Dual-polarization pump rejection filter in silicon nitride technology.

On-chip pump rejection filters are key building blocks in a variety of applications exploiting nonlinear phenomena, including Raman spectroscopy and photon-pair generation. Ultrahigh rejection has been achieved in the silicon technology by non-coherent cascading of modal-engineered Bragg filters. However, this concept cannot be directly applied to silicon nitride waveguides as the comparatively lower index contrast hampers the suppression of residual light propagating in the orthogonal polarization, limiting the achievable rejection. Here, we propose and demonstrate a novel, to the best of our knowledge, strategy to overcome this limitation based on non-coherent cascading of the modal- and polarization-engineered Bragg filters. Based on this concept, we experimentally demonstrate a rejection exceeding 60 dB for both polarizations, with a bandwidth of 4.4 nm. This is the largest rejection reported for silicon nitride Bragg gratings supporting both polarizations.

Near perfect two-photon interference out of a down-converter on a silicon photonic chip.

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.

Plug-and-play generation of non-Gaussian states of light at a telecom wavelength.

In the context of emerging quantum technologies, this work marks an important progress towards practical quantum optical systems in the continuous variable regime. It shows the feasibility of experiments where non-Gaussian state generation entirely relies on plug-and-play components from guided-wave optics technologies. This strategy is successfully demonstrated with the heralded preparation of low amplitude Schrödinger cat states via single-photon subtraction from a squeezed vacuum. All stages of the experiment are based on off-the-shelf fiber components. This leads to a stable, compact, and easily re-configurable realization, fully compatible with existing fiber networks and, more in general, with future out-of-the-laboratory applications.

Design and fabrication of dispersion controlled highly nonlinear fibers for far-detuned four-wave mixing frequency conversion.

We report on the conception, fabrication and characterization of a new concept of optical fiber enabling a precise control of the ratio between the 2nd and 4th-order of chromatic dispersion (respectively ß2 and ß4) at 1.55 µm which is at the heart of the Four-Wave-Mixing (FWM) generation. For conventional highly nonlinear fiber the sensitivity of this ratio to fiber geometry fluctuations is very critical, making the fabrication process challenging. The new design fiber reconciles the accurate control of chromatic dispersion properties and fabrication by standard stack and draw method, allowing a robust and reliable method against detrimental fluctuations parameters during the fabrication process. Experimental frequency conversion with FWM in the new design fiber is demonstrated.

Strong pump rejection filter for polarization-diverse silicon platforms.

Integrated wavelength filters with high optical rejection are key components in several silicon photonics circuits, including quantum photon-pair sources and spectrometers. Non-coherent cascading of modal-engineered Bragg filters allows for remarkable optical rejections in structures that only support transverse-electric (TE) polarized modes such as uncladded 220-nm-thick silicon. However, the restriction to TE-only platforms limits the versatility of the non-coherent cascading approach. Here, we propose and experimentally demonstrate a new, to the best of our knowledge, approach for high-rejection filters in polarization-diverse platforms by combining non-coherent cascading of modal-engineered Bragg filters and anisotropy-engineered metamaterial bends. Bragg filters provide a high rejection of the TE mode, while the metamaterial bends remove any residual power propagating in the transverse-magnetic (TM) mode, without any penalty in terms of insertion loss or device footprint. Based on this strategy, we demonstrate optical rejection exceeding 60 dB in 300-nm-thick, cladded silicon waveguides.

Supermode-based second harmonic generation in a nonlinear interferometer.

We demonstrate supermode-based second harmonic generation in an integrated nonlinear interferometer made of linear and nonlinear directional couplers. We use a fully-fibered pump shaper to demonstrate second harmonic generation pumped by the symmetric or anti-symmetric fundamental spatial modes. The selection of the pumping mode and thus of a specific SHG spectral profile is achieved through the selection of the fundamental wavelength and via a robust phase setting scheme. We use two methods: either post-selecting or actively setting the pumping mode. Such modal phase matching paves the way for classical and quantum applications of coupled nonlinear photonic circuits, where multimode excitation, encoding and detection are a route for multiplexing and scaling up light-processing.

Silicon subwavelength modal Bragg grating filters with narrow bandwidth and high optical rejection.

Waveguide Bragg grating filters with narrow bandwidths and high optical rejections are key functions for several advanced silicon photonics circuits. Here, we propose and demonstrate a new, to the best of our knowledge, Bragg grating geometry that provides a narrowband and high rejection response. It combines the advantages of subwavelength and modal engineering. As a proof-of-concept demonstration, we implement the proposed Bragg filters in 220-nm-thick Si technology with a single etch step. We experimentally show flexible control of the filter selectivity, with measured null-to-null bandwidths below 2 nm, and strength of 60 dB rejection with a null-to-null bandwidth of 1.8 nm.

Morphing Supermodes: A Full Characterization for Enabling Multimode Quantum Optics.

In this Letter, we present a universal approach enabling the full characterization of the quantum properties of a multimode optical system in terms of squeezing and morphing supermodes. These are modes undergoing a continuous evolution that allow uncoupling the system dynamics in terms of statistically independent physical observables. This dynamical feature, never considered so far, enables the description and investigation of an extremely broad variety of key resources for experimental quantum optics, ranging from optical parametric oscillators to silicon-based microring resonators, as well as optomechanical systems.

Quantum optical frequency up-conversion for polarisation entangled qubits: towards interconnected quantum information devices.

Realising a global quantum network requires combining individual strengths of different quantum systems to perform universal tasks, notably using flying and stationary qubits. However, transferring coherently quantum information between different systems is challenging as they usually feature different properties, notably in terms of operation wavelength and wavepacket. To circumvent this problem for quantum photonics systems, we demonstrate a polarisation-preserving quantum frequency conversion device in which telecom wavelength photons are converted to the near infrared, at which a variety of quantum memories operate. Our device is essentially free of noise, which we demonstrate through near perfect single photon state transfer tomography and observation of high-fidelity entanglement after conversion. In addition, our guided-wave setup is robust, compact, and easily adaptable to other wavelengths. This approach therefore represents a major building block towards advantageously connecting quantum information systems based on light and matter.

Subwavelength engineering and asymmetry: two efficient tools for sub-nanometer-bandwidth silicon Bragg filters.

Bragg filters stand as key building blocks of the silicon-on-insulator (SOI) photonics platform, allowing the implementation of advanced on-chip signal manipulation. However, achieving narrowband Bragg filters with large rejection levels is often hindered by fabrication constraints and imperfections. Here, we show that the combination of single-side corrugation asymmetry and subwavelength engineering provides a narrowband response with large corrugations, overcoming minimum feature size constraints of conventional Si Bragg filters. We comprehensively study the impact of the corrugation asymmetry in conventional and subwavelength single-etched SOI Bragg filters, showing their potential for bandwidth reduction. Finally, we experimentally demonstrate novel subwavelength geometry based on shifted corrugation teeth, achieving null-to-null bandwidths and rejections of 0.8 nm and 40 dB for the symmetric configuration and 0.6 nm and 15 dB for the asymmetric case.