platform*

Trapped ions are excellent candidates for quantum computing and quantum networks because of their long coherence times, ability to generate entangled photons as well as high fidelity single- and two-qubit gates. To scale up trapped-ion quantum computing, we need to develop a Bell-state analyzer on a reconfigurable platform that can herald high fidelity entanglement between remote ions. Thin-film lithium niobate is an attractive platform for its large transparency window and high electro-optic coefficient. However, trapped ions naturally emit polarization-encoded photonic qubits, while thin-film lithium niobate devices are polarization-sensitive due to the large mode anisotropy created during fabrication. In this work, we design a photonic Bell-state analyzer on a thin film lithium niobate platform for polarization-encoded qubits. We optimize the dimensions of the bell state analyzer 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.