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We experimentally investigate on-chip control and analysis of spatially multimode nonlinear interactions in silicon nitride waveguide circuits. Using widely different dispersion of transverse supermodes in a strongly-coupled dual-core waveguide section, and using integrated pairs of input and output single-mode waveguides, we enable controlled excitation of nonlinear processes in multiple supermodes, while a basic physical mode decomposition aids the identification of parallel and cascaded processes. Pumping with ultrashort pulses at 1.5-$\mu$m wavelength (around 195-THz light frequency), we observe simultaneous dual-supermode, near-infrared supercontinuum generation having different spectral widths, in parallel with third-harmonic generation at around 515 nm (582 THz). Cascaded four-wave mixing with supercontinuum components upconverts the third-harmonic radiation toward a set of four shorter blue wavelengths emitted in the range between 485 and 450 nm (617 to 661 THz). The approach taken here, i.e., using chip-integrated spatial multiplexing and demultiplexing for excitation and analysis of broadband transverse nonlinear conversion, can be an advanced tool for better understanding and control in multimode nonlinear optics, such as for extending frequency conversion to wider spectral ranges via extra phase matching paths., Comment: 13 pages, 8 figures

Mode-locked lasers are of interest for applications such as biological imaging, non-linear frequency conversion, and single-photon generation. In the infrared, chip-integrated mode-locked lasers have been demonstrated through integration of laser diodes with low-loss photonic circuits. However additional challenges, such as a higher propagation loss and smaller alignment tolerances have prevented the realization of such lasers in the visible range. Here, we demonstrate the first chip-integrated mode-locked diode laser in the visible using an integrated photonic circuit for cavity extension. Based on a gallium arsenide gain chip and a low-loss silicon nitride feedback circuit, the laser is passively mode-locked using a saturable absorber implemented by focused ion beam milling. At a center wavelength of 642 nm, the laser shows an average output power of 3.4 mW, with a spectral bandwidth of 1.5 nm at a repetition rate of 7.84 GHz., Comment: 7 pages, 4 figures

Thin-film lithium niobate (TFLN) has emerged as a promising platform for the realization of high performance chip-scale optical systems, spanning a range of applications from optical communications to microwave photonics. Such applications rely on the integration of multiple components onto a single platform. However, while many of these components have already been demonstrated on the TFLN platform, to date, a major bottleneck of the platform is the existence of a tunable, high-power, and narrow-linewidth on-chip laser. Here, we address this problem using photonic wire bonding to integrate optical amplifiers with a thin-film lithium niobate feedback circuit, and demonstrate an extended cavity diode laser yielding high on-chip power of 78 mW, side mode suppression larger than 60 dB and wide wavelength tunability over 43 nm. The laser frequency stability over short timescales shows an ultra-narrow intrinsic linewidth of 550 Hz. Long-term recordings indicate a high passive stability of the photonic wire bonded laser with 58 hours of mode-hop-free operation, with a trend in the frequency drift of only 4.4 MHz/h. This work verifies photonic wire bonding as a viable integration solution for high performance on-chip lasers, opening the path to system level upscaling and Watt-level output powers., Comment: 10 pages, 4 figures; updated long-term stability measurements with new and improved data

We demonstrate absorber-free passive and hybrid mode-locking at sub-GHz repetition rates using a hybrid integrated extended cavity diode laser around 1550 nm. The laser is based on InP as gain medium and a long Si$_3$N$_4$ feedback circuit, with three highly frequency selective microring resonators. The feedback resonators not only increases the cavity length up to 0.6 m to achieve sub-GHz repetition rates but also serve as a dispersive narrowband mirror for sharp spectral filtering, which enables Fourier domain mode-locking. We observe passive mode-locking with repetition rates below 500 MHz, with $\approx$ 15 comb lines at around 0.2 mW total power. To stabilize the repetition rate, hybrid mode-locking is demonstrated by weak RF modulation of the diode current at frequencies around 500 MHz. The RF injection reduces the Lorentzian linewidth component from 8.9 kHz to a detection limited value around 300 mHz. To measure the locking range of the repetition rate, the injected RF frequency is tuned with regard to the passive mode-locking frequency and the injected RF power is varied. The locking range increases approximately as a square-root function of the injected RF power. At 1 mW injection a wide locking range of about 80 MHz is obtained. We observe the laser maintaining stable mode-locking also when the DC diode pump current is increased from 40 mA to 190 mA, provided that the cavity length is maintained constant with thermo-refractive tuning., Comment: This manuscript is under review for publication with optics express

