We report an efficient energy-time entangled photon-pair source based on four-wave mixing in a CMOS-compatible silicon photonics ring resonator. Thanks to suitable optimization, the source shows a large spectral brightness of 400 pairs of entangled photons /s/MHz for 500 μW pump power, compatible with standard telecom dense wavelength division multiplexers. We demonstrate high-purity energy-time entanglement, i.e., free of photonic noise, with near perfect raw visibilities (> 98%) between various channel pairs in the telecom C-band. Such a compact source stands as a path towards more complex quantum photonic circuits dedicated to quantum communication systems.

The high index contrast of the silicon-on-insulator (SOI) platform allows the realization of ultracompact photonic circuits. However, this high contrast hinders the implementation of narrow-band Bragg filters. These typically require corrugations widths of a few nanometers or double-etch geometries, hampering device fabrication. Here we report, for the first time, on the realization of SOI Bragg filters based on sub-wavelength index engineering in a differential corrugation width configuration. The proposed double periodicity structure allows narrow-band rejection with a single etch step and relaxed width constraints. Based on this concept, we experimentally demonstrate a singleetch, 220 nm thick, Si Bragg filter featuring a corrugation width of 150 nm, a rejection bandwidth of 1.1 nm and an extinction ratio exceeding 40 dB. This represents a ten-fold width increase compared to conventional single-periodicity, single-etch counterparts with similar bandwidths. The silicon-on-insulator (SOI) platform with submicrometric thick Si layer has shown outstanding results in the miniaturization of photonic circuits [1]. Highquality materials and mature fabrication processes, together with the potential to leverage already existing CMOS facilities, make it a promising candidate for the large volume production of performant photonic devices. In addition to datacom [2] or sensing applications [3,4], SOI shows a great potential for the generation and manipulation of photonic entanglement [5][6][7][8][9][10]. Such a technology would enable monolithic integration of quantumprocessing circuits, opening new routes for envisioned quantum-based applications, including quantum key distribution [11] and optical quantum computing [12].

The fruitful association of quantum and integrated photonics holds the promise to produce, manipulate, and detect quantum states of light using compact and scalable systems. Integrating all the building-blocks necessary to produce high-quality photonic entanglement in the telecom wavelength range out of a single chip remains a major challenge, mainly due to the limited performance of on-chip light rejection filters. We report a stand-alone, telecom-compliant, device that integrates, on a single substrate, a nonlinear photon-pair generator and a passive pump rejection filter. Using standard channel-grid fiber demultiplexers, we demonstrate the first entanglement qualification of such a integrated circuit, showing the highest raw quantum interference visibility for time-energy entangled photons over two telecom-wavelength bands. Genuinely pure maximally entangled states can therefore be generated thanks to the high-level of noise suppression obtained with the pump filter. These results will certainly further promote the development of more advanced and scalable photonic-integrated quantum systems compliant with telecommunication standards.Quantum information science (QIS) exploits the fundamental properties of quantum physics to code and manipulate quantum states. QIS is regarded as the most promising pathway towards disruptive technologies, envisioning major improvements in processing capabilities and communication security [1,2]. However, practical implementations, such as quantum key distribution systems or quantum processors, require a large amount of compatible building-blocks [3][4][5][6]. Integrated photonics provides efficient and reliable solutions for realizing advanced quantum communication systems based on both linear and nonlinear elements [7][8][9][10][11][12][13]. Still, all these realizations face a crucial limitation as soon as on-chip suppression of photonic noise is concerned due to the substantially higher pump intensity compared to that of the photon-pairs. Most of the time, this operation is externalized, using fiber or bulk optical components, and hinders the benefit of both the compactness and stability of the whole system [14].CMOS-compatible technologies hold the promise of bringing quantum photonics one step further with optical circuits showing higher integration levels [15]. A few experiments based on this technology have been carried out to address the pump rejection challenge using on-chip solutions [16][17][18][19][20][21][22]. On-chip pump rejection has been demonstrated based on a semiconductor quantum dot integrated in a CMOS photonic circuit, but emitting out of the telecom range. The other strategies suffer from two main limitations: i) the continuous monitoring of the filter response to maintain proper performance [16][17][18], and ii) prohibitive additional interconnection losses between components [19, 20] (up to 9 dB[21]). Moreover, all these solutions have reported temporal correlation measurements for revealing the degree of indistinguishability * laurent.labonte@univ-cote...