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...

Selective optical filters with high rejection levels are of fundamental importance for a wide range of advanced photonic circuits. However, the implementation of high-rejection on-chip optical filters is seriously hampered by phase errors arising from fabrication imperfections. Due to coherent interactions, unwanted phase-shifts result in detrimental destructive interferences that distort the filter response, whatever the chosen strategy (resonators, interferometers, Bragg filters, etc.). State-of-the-art high-rejection filters partially circumvent the sensitivity to phase errors by means of active tuning, complicating device fabrication and operation. Here, a new approach based on coherency-broken Bragg filters is proposed to overcome this fundamental limitation. Non-coherent interaction among modal-engineered waveguide Bragg gratings separated by single-mode waveguides is exploited to yield effective cascading, even in the presence of phase errors. This technologically independent approach allows seamless combination of filter stages with moderate performance free of active control, providing a dramatic increase of on-chip rejection. Based on this concept, on-chip non-coherent cascading of Si Bragg filters is experimentally demonstrated, achieving a light rejection exceeding 80 dB, the largest value reported for an all-passive silicon filter.

Entanglement is a key resource in quantum information science and associated emerging technologies. Photonic systems offer a large range of exploitable entanglement degrees of freedom (DOF) such as frequency, time, polarization, and spatial modes. Hyperentangled photons exploit multiple DOF simultaneously to enhance the performance of quantum information protocols. Here, we report a fully guided-wave approach for generating polarization and energy-time hyperentangled photons at telecom wavelengths. Moreover, by demultiplexing the broadband emission spectrum of the source into five standard telecom channel pairs, we demonstrate compliance with fibre network standards and improve the effective bit rate capacity of the quantum channel up to one order of magnitude. In all channel pairs, we observe a violation of a generalised Bell inequality by more than 27 standard deviations, underlining the relevance of our approach.

Integrated entangled photon-pair sources are key elements for enabling large-scale quantum photonic solutions and address the challenges of both scaling-up and stability. Here we report the first demonstration of an energy-time entangled photon-pair source based on spontaneous parametric down-conversion in silicon-based platform–stoichiometric silicon nitride (Si3N4)–through an optically induced second-order (χ(2)) nonlinearity, ensuring type-0 quasi-phase-matching of fundamental harmonic and its second-harmonic inside the waveguide. The developed source shows a coincidence-to-accidental ratio of 1635 for 8 µW pump power. We report two-photon interference with remarkable near-perfect visibility of 99.36±1.94%, showing high-quality photonic entanglement without excess background noise. This opens a new horizon for quantum technologies requiring the integration of a large variety of building functionalities on a single chip.

Shaping single-mode operation in high-power fibers requires a precise knowledge of the gain-medium optical properties. This requires precise measurements of the refractive index differences (Δn) between the core and the cladding of the fiber. We exploit a quantum optical method based on low-coherence Hong-Ou-Mandel interferometry to perform practical measurements of the refractive index difference using broadband energy-time entangled photons. The precision enhancement reached with this method is benchmarked with a classical method based on single photon interferometry. We show in classical regime an improvement by an order of magnitude of the precision compared to already reported classical methods. Strikingly, in the quantum regime, we demonstrate an extra factor of 4 on the precision enhancement, exhibiting a state-of-the-art Δn precision of 6 × 10−7. This work sets the quantum photonics metrology as a powerful characterization tool that should enable a faster and reliable design of materials dedicated to light amplification.

Sub-wavelength grating (SWG) metamaterials have garnered a great interest for their singular capability to shape the propagation of light. However, practical SWG implementations are limited by fabrication constraints, such as minimum feature size. Here, we present a new nanophotonic waveguide grating concept that exploits phase-matching engineering to suppress diffraction effects for a period three times larger than those with SWG approaches. This long-period grating not only facilitates fabrication, but also enables a new diffraction-less regime with additional degrees of freedom to control light propagation. More specifically, the proposed phase-matching engineering enables selective diffraction suppression, providing new tools to shape propagation in the grating. We harness this flexible diffraction control to yield single-mode propagation in, otherwise, highly multimode waveguides, and to implement Bragg filters that combine highly-diffractive and diffraction-less regions to dramatically increase light rejection. Capitalizing on this new concept, we experimentally demonstrate a Si membrane Bragg filter with record rejection value exceeding 60 dB. These results demonstrate the potential of the proposed long-period grating for the engineering of diffraction in nanophotonic waveguides and pave the way for the development of a new generation of high-performance Si photonics devices.