Deep Neural Networks (DNNs) require large amounts of annotated training data for a good performance. Often this data is generated using manual labeling (error-prone and time-consuming) or rendering (requiring geometry and material information). Both approaches make it difficult or uneconomic to apply them to many small-scale applications. A fast and straightforward approach of acquiring the necessary training data would allow the adoption of deep learning to even the smallest of applications. Chroma keying is the process of replacing a color (usually blue or green) with another background. Instead of chroma keying, we propose luminance keying for fast and straightforward training image acquisition. We deploy a black screen with high light absorption (99.99\%) to record roughly 1-minute long videos of our target objects, circumventing typical problems of chroma keying, such as color bleeding or color overlap between background color and object color. Next we automatically mask our objects using simple brightness thresholding, saving the need for manual annotation. Finally, we automatically place the objects on random backgrounds and train a 2D object detector. We do extensive evaluation of the performance on the widely-used YCB-V object set and compare favourably to other conventional techniques such as rendering, without needing 3D meshes, materials or any other information of our target objects and in a fraction of the time needed for other approaches. Our work demonstrates highly accurate training data acquisition allowing to start training state-of-the-art networks within minutes., Comment: 32. International Conference in Central Europe on Computer Graphics, Visualization and Computer Vision'2024

We present hybrid-integrated extended cavity diode lasers tunable around 637 nm, with a gain-wide spectral coverage of 8 nm. This tuning range allows addressing the zero-phonon line of nitrogen vacancy centers and includes the wavelength of HeNe lasers (633 nm). The lasers provide wide mode-hop free tuning up to 97 GHz and a narrow intrinsic linewidth down to 10 kHz. The maximum output power is 2.5 mW in a single-mode fiber, corresponding to an on-chip power of 4.0 mW. Full integration and packaging in a standard housing with fiber pigtails provide high intrinsic stability and will enable integration into complex optical systems., Comment: 11 pages, 7 figures

We selected sources with a steep soft-X-ray-band spectrum with a photon index larger than 2.5 -- measured by eROSITA on board the Spectrum-Roentgen-Gamma (SRG) -- from the eFEDS AGN catalogue as candidates of highly accreting supermassive black holes, and investigated their multi-wavelength properties. Among 601 bright AGN with 0.2-5 keV counts of greater than 100, 83 sources (~14%) are classified as steep-spectrum sources. These sources have typical 0.5-2 keV luminosities of L(SX) ~ 1e44 erg/s and the majority of them are found at redshifts below z=1. In comparison with sources with flatter spectra, these sources have, on average, a UV (or optical) to 2 keV luminosity ratio that is larger by ~0.3 dex and bluer optical-to-UV continuum emission. They also appear to be radio quiet based on the detection rate in the FIRST and VLASS surveys. Their host galaxies -- at least in the redshift range of z=0.2-0.8, where the AGN-galaxy decomposition results from the Subaru Hyper Suprime-Cam imaging are available -- tend to be late-type and have smaller stellar masses than those of sources with flatter spectra. These properties are similar to those found in nearby narrow-line Seyfert 1 galaxies, in agreement with the picture that they are AGN with elevated accretion rates and are in the early growth phase of black hole and galaxy co-evolution. However, the steep-spectrum sources are not exclusively narrow-line Seyfert 1 galaxies; indeed many are broad-line Seyfert 1 galaxies, as found by a catalogue search. This suggests that these steep-spectrum sources may be black holes generally with high accretion rates but of a wide mass range, including a few objects emitting at L(SX)>1e45 erg/s, of which black hole masses can be close to 10^9 M_sun., Comment: 8 pages, accepted for publication in A&A

