Integrated Bright Source of Polarization-Entangled Photons Using Lithium Niobate Photonic Chips

This article demonstrates the creation of a compact and efficient source of polarization-entangled photon pairs using thin-film lithium niobate (TFLN) integrated photonics. The device utilizes a combination of a multimode interferometer (MMI), two periodically poled lithium niobate (PPLN) waveguides, and a polarization splitter-rotator (PSR) to generate high-quality polarization-entangled Bell state photons via continuous-wave (CW) pumping. This approach surpasses other integrated platforms, including silicon photonics, in terms of photon pair generation rate, achieving a remarkable 508.5 MHz/mW on-chip brightness.

Key points from the article include:

1. Device Design and Entanglement Generation: The device generates polarization-entangled photon pairs by employing PPLN waveguides for spontaneous parametric down-conversion (SPDC) and a PSR to convert spatial-mode entanglement into polarization entanglement. The device uses a CW pump laser, and the generated photon pairs exhibit high polarization entanglement quality.

2. Experimental Results: The experiment demonstrates high-quality entangled photon pairs with purity of 0.901, concurrence of 0.900, and fidelity of 0.944. These values confirm the successful generation of polarization-entangled Bell states, making the device suitable for quantum communication and other photonic quantum technologies.

3. High Brightness: The on-chip photon pair generation rate of 508.5 MHz/mW significantly exceeds the performance of silicon-based platforms. This high brightness makes the TFLN-based device highly effective for practical quantum applications.

4. Quantum State Tomography: Quantum state tomography (QST) was conducted to fully characterize the polarization states of the generated photons. The results from QST show that the two photons are strongly entangled in their polarization states, supporting the high quality of the generated Bell state.

5. Comparison with Other Platforms: The TFLN device's performance, including brightness and entanglement quality, is compared with other integrated photonic platforms, demonstrating the advantages of using TFLN in quantum photonic applications. This device achieves superior performance compared to previous silicon-based devices and is a promising platform for scalable quantum photonics.

6. Future Directions: The study suggests potential improvements in the device's performance, such as better control over polarization crosstalk and optimization of phase control. Additionally, the TFLN platform has the potential for broader applications in quantum communication, sensing, and high-capacity quantum networks.

In conclusion, this work highlights the successful realization of a bright and compact source of polarization-entangled photon pairs using TFLN integrated photonics, offering a significant advancement for integrated quantum technologies.