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Silicon carbide (SiC) is rapidly emerging as a leading platform for the implementation of nonlinear and quantum photonics. Here, we find that commercial SiC, which hosts a variety of spin qubits, possesses low optical absorption that can enable SiC integrated photonics with quality factors exceeding 10 7 . We fabricate multimode microring resonators with quality factors as high as 1.1 million, and observe low-threshold ( 8.5 ± 0.5 m W ) optical parametric oscillation using the fundamental mode as well as optical microcombs spanning 200 nm using a higher-order mode. Our demonstration is an essential milestone in the development of photonic devices that harness the unique optical properties of SiC, paving the way toward the monolithic integration of nonlinear photonics with spin-based quantum technologies.

Correlated magnetic noise from Schumann resonances threatens to contaminate the observation of a stochastic gravitational-wave background in interferometric detectors. In previous work, we reported on the first effort to eliminate global correlated noise from the Schumann resonances using Wiener filtering, demonstrating as much as a factor of two reduction in the coherence between magnetometers on different continents. In this work, we present results from dedicated magnetometer measurements at the Virgo and KAGRA sites, which are the first results for subtraction using data from gravitational-wave detector sites. We compare these measurements to a growing network of permanent magnetometer stations, including at the LIGO sites. We show the effect of mutual magnetometer attraction, arguing that magnetometers should be placed at least one meter from one another. In addition, for the first time, we show how dedicated measurements by magnetometers near to the interferometers can reduce coherence to a level consistent with uncorrelated noise, making a potential detection of a stochastic gravitational-wave background possible.

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The recent discovery of merging black holes suggests that a stochastic gravitational-wave background is within reach of the advanced detector network operating at design sensitivity. However, correlated magnetic noise from Schumann resonances threatens to contaminate observation of a stochastic background. In this paper, we report on the first effort to eliminate intercontinental correlated noise from Schumann resonances using Wiener filtering. Using magnetometers as proxies for gravitational-wave detectors, we demonstrate as much as a factor of two reduction in the coherence between magnetometers on different continents. While much work remains to be done, our results constitute a proof-of-principle and motivate follow-up studies with a dedicated array of magnetometers. PACS numbers:Introduction. A stochastic gravitational-wave background (SGWB) is a potential signal source for groundbased, second-generation interferometric gravitationalwave detectors such as Advanced LIGO [1] and Advanced Virgo [2]. An astrophysical SGWB could be produced by objects such as compact binary coalescences, pulsars, magnetars, or core-collapse supernovae. A cosmological background could be generated by various physical processes in the early universe [3,4]. Previous analyses have achieved interesting constraints on these processes [3][4][5]. In particular, with the recent discovery of a binary blackhole merger [6], there is a chance of observing a SGWB from these systems [7].

The ability to shape photon emission facilitates strong photon-mediated interactions between disparate physical systems, thereby enabling applications in quantum information processing, simulation and communication. Spectral control in solid state platforms such as color centers, rare earth ions, and quantum dots is particularly attractive for realizing such applications on-chip. Here we propose the use of frequency-modulated optical transitions for spectral engineering of single photon emission. Using a scattering-matrix formalism, we find that a two-level system, when modulated faster than its optical lifetime, can be treated as a single-photon source with a widely reconfigurable photon spectrum that is amenable to standard numerical optimization techniques. To enable the experimental demonstration of this spectral control scheme, we investigate the Stark tuning properties of the silicon vacancy in silicon carbide, a color center with promise for optical quantum information processing technologies. We find that the silicon vacancy possesses excellent spectral stability and tuning characteristics, allowing us to probe its fast modulation regime, observe the theoretically-predicted two-photon correlations, and demonstrate spectral engineering. Our results suggest that frequency modulation is a powerful technique for the generation of new light states with unprecedented control over the spectral and temporal properties of single photons.

An outstanding challenge for color center-based quantum information processing technologies is the integration of optically-coherent emitters into scalable thin-film photonics. Here, we report on the integration of near-transform-limited silicon vacancy (VSi) defects into microdisk resonators fabricated in a CMOS-compatible 4H-Silicon Carbide-on-Insulator platform. We demonstrate a single-emitter cooperativity of up to 0.8 as well as optical superradiance from a pair of color centers coupled to the same cavity mode. We investigate the effect of multimode interference on the photon scattering dynamics from this multi-emitter cavity quantum electrodynamics system. These results are crucial for the development of quantum networks in silicon carbide and bridge the classicalquantum photonics gap by uniting optically-coherent spin defects with wafer-scalable, state-of-theart photonics.Color centers 1-3 are among the leading contenders for the realization of distributed quantum information processing, including communication 4,5 and computation 6 , combining a long-lived multi-qubit spin register 7 with a photonic interface in the solid state. To continue scaling up quantum networks while maintaining high entanglement generation rates, the intrinsically weak interaction between photons and color centers must be enhanced via integration into photonic resonators 5,8-13 . Efforts in cavity integration have already enabled milestone demonstrations such as cavity-mediated coherent interaction between two emitters 9 , single-emitter cooperativity exceeding 100 and spin-memory-assisted quantum communication 5 . The ultimate goal of quantum computation and error-protected communication 14 requires the realization of photonic circuits with high complexity and minimal inter-node loss, and will require bringing together all integrated photonics expertise developed in the past two decades. 15 Yet color center technologies cannot at present take advantage of the state of the art in integrated photonics, due to two central challenges. First, thin-film-oninsulator photonics technologies have been incompatible with high-quality color centers: this motivated the focus on bulk-crystal-carving methods 8,16-19 , suitable for fabrication of individual devices but restrictive in terms of large-scale monolithic photonic circuits. Second, inversion symmetry, which protects optical transitions from electric fields (to first order 20,21 ), had been widely considered to be a prerequisite for color centers to maintain optical coherence in nanophotonic structures. This notion motivates the dominant focus on group-IV color centers in diamond (SiV, SnV, GeV) 22 , and eliminates from consideration an entire class of materials that lack crystal inversion symmetry. Among these materials is silicon carbide (SiC) 23 , which has otherwise emerged as the top contender for wafer-scale integration of color centers with excellent spin-optical properties (such as the sili-con vacancy (V Si ) 19,24-27 and the divacancy 28,29 ). This