The eROSITA instrument aboard the Spectrum Roentgen Gamma (SRG) satellite has performed its first all-sky survey between December 2019 and June 2020. This paper presents the resulting hard X-ray (2.3-5 keV) sample, the first created from an all-sky imaging survey in the 2-8 keV band, for sources within western galactic sky. The 5466 hard X-ray selected sources detected with eROSITA are presented and discussed. The Bayesian statistics-based code NWAY is used to identify the counterparts for the X-ray sources. These sources are classified based on their multiwavelength properties, and the literature is searched to identify spectroscopic redshifts, which further inform the source classification. A total of 2547 sources are found to have good-quality counterparts, and 111 of these are detected only in the hard band. Comparing with other hard X-ray selected surveys, the eROSITA hard sample covers a larger redshift range and probes dimmer sources, providing a complementary and expanded sample as compared to Swift-BAT. Examining the column density distribution of missed and detected eROSITA sources present in the follow-up catalog of Swift BAT 70 month sources, it is demonstrated that eROSITA can detect obscured sources with column densities $>10^{24}$ cm$^{-2}$, but that the completeness drops rapidly after $10^{23}$ cm$^{-2}$. A sample of hard-only sources, many of which are likely to be heavily obscured AGN, is also presented and discussed. X-ray spectral fitting reveals that these sources have extremely faint soft X-ray emission and their optical images suggest that they are found in more edge-on galaxies with lower b/a. The resulting X-ray catalog is demonstrated to be a powerful tool for understanding AGN, in particular heavily obscured AGN found in the hard-only sample., Comment: 20 pages, 21 figures. Submitted to A&A. Associated with eROSITA data release. Associated catalogs will be released after journal publication

During its calibration and performance verification phase, the eROSITA instrument aboard the SRG satellite performed a uniform wide--area X-ray survey of approximately 140 deg$^{2}$ in a region of the sky known as the eROSITA Final Equatorial Depth Survey (eFEDS). The primary aim of eFEDS is to demonstrate the scientific performance to be expected at the end of the 8-pass eROSITA all sky survey. This will provide the first focussed image of the whole sky in the hard X-ray ($>2$~keV) bandpass. The expected source population in this energy range is thus of great interest, particularly for AGN studies. We use the 2.3--5 keV selection presented by Brunner et al. (2022) to construct a sample of 246 point-like hard X-ray sources for further study and characterization. These are classified as either extragalactic ($\sim 90$~\%) or Galactic ($\sim 10$~\%), with the former consisting overwhelmingly of AGN and the latter active stars. We concentrate our further analysis on the extragalactic/AGN sample, describing their X-ray and multiwavelength properties and comparing them to the eFEDS main AGN sample selected in the softer 0.2-2.3 keV band. The eROSITA hard band selects a subsample of sources that is a factor $>10$ brighter than the eFEDS main sample. The AGN within the hard population reach up to $z=3.2$ but on the whole are relatively nearby, with median $z$=0.34 compared to $z$=0.94 for the main sample. The hard survey probes typical luminosities in the range $\log L_{\rm X} = 43-46$. X-ray spectral analysis shows significant intrinsic absorption (with $\log N_{\rm H}>21$) in $\sim 20$~\% of the sources, with a hard X-ray power law continuum with mean $<\Gamma>=1.83\pm0.04$, typical of AGN, but slightly harder than the soft-selected eROSITA sample. (abridged), Comment: Submitted to A&A as part of eROSITA Data Release 1

Although flux variability is one of the defining properties of accretion flows onto supermassive black holes, its dependence on physical parameters such as the mass of the compact object and the Eddington ratio remain under discussion. In this paper we address this issue using the structure function statistic to measure the variability at X-ray wavelengths of a sample of optically selected QSOs with available black hole masses and Eddington ratios. We present a new Bayesian methodology for estimating the structure function tailored to the Poisson nature of the X-ray data. This is applied to 15,548 SDSS DRQ16 QSOs with repeat observations in the XMM-Newton archive and/or the SRG/eROSITA All Sky Survey. The X-ray structure function monotonically increases to time intervals of about 10-15 years, suggesting a preference for scenarios in which instabilities of the accretion disk contribute to the X-ray variability on long timescales. Additionally, there is evidence that the amplitude of the stochastic X-ray flux variations rises with decreasing black hole mass and Eddington ratio. This finding imposes stringent constraints on empirical models of Active Galactic Nuclei variability derived from local samples, emphasizing the significance of high-redshift population studies for comprehending the stochastic flux variations in active black holes., Comment: 14 pages, 13 figure, MNRAS accepted, revision following refereeing process. The code used in this paper is available at https://github.com/ageorgakakis/StructureFunction

The extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spectrum-Roentgen-Gamma (SRG) mission with its first All-Sky Survey (eRASS1) has offered an unprecedented, comprehensive view of the variable X-ray sky. With enhanced sensitivity, broader energy coverage, and improved resolution compared to prior surveys, the eRASS1 Data Release 1 (DR1) catalogue underwent a variability analysis, focusing on a substantial subset of 128,669 sources. We performed multiple variability tests, utilizing conventional normalized excess variance, maximum amplitude variability, and Bayesian excess variance methods. Among the 128,669 DR1 sources, our research identified 557 objects exhibiting variability through NEV and AMPLMAX tests. After applying suitable thresholds, 108 sources demonstrated significant variability via NEV, while 73 did so through AMPLMAX. The utilization of the bexvar method extended our detection capabilities to lower count rates, unveiling a total of 1307 sources manifesting variability. Furthermore, our comparative analysis spanning 2.5 years encompassed observations from consecutive eROSITA surveys, eRASS2, eRASS3, eRASS4, and eRASS5. Notably, the Gamma-ray burst afterglow GRB 200120A, which was the most variable DR1 source, was as expected absent in subsequent eROSITA survey scans. Observations of the Low-Mass X-ray Binary GX 339-4 across various eROSITA survey scans unveiled substantial variability. These outbursts involve the movement of the inner radius of the accretion disk, fluctuating inward and outward. Combining eROSITA and MAXI data reveals that the most effective tracer for monitoring the onset of the outbursts is the softest eROSITA band. Magnetically active stars are commonly found among the more variable X-ray sources. We analyzed the AGN sample to identify variability patterns and instances of efficiency limit violations., Comment: 13 pages, 10 figures

The eROSITA telescope array aboard the Spektrum Roentgen Gamma (SRG) satellite began surveying the sky in December 2019, with the aim of producing all-sky X-ray source lists and sky maps of an unprecedented depth. Here we present catalogues of both point-like and extended sources using the data acquired in the first six months of survey operations (eRASS1; completed June 2020) over the half sky whose proprietary data rights lie with the German eROSITA Consortium. We describe the observation process, the data analysis pipelines, and the characteristics of the X-ray sources. With nearly 930000 entries detected in the most sensitive 0.2-2.3 keV energy range, the eRASS1 main catalogue presented here increases the number of known X-ray sources in the published literature by more than 60%, and provides a comprehensive inventory of all classes of X-ray celestial objects, covering a wide range of physical processes. A smaller catalogue of 5466 sources detected in the less sensitive but harder 2.3-5 keV band is the result of the first true imaging survey of the entire sky above 2 keV. We show that the number counts of X-ray sources in eRASS1 are consistent with those derived over narrower fields by past X-ray surveys of a similar depth, and we explore the number counts variation as a function of the location in the sky. Adopting a uniform all-sky flux limit (at 50% completeness) of F_{0.5-2 keV} > 5 \times 10^{-14}$ erg\,s$^{-1}$\,cm$^{-2}$, we estimate that the eROSITA all-sky survey resolves into individual sources about 20% of the cosmic X-ray background in the 1-2 keV range. The catalogues presented here form part of the first data release (DR1) of the SRG/eROSITA all-sky survey. Beyond the X-ray catalogues, DR1 contains all detected and calibrated event files, source products (light curves and spectra), and all-sky maps. Illustrative examples of these are provided., Comment: 39 pages, 23 figures. Accepted for publication in A&A. Accompanying eROSITA-DE Data Release 1

