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We investigate pattern formation in two-dimensional Bose-Einstein condensates (BECs) caused by periodic driving of the interatomic interaction. We show that this modulation generically leads to a stable square grid density pattern, due to nonlinear effects beyond the initial Faraday instability. We take the amplitudes of two waves parametrizing the two-dimensional density pattern as order parameters in pattern formation. For these amplitudes, we derive a set of coupled time evolution equations from the Gross--Pitaevskii (GP) equation with a time-periodic interaction. We identify the fixed points of the time evolution and show by stability analysis that the inhomogeneous density exhibits a square grid pattern, which can be understood as a manifestation of a stable fixed point. Our stability analysis establishes the pattern in BECs as a nonequilibrium steady state., Comment: 7 pages, 3 figures. Supplemental material: 9 pages

The formation of patterns in driven systems has been studied extensively, and their emergence can be connected to a fine balance of instabilities and stabilization mechanisms. While the early phase of pattern formation can be understood on the basis of linear stability analyses, the long-time dynamics can only be described by accounting for the interactions between the excitations generated by the drive. Here, we observe the stabilization of square patterns in an interaction-driven, two-dimensional Bose-Einstein condensate. These patterns emerge due to inherent high-order processes that become relevant in the regime of large phonon occupations. Theoretically, this can be understood as the emergence of a stable fixed point of coupled nonlinear amplitude equations, which include phonon-phonon interactions. We experimentally probe the predicted flows towards such a stable fixed-point, as well as repulsion from a saddle fixed-point, using the experimental control unique to quantum gases., Comment: Version as accepted in PRX

There is a wide range of modal logics whose semantics goes beyond relational structures, and instead involves, e.g., probabilities, multi-player games, weights, or neighbourhood structures. Coalgebraic logic serves as a unifying semantic and algorithmic framework for such logics. It provides uniform reasoning algorithms that are easily instantiated to particular, concretely given logics. The COOL 2 reasoner provides an implementation of such generic algorithms for coalgebraic modal fixpoint logics. As concrete instances, we obtain in particular reasoners for the aconjunctive and alternation-free fragments of the graded $\mu$-calculus and the alternating-time $\mu$-calculus. We evaluate the tool on standard benchmark sets for fixpoint-free graded modal logic and alternating-time temporal logic (ATL), as well as on a dedicated set of benchmarks for the graded $\mu$-calculus., Comment: Final version (corrected slight mistake in Rabin-type formula series)

We report on the first observation of photoassociation near the ${}^2 S _\frac 1 2 + {}^3P_1$-asymptote in a mixture of ${}^{87}$Rb and ${}^{170}$Yb. In a search spanning binding energies between 0.1 GHz and 11 GHz, a single pair of interspecies PA resonances is detected around 3.1 GHz. These resonances are characterized by extracting PA rates, binding energies and Zeeman shift coefficients. Using one of these resonances, 2-photon-photoassociaton is performed, improving on previous measurements of the binding energies of the two least bound states in the electronic ground state and demonstrating intercombination line photoassociation as a powerful spectroscopic tool. We discuss implications for pathways towards RbYb molecules in the absolute ground state., Comment: 8 pages, 4 figures

We present an experimental study of the Rydberg $^1S_0$- and $^1P_1$- series of ytterbium for principal quantum numbers in the range of $n=70$ to $90$. The study is performed using trap loss spectroscopy in a magneto-optical trap operating on the $6^1S_0 \rightarrow 6^1P_1$ transition at $399\,\text{nm}$. Compared to the commonly used Rydberg spectroscopy method using field-ionization and ion detection, trap loss spectroscopy is significantly simpler and requires a less sophisticated experimental setup. Using this method we determine relative values of the scalar and tensor electric polarizabilities of both, $6sns\,^1S_0$- and $6snp\,^1P_1$- Rydberg states.

