Robust and bright polarization-entangled photon sources exploiting non-critical phase matching without periodic poling

TL;DR

This paper presents a robust, bright polarization-entangled photon source using non-critical phase matching in a bulk KTP crystal, avoiding complex periodic poling and enabling scalable quantum information applications.

Contribution

It introduces a non-critical phase matching method with a Sagnac interferometer for high-brightness entangled photon generation without periodic poling.

Findings

Achieved 25.1 kHz/mW brightness with high purity and fidelity

Demonstrated tolerance to pump angle and temperature variations

Produced all four Bell states experimentally

Abstract

Entangled photon sources are essential for quantum information applications, including quantum computation, quantum communication, and quantum metrology. Periodically poled (PP) crystals are commonly used to generate bright photon sources through quasi-phase matching. However, fabricating uniform micron-scale periodic structures poses significant technical difficulties, typically limiting the crystal thickness to less than a millimeter. Here, we adopt non-critical phase matching to produce a robust and bright polarization-entangled photon source based on a Sagnac interferometer. This method is tolerant of variations in pump incidence angles and temperature, and theoretically offers about a 2.5-fold brightness enhancement compared to quasi-phase matching. Additionally, the absence of periodic poling allows for a larger crystal cross-section. Using a bulk KTP crystal without a PP structure, we experimentally produce the four Bell states with a brightness of 25.1 kHz/mW, achieving purity, concurrence, and fidelity values close to 0.99. We believe our scheme will serve as a key building block for scalable and practical photonic quantum information applications.