Recent years have seen broad observational support for the presence of a clumpy component within the circumnuclear gas around SMBHs. In the X-ray band, individual clouds can manifest themselves when they transit the line of sight to the X-ray corona, temporarily obscuring the X-ray continuum and thereby indicating the characteristics and location of these clouds. X-ray flux monitoring with SRG/eROSITA has revealed that in the Seyfert 1 AGN EC 04570-5206, the soft X-ray flux dipped abruptly for about 10-18 months over 2020-2021, only to recover and then drop a second time by early 2022. Here, we investigate whether these flux dips and recoveries could be associated with cloud occultation events. We complemented the eROSITA scans with multiwavelength follow-up observations, including X-ray/UV observations with Swift, XMM-Newton, and NICER, along with ground-based optical photometric and spectroscopic observations to investigate the spectral and flux variability. XMM-Newton spectra confirm that the soft X-ray flux dips were caused by partial-covering obscuration by two separate clouds. The 2020-2021 event was caused by a cloud with column density near 1e22 /cm2 and a covering fraction near 0.6. The cloud in the 2022 event had a column density near 3e23 /cm2 and a covering fraction near 0.8. The optical/UV continuum flux varied minimally and the optical emission line spectra showed no variability in Balmer profiles or intensity. The transiting gas clouds are neutral or lowly-ionized, while the lower limits on their radial distances are commensurate with the dust sublimation zone (cloud 1) or the optical broad line region (cloud 2). One possible explanation is a dust-free, outflowing wind with embedded X-ray clumps. These events are the first cloud obscuration events detected in a Seyfert galaxy using eROSITA's X-ray monitoring capabilities., Comment: Accepted for publication in Astronomy and Astrophysics

The evolution of synchrotrons towards higher brilliance beams has increased the possible sample-to-detector propagation distances for which the source confusion circle does not lead to geometrical blurring. This makes it possible to push near-field propagation driven phase contrast enhancement to the limit, revealing low contrast features which would otherwise remain hidden under an excessive noise. Until today this possibility was hindered, in many objects of scientific interest, by the simultaneous presence of strong phase gradient regions and low contrast features. The strong gradients, when enhanced with the now possible long propagation distances, induce such strong phase effects that the linearisation assumptions of current state-of-the-art single-distance phase retrieval filters are broken, and the resulting image quality is jeopardized. Here, we introduce a new iterative phase-retrieval algorithm and compare it with the Paganin phase-retrieval algorithm, in both the monochromatic and polychromatic cases, obtaining superior image quality. In the polychromatic case the comparison was done with an extrapolated Paganin algorithm obtained by reintroducing, into our phase retrieval algorithm, the linearization approximations underlying the Paganin forward model. Our work provides an innovative algorithm which efficiently performs the phase retrieval task over the entire near-field range, producing images of exceptional quality for mixed attenuation objects.

Brillouin enhanced four wave mixing in the form of a Brillouin dynamic grating (BDG) enables a uniquely tunable filter, whose properties can be tuned by purely optical means. This makes the BDG a valuable tool in microwave photonics (MWP). BDGs have been studied extensively in fibers, but the only observation in an integrated platform required exotic materials. Unlocking BDG in a standard and mature platform will enable its integration into large-scale circuits. Here we demonstrate the first observation of a BDG in a silicon nitride (Si$_3$N$_4$) waveguide. We also present a new, optimized design, which will enhance the BDG response of the waveguide, unlocking a path to large-scale integration into MWP circuits.

We investigate on-chip spectral control of supercontinuum generation, taking advantage of the additional spatial degree of freedom in strongly-coupled dual-core waveguides. Using numerical integration of the multi-mode generalized nonlinear Schr\"odinger equation, we show that, with proper waveguide cross-section design, selective excitation of supermodes can vary the dispersion to its extremes, i.e., all-normal or anomalous dispersion can be selected via phase shifting in a Mach-Zehnder input circuit. The resulting control allows to provide vastly different supercontinuum spectra with the same waveguide circuit.

Context. The soft excess, a surplus of X-ray photons above 2 keV with respect to a power law, is a feature of debated physical origin found in the X-ray spectra of many type-1 active galactic nuclei (AGN). The eROSITA instrument aboard the Spectrum-Roentgen-Gamma (SRG) mission will provide an all-sky census of AGN suitable for spectral analysis. Aims. The primary goal of this work is to test a variety of models for the soft X-ray emission of AGN (thermal emission, non-thermal emission, ionised absorption, or neutral partial covering absorption) to help identify the physical origin of the soft X-ray spectral complexity. Differences between these models are examined in the context of this sample to understand the physical properties. Methods. We used Bayesian X-ray analysis to fit a sample of 200 AGN from the eFEDS hard X-ray--selected sample with a variety of phenomenological and physically motivated models. Model selection was performed using the Bayes factor to compare the applicability of each model. Results. We find that 29 sources have evidence for a soft excess at a confidence level >97.5%, all of which are better modelled by an additional soft power law than by thermal blackbody emission. We find 23 of these sources prefer a warm corona model, while six sources prefer relativistic blurred reflection. Additionally many sources show evidence for complex absorption, with 29 preferring a warm absorber and 25 a partial covering absorber. Sources with a soft excess show a significantly higher Eddington ratio than those with warm absorbers. We discuss the implication of these results for the physical processes in the central regions of AGN. Conclusions. Spectral fitting with Bayesian statistics is ideal for the identification of complex absorption and soft excesses in the X-ray spectra of AGN and can allow one to distinguish between different physical interpretations. (Abridged), Comment: 27 pages, 30 figures, plus 2 appendices. Accepted for publication by A&A