Quantum information processing (QIP) with solid state spin qubits strongly depends on the efficient initialisation of the qubit's desired charge state. While the negatively charged tin-vacancy ($\text{SnV}^{-}$) centre in diamond has emerged as an excellent platform for realising QIP protocols due to long spin coherence times at liquid helium temperature and lifetime limited optical transitions, its usefulness is severely limited by termination of the fluorescence under resonant excitation [1,2,3]. Here, we unveil the underlying charge cycle of group IV-vacancy (G4V) centres and exploit it to demonstrate highly efficient initialisation of the desired negative charge state of single SnV centres while preserving long term stable optical resonances. We furthermore all-optically probe the coherence of the ground state spins by means of coherent population trapping and find a spin dephasing time of 5$\mu$s. Additionally, we demonstrate proof-of-principle single shot spin state readout without the necessity of a magnetic field aligned to the symmetry axis of the defect.

4GREAT is an extension of the German Receiver for Astronomy at Terahertz frequencies (GREAT) operated aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). The spectrometer comprises four different detector bands and their associated subsystems for simultaneous and fully independent science operation. All detector beams are co-aligned on the sky. The frequency bands of 4GREAT cover 491-635, 890-1090, 1240-1525 and 2490-2590 GHz, respectively. This paper presents the design and characterization of the instrument, and its in-flight performance. 4GREAT saw first light in June 2018, and has been offered to the interested SOFIA communities starting with observing cycle 6., Comment: Accepted for publication in IEEE Transactions on Terahertz Science and Technology

Recently deep generative models have achieved impressive progress in modeling the distribution of training data. In this work, we present for the first time a generative model for 4D light field patches using variational autoencoders to capture the data distribution of light field patches. We develop a generative model conditioned on the central view of the light field and incorporate this as a prior in an energy minimization framework to address diverse light field reconstruction tasks. While pure learning-based approaches do achieve excellent results on each instance of such a problem, their applicability is limited to the specific observation model they have been trained on. On the contrary, our trained light field generative model can be incorporated as a prior into any model-based optimization approach and therefore extend to diverse reconstruction tasks including light field view synthesis, spatial-angular super resolution and reconstruction from coded projections. Our proposed method demonstrates good reconstruction, with performance approaching end-to-end trained networks, while outperforming traditional model-based approaches on both synthetic and real scenes. Furthermore, we show that our approach enables reliable light field recovery despite distortions in the input.

The recently discovered negatively charged tin-vacancy centre in diamond is a promising candidate for applications in quantum information processing (QIP). We here present a detailed spectroscopic study encompassing single photon emission and polarisation properties, the temperature dependence of emission spectra as well as a detailed analysis of the phonon sideband and Debye-Waller factor. Using photoluminescence excitation spectroscopy (PLE) we probe an energetically higher lying excited state and prove fully lifetime limited linewidths of single emitters at cryogenic temperatures. For these emitters we also investigate the stability of the charge state under resonant excitation. These results provide a detailed insight into the spectroscopic properties of the $\text{SnV}^-$ centre and lay the foundation for further studies regarding its suitability in QIP.

Nitrogen-vacancy (NV) centers feature outstanding properties like a spin coherence time of up to one second as well as a level structure offering the possibility to initialize, coherently manipulate and optically read-out the spin degree of freedom of the ground state. However, only about three percent of their photon emission are channeled into the zero phonon line (ZPL), limiting both the rate of indistinguishable single photons and the signal-to-noise ratio (SNR) of coherent spin-photon interfaces. We here report on the enhancement of the SNR of the optical spin read-out achieved by tuning the mode of a two-dimensional photonic crystal (PhC)cavity into resonance with the NV-ZPL. PhC cavities are fabricated by focused ion beam (FIB) milling in thin reactive ion (RIE) etched ultrapure single crystal diamond membranes featuring modes with Q-factors of up to 8250 at mode volumes below one cubic wavelength. NV centers are produced in the cavities in a controlled fashion by a high resolution atomic force microscope (AFM) implantation technique. On cavity resonance we observe a lifetime shortening from 9.0ns to 8.0ns as well as an enhancement of the ZPL emission by almost one order of magnitude. Although on resonance the collection efficiency of ZPL photons and the spin-dependent fluorescence contrast are reduced, the SNR of the optical spin read-out is almost tripled for the cavity-coupled NV centers.

