Programmable Bell State Generation in an Integrated Thin Film Lithium Niobate Circuit

TL;DR

This paper demonstrates a reconfigurable lithium niobate photonic circuit capable of generating high-fidelity entangled Bell states on-chip, advancing integrated quantum photonics with high-brightness sources and programmable interferometers.

Contribution

It introduces a reconfigurable LNOI circuit integrating photon pair sources and interferometers for on-chip entanglement generation, a significant step forward in scalable quantum photonics.

Findings

Photon pair sources with 26 MHz nm$^{-1}$ mW$^{-1}$ brightness.

Interference visibility of 99% for indistinguishability.

Bell states prepared with over 90% fidelity.

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 generation of entangled states. The pair sources achieve a source brightness of 26 MHz nm$^{-1}$ mW$^{-1}$ while maintaining a coincidence-to-accidental ratio above 100. We successfully interfere the two sources with $99 \pm 0.7$ % visibility, demonstrating the indistinguishability required for producing entanglement on-chip. We show 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.