Hybrid integrated diode lasers based on combining semiconductor optical amplifiers with low-loss Si3N4-based feedback circuits enable great laser performance for advanced photonic circuits. In particular, using high-Q Si3N4 ring resonators for frequency-selective feedback provides wide spectral coverage, mode-hop free tuning, and high frequency stability on short timescales, showing as ultra-narrow intrinsic linewidths. However, many applications also require long-term stability, which can be provided by locking the laser frequency to a suitable reference. We present the stabilization of a hybrid-integrated laser, which is widely tunable around the central wavelength of 1550 nm, to a fiber-based optical frequency discriminator (OFD) and to an acetylene absorption line. By locking the laser to the OFD, the laser's fractional frequency stability is improved down to 2.1$\cdot$10$^{-12}$ over an averaging time of 0.5 ms. For absolute stability over longer times of several days, we successfully lock the laser frequency to an acetylene absorption line. This limits the frequency deviations of the laser to a range of less than 12 MHz over 5 days., Comment: 12 pages, 6 figures

Hybrid integrated diode lasers have so far been realized using silicon, polymer, and silicon nitride (Si3N4) waveguide platforms for extending on-chip tunable light engines from the infrared throughout the visible range. Here we demonstrate the first hybrid integrated laser using the aluminum oxide (Al2O3) deep-UV capable waveguide platform. By permanently coupling low-loss Al2O3 frequency-tunable Vernier feedback circuits with GaN double-pass amplifiers in a hermetically sealed housing, we demonstrate the first extended cavity diode laser (ECDL) in the near UV. The laser shows a maximum fiber-coupled output power of 0.74 mW, corresponding to about 3.5 mW on chip, and tunes more than 4.4 nm in wavelength from 408.1 nm to 403.7 nm. Integrating stable, single-mode and tunable lasers into a deep-UV platform opens a new path for chip-integrated photonic applications., Comment: 14 pages, 3 figures

We present the first look at the spectral and timing analysis of Narrow-Line Seyfert 1 Galaxies (NLS1s) with the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spectrum-Roentgen-Gamma (SRG) mission. The sample of about 1,200 NLS1s was obtained via cross-match of the first eROSITA All-Sky Survey (eRASS1) catalogue with the catalogue of spectroscopically selected NLS1s from the Sloan Digital Sky Survey (SDSS) DR12 by Rakshit et al. [ApJS, 229, 39 (2017)]. The X-ray spectral analysis is based on a simple power-law fit. The photon index distribution has a mean value of about 2.81+-0.03, as expected from previous X-ray studies of NLS1s. Interestingly, it is positively skewed, and about 10 percent of the sources are located in the super-soft tail of photon indices larger than 4. These sources are of further interest as their source counts run into the X-ray background at values at around 1 keV. We argue that ionised outflows have been detected by eROSITA and may account for some of the extreme spectral steepness, which is supported by correlations found between the photon index and optical outflows parameters. We analysed the intrinsic X-ray variability of the eRASS1 to eRASS3 light curves of the sample but do not find significant variability neither during the individual survey scans nor between them., Comment: 11 pages, 10 figures, accepted for publication in Section 4. Extragalactic astronomy of Astronomy and Astrophysics