We investigate native nitrogen (NV) and silicon vacancy (SiV) color centers in commercially available, heteroepitaxial, wafer-sized, mm thick, single-crystal diamond. We observe single, native NV centers with a density of roughly 1 NV per $\mu m^3$ and moderate coherence time ($T_2 = 5 \mu s$) embedded in an ensemble of SiV centers. Low-temperature spectroscopy of the SiV zero phonon line fine structure witnesses high crystalline quality of the diamond especially close to the growth surface, consistent with a reduced dislocation density. Using ion implantation and plasma etching, we verify the possibility to fabricate nanostructures with shallow color centers rendering our diamond material promising for fabrication of nanoscale sensing devices. As this diamond is available in wafer-sizes up to $100 mm$ it offers the opportunity to up-scale diamond-based device fabrication.

Strong light-matter interactions are critical for quantum technologies based on light, such as memories or nonlinear interactions. Solid state materials will be particularly important for such applications, because of the relative ease of fabrication of components. Silicon vacancy centers (SiV) in diamond feature especially narrow inhomogeneous spectral lines, which are rare in solid materials. Here, we demonstrate resonant coherent manipulation, stimulated Raman adiabatic passage, and strong light-matter interaction via four-wave mixing of a weak signal field in an ensemble of SiV centers.

Segmented magnetic nanowires are a promising route for the development of three dimensional data storage techniques. Such devices require a control of the coercive field and the coupling mechanisms between individual magnetic elements. In our study, we investigate electrodeposited nanomagnets within host templates using vibrating sample magnetometry and observe a strong dependence between nanowire length and coercive field (25 nm to 5 $\mu$m) and diameter (25 nm to 45 nm). A transition from a magnetization reversal through coherent rotation to domain wall propagation is observed at an aspect ratio of approximately 2. Our results are further reinforced via micromagnetic simulations and angle dependent hysteresis loops. The found behavior is exploited to create nanowires consisting of a fixed and a free segment in a spin-valve like structure. The wires are released from the membrane and electrically contacted, displaying a giant magnetoresistance effect that is attributed to individual switching of the coupled nanomagnets. We develop a simple analytical model to describe the observed switching phenomena and to predict stable and unstable regimes in coupled nanomagnets of certain geometries.

Plasmonic nanoparticles influence the absorption and emission processes of nearby emitters due to local enhancements of the illuminating radiation and the photonic density of states. Here, we use the plasmon resonance of metal nanoparticles in order to enhance the stimulated depletion of excited molecules for super-resolved microscopy. We demonstrate stimulated emission depletion (STED) microscopy with gold nanorods with a long axis of only 26 nm and a width of 8 nm that provide an enhancement of the resolution compared to fluorescent-only probes without plasmonic components irradiated with the same depletion power. These novel nanoparticle-assisted STED probes represent a ~2x10^3 reduction in probe volume compared to previously used nanoparticles and we demonstrate their application to the first plasmon-assisted STED cellular imaging. We also discuss their current limitations., Comment: 30 pages, 15 figures, main text and supplementary information

Recently, the negatively charged silicon vacancy centre (SiV) in diamond has emerged as a novel promising system for quantum information processing (QIP) due to its superior spectral properties and advantageous electronic structure: The SiV offers an optically accessible {\Lambda}-type level structure with a large orbital level splitting even without the need of an external magnetic field and previous studies determined the ground state coherence time of the centre to be on the order of 35-45ns. While these studies already indicate the feasibility of an ultrafast all-optical coherent control of the SiV, no experimental realization has been presented so far. Here, we report on optical Rabi oscillations and Ramsey interference between the ground and the excited state of the SiV using 12 ps long, resonant rotation pulses. Moreover, utilizing a {\Lambda}-scheme, we demonstrate Raman-based coherent Rabi rotations and Ramsey interference within the ground state manifold using a single, 1ps long off-resonant pulse. These measurements prove the accessibility of the complete set of single-qubit operations relying solely on optical fields and pave the way for high-speed QIP applications using SiV centres., Comment: 8 pages, 4 figures

Iron nanowires with a square cross section are grown by pulsed electrodeposition within a newly developed nanochannel template that allows for easy characterization. Measurements of the magnetoresistance as a function of magnetic field and temperature are performed within a large parameter window allowing for the investigation of the magnonic contribution to the magnetoresistance