Quantum frequency combs from chip-scale integrated sources are promising candidates for scalable and robust quantum information processing (QIP). However, to use these quantum combs for frequency domain QIP, demonstration of entanglement in the frequency basis, showing that the entangled photons are in a coherent superposition of multiple frequency bins, is required. We present a verification of qubit and qutrit frequency-bin entanglement using an on-chip quantum frequency comb with 40 mode pairs, through a two-photon interference measurement that is based on electro-optic phase modulation. Our demonstrations provide an important contribution in establishing integrated optical microresonators as a source for high-dimensional frequency-bin encoded quantum computing, as well as dense quantum key distribution.

Optical resonators with high quality factors (Qs) are promising for a variety of applications due to the enhanced nonlinearity and increased photonic density of states at resonances. In particular, frequency combs (FCs) can be generated through four-wave mixing in high-Q microresonators made from Kerr nonlinear materials such as silica, silicon nitride, magnesium fluoride, and calcium fluoride. These devices have potential for on-chip frequency metrology and high-resolution spectroscopy, high-bandwidth radiofrequency information processing, and high-data-rate telecommunications. Silicon nitride microresonators are attractive due to their compatibility with integrated circuit manufacturing; they can be cladded with silica for long-term stable yet tunable operation, and allow multiple resonators to be coupled together to achieve novel functionalities. Despite previous demonstrations of high-Q silicon nitride resonators, FC generation using silicon nitride microresonator chips still requires pump power significantly higher than those in whispering gallery mode resonators made from silica, magnesium, and calcium fluorides, which all have shown resonator Qs between 0.1 and 100 billion. Here, we report on a fabrication procedure that leads to the demonstration of "finger-shaped" Si 3 N 4 microresonators with intrinsic Qs up to 17 million at a free spectrum range (FSR) of 24.7 GHz that are suitable for telecommunication and microwave photonics applications. The frequency comb onset power can be as low as 2.36 mW and broad, single FSR combs can be generated at a low pump power of 24 mW, both within reach of on-chip semiconductor lasers. Our demonstration is an important step toward a fully integrated on-chip FC source. Kerr comb generation in microresonators starts when an external continuous-wave (CW) laser is tuned into a cavity resonance; this causes intracavity power to build, which enables additional cavity modes to oscillate through nonlinear wave mixing [10]. FC formation has now been demonstrated in a variety of Kerr nonlinear materials such as silica [9,14-18], silicon nitride (Si 3 N 4 ) [19-21], aluminum nitride [22], CaF 2 [23], and MgF 2 [24]. Recently, dissipative Kerr solitons have also been demonstrated in MgF 2 and Si 3 N 4 optical microresonators [25,26]. Out of these materials, stoichiometric Si 3 N 4 has distinctive 2334-2536/16/111171-10 Journal

Ultra-compact, densely integrated optical components manufactured on a CMOS-foundry platform are highly desirable for optical information processing and electronic-photonic co-integration. However, the large spatial extent of evanescent waves arising from nanoscale confinement, ubiquitous in silicon photonic devices, causes significant cross-talk and scattering loss. Here, we demonstrate that anisotropic all-dielectric metamaterials open a new degree of freedom in total internal reflection to shorten the decay length of evanescent waves. We experimentally show the reduction of cross-talk by greater than 30 times and the bending loss by greater than 3 times in densely integrated, ultra-compact photonic circuit blocks. Our prototype all-dielectric metamaterial-waveguide achieves a low propagation loss of approximately 3.7±1.0 dB/cm, comparable to those of silicon strip waveguides. Our approach marks a departure from interference-based confinement as in photonic crystals or slot waveguides, which utilize nanoscale field enhancement. Its ability to suppress evanescent waves without substantially increasing the propagation loss shall pave the way for all-dielectric metamaterial-based dense integration.

Kerr nonlinearity-based frequency combs and solitons have been generated from on-chip microresonators. The initiation of the combs requires global or local anomalous dispersion which leads to many limitations, such as material choice, film thickness, and spectral ranges where combs can be generated, as well as fabrication challenges. Using a concentric racetrack-shaped resonator, we show that such constraints can be lifted and resonator dispersion can be engineered to be anomalous over moderately broad bandwidth. We demonstrate anomalous dispersion in a 300 nm thick silicon nitride film, suitable for semiconductor manufacturing but previously thought to result in waveguides with high normal dispersion. Together with a mode-selective, tapered coupling scheme, we generate coherent mode-locked frequency combs. Our method can realize anomalous dispersion for resonators at almost any wavelength and simultaneously achieve material and process compatibility with semiconductor manufacturing.

We present a direct strip-slot waveguide mode coupler without any auxiliary structures. Contrary to popular belief, an apparent mode mismatch between strip and slot waveguide does not deteriorate conversion efficiency. Separated electric and magnetic field distributions in a slot waveguide lead to highly efficient modal coupling in the direct strip-slot coupler and result in high conversion efficiency. Accurate experimental characterization shows that the direct strip-slot waveguide mode coupler is capable of up to 96% conversion efficiency with a broad bandwidth. Being simplest and of high efficiency, the direct strip-slot waveguide mode coupler can encourage potential applications of slot waveguides.

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We present a monolithic InP-based photonic integrated circuit (PIC) consisting of a widely tunable laser master oscillator feeding an array of integrated semiconductor optical amplifiers that are interferometrically combined on-chip in a single-mode waveguide. We demonstrate a stable and efficient on-chip coherent beam combination and obtain up to 240 mW average power from the monolithic PIC, with 30–50 kHz Schawlow-Townes linewidths and >180 mW average power across the extended C-band. We also explored hybrid integration of the InP-based laser and amplifier array PIC with a high quality factor silicon nitride microring resonator. We observe lasing based on gain from the interferometrically combined amplifier array in an external cavity formed via feedback from the silicon nitride microresonator chip; this configuration results in narrowing of the Schawlow-Townes linewidth to ∼3 kHz with 37.9 mW average power at the SiN output facet. This work demonstrates a new approach toward high power, narrow linewidth sources that can be integrated with on-chip single-mode waveguide platforms for potential applications in nonlinear integrated photonics.

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