Integrated photonics has recently become a leading platform for the realization and processing of optical entangled quantum states in compact, robust and scalable chip formats with applications in long-distance quantum-secured communication, quantum-accelerated information processing and non-classical metrology. However, the quantum light sources developed so far have relied on external bulky excitation lasers making them impractical, not reproducible prototype devices, hindering scalability and the transfer out of the lab into real-world applications. Here we demonstrate a fully integrated quantum light source, which overcomes these challenges through the combined integration of a laser cavity, a highly efficient tunable noise suppression filter ($> 55$ dB) exploiting the Vernier effect and a nonlinear microring for entangled photon pair generation through spontaneous four-wave mixing. The hybrid quantum source employs an electrically-pumped InP gain section and a Si$_3$N$_4$ low-loss microring filter system, and demonstrates high performance parameters, i.e., a pair emission over four resonant modes in the telecom band (bandwidth $\sim 1$ THz), and a remarkable pair detection rate of $\sim 620$ Hz at a high coincidence-to-accidental ratio of $\sim 80$. The source directly creates high-dimensional frequency-bin entangled quantum states (qubits/qudits), verified by quantum interference measurements with visibilities up to $96\%$ (violating Bell-inequality) and by density matrix reconstruction through state tomography showing fidelities of up to $99\%$. Our approach, leveraging a hybrid photonic platform, enables commercial-viable, low-cost, compact, light-weight, and field-deployable entangled quantum sources, quintessential for practical, out-of-lab applications, e.g., in quantum processors and quantum satellite communications systems.

Fully integrated supercontinuum sources on-chip are critical to enabling applications such as portable and mechanically-stable medical imaging devices, chemical sensing and LiDAR. However, the low-efficiency of current supercontinuum generation schemes prevent full on-chip integration. In this letter, we present a scheme where the input energy requirements for integrated supercontinuum generation is drastically lowered by orders of magnitude, for bandwidth generation of the order of 500 to 1000 nm. Through sign-alternating the dispersion in a CMOS compatible silicon nitride waveguide, we achieve an efficiency enhancement by factors reaching 3800. We show that the pulse energy requirement for large bandwidth supercontinuum generation at high spectral energy (e.g., 1/e level) is lowered from nanojoules to 6 picojoules. The lowered pulse energy requirements enables that chip-integrated laser sources, such as mode-locked heterogeneously or hybrid integrated diode lasers, can be used as a pump source, enabling fully integrated on-chip high-bandwidth supercontinuum sources.

The eROSITA X-ray telescope on board the Spectrum-Roentgen-Gamma (SRG) observatory performed calibration and performance verification (CalPV) observations between September 2019 and December 2019, ahead of the planned four-year all-sky surveys. Most of them were deep, pointing-mode observations. We present here the X-ray catalog detected from the set of extra-galactic CalPV observations released to the public by the German eROSITA consortium, and the multiband counterparts of these X-ray sources. We developed a source detection method optimized for point-like X-ray sources by including extended X-ray emission in the background measurement. The multiband counterparts were identified using a Bayesian method from the CatWISE catalog. Combining 11 CalPV fields, we present a catalog containing 9515 X-ray sources, whose X-ray fluxes were measured through spectral fitting. CatWISE counterparts are presented for 77% of the sources. Significant variabilities are found in 99 of the sources, which are also presented with this paper. Most of these fields show similar number counts of point sources as typical extragalactic fields, and a few harbor particular stellar populations., Comment: to be published in A&A, 23 pages, 14 figures

We present a tunable, hybrid waveguide-fiber optical parametric oscillator (OPO) synchronously pumped by an ultra-fast fiber laser exploiting four-wave mixing (FWM) generated in silicon nitride waveguides. Parametric oscillation results in a 35 dB enhancement of the idler spectral power density in comparison to spontaneous FWM, with the ability of wide wavelength tuning over 86 nm in the O-band. Measurements of the oscillation threshold and the efficiency of the feedback loop reveal how an integration of the OPO on a single silicon nitride chip can be accomplished at standard repetition rates of pump lasers in the order of 100 MHz.

