# Programmable Bell state generation in an integrated thin film lithium niobate circuit

## TL;DR

This paper presents a reconfigurable quantum photonic circuit using lithium niobate to generate entangled photon pairs with high fidelity and brightness.

## Contribution

The work introduces a programmable LNOI circuit that enables high-fidelity generation of Bell states with integrated photon pair sources.

## Key findings

- The circuit achieves a source brightness of 26 MHz nm−1mW−1 with a coincidence-to-accidental ratio above 100.
- Photon sources are interfered with 99.0 ± 0.7% visibility, demonstrating indistinguishability for entanglement.
- Maximally entangled Bell states are prepared with fidelity above 90% using quantum state tomography.

## Abstract

Entanglement is central to quantum technologies such as cryptography, sensing, and computing. Photon pairs generated via nonlinear optical processes are excellent for preparing entangled states due to their long coherence times and compatibility with fiber optic networks. Steady progress in nanofabrication has positioned lithium niobate-on-insulator (LNOI) as a leading platform for monolithic integration of photon pair sources into optical circuits, leveraging its strong second-order nonlinearity. Here, we present a reconfigurable photonic integrated circuit on LNOI, which combines two on-chip photon pair sources with programmable interferometers, enabling the generation of entangled states. The photon pair sources achieve a source brightness of 26 MHz nm−1mW−1 while maintaining a coincidence-to-accidental ratio above 100. We successfully interfere the two sources with 99.0 ± 0.7% visibility, demonstrating the indistinguishability required for producing entanglement on-chip. We show the preparation of any of the maximally entangled Bell states with fidelity above 90% verified by quantum state tomography. These results establish LNOI as a compelling, scalable platform to explore integrated quantum photonic technologies enabled by high-brightness sources of entangled quantum states.

We demonstrate a programmable thin-film lithium niobate photonic circuit capable of high-brightness generation and reconfigurable projection of path-encoded Bell states with high fidelity.