The X-ray telescope eROSITA onboard the newly launched SRG mission serendipitously observed the galaxy cluster A3408 ($z=0.0420$) during the PV observation of the AGN 1H0707-495. Despite its brightness and large extent, it has not been observed by any modern X-ray observatory. A neighbouring cluster in NW direction, A3407 ($z=0.0428$), appears to be close at least in projection ($\sim 1.7$ Mpc). This cluster pair could be in a pre- or post-merger state. We perform a detailed X-ray analysis of A3408. We construct particle background subtracted and exposure corrected images and surface brightness profiles in different sectors. The spectral analysis is performed out to $1.4r_{500}$. Additionally, a temperature map is presented depicting the distribution of the ICM temperature. Furthermore, we make use of data from the RASS to estimate some bulk properties of A3408 and A3407, using the growth curve analysis method and scaling relations. The imaging analysis shows a complex morphology of A3408 with a strong elongation in SE-NW direction. This is quantified by comparing the surface brightness profiles of the NW, SW, SE and NE directions, where the NW and SE directions show a significantly higher surface brightness compared to the other directions. We determine a gas temperature ${\rm k_B}T_{500}=(2.23\pm0.09)$ keV. The T-profile reveals a hot core within $2'$ of the emission peak, ${\rm k_B}T=3.04^{+0.29}_{-0.25}$ keV. Employing a M-T relation, we obtain $M_{500}=(9.27\pm0.75)\times 10^{13}M_{\odot}$ iteratively. The $r_{200}$ of A3407 and A3408 are found to overlap in projection which makes ongoing interactions plausible. The 2d T-map reveals higher temperatures in W than in E direction. A3407 and A3408 are likely in a pre-merger state, affecting the ICM properties, i.e., increased temperatures in the direction of A3407 indicate adiabatic compression or shocks due to the interaction., Comment: 10 pages, 7 figures (main text), 2 figures (appendix). Submitted to A&A for the Special Issue: The Early Data Release of eROSITA and Mikhail Pavlinsky ART-XC on the SRG Mission

The reliability of detecting source variability in sparsely and irregularly sampled X-ray light curves is investigated. This is motivated by the unprecedented survey capabilities of eROSITA onboard SRG, providing light curves for many thousand sources in its final-depth equatorial deep field survey. Four methods for detecting variability are evaluated: excess variance, amplitude maximum deviations, Bayesian blocks and a new Bayesian formulation of the excess variance. We judge the false detection rate of variability based on simulated Poisson light curves of constant sources, and calibrate significance thresholds. Simulations with flares injected favour the amplitude maximum deviation as most sensitive at low false detections. Simulations with white and red stochastic source variability favour Bayesian methods. The results are applicable also for the million sources expected in eROSITA's all-sky survey., Comment: Variability analysis tools available https://github.com/JohannesBuchner/bexvar/. 15 min Talk: https://youtu.be/xBC1S9MTH4w. Accepted in A&A, Special Issue: The Early Data Release of eROSITA and Mikhail Pavlinsky ART-XC on the SRG Mission

The eROSITA X-ray telescope on board the Spectrum-Roentgen-Gamma (SRG) satellite has started to detect new X-ray sources over the full sky at an unprecedented rate. Understanding the performance and selection function of the source detection is important for the subsequent scientific analysis of the eROSITA catalogs. Through simulations, we test and optimize the eROSITA source detection procedures, and we characterize the detected catalog quantitatively. Taking the eROSITA Final Equatorial-Depth Survey (eFEDS) as an example, we ran extensive photon-event simulations based on our best knowledge of the instrument characteristics, the background spectrum, and the population of astronomical X-ray sources. We introduce a method of analyzing source detection completeness, purity, and efficiency based on the origin of each photon. According to the source detection efficiency measured in the simulation, we chose a two-pronged strategy to build eROSITA X-ray catalogs, creating a main catalog using only the most sensitive band (0.2-2.3 keV) and an independent hard-band-selected catalog using multiband detection in a range up to 5 keV. Because our mock data are highly representative of the real eFEDS data, we used the mock catalogs to measure the completeness and purity of the eFEDS catalogs as a function of multiple parameters, such as detection likelihood, flux, and luminosity. These measurements provide a basis for choosing the eFEDS catalog selection thresholds. The mock catalogs (available with this paper) can be used to construct the selection function of active galactic nuclei and galaxy clusters. A direct comparison of the output and input mock catalogs also gives rise to a correction curve that converts the raw point-source flux distribution into the intrinsic number counts distribution., Comment: Accepted by A&A for the Special Issue: The Early Data Release of eROSITA and Mikhail Pavlinsky ART-XC on the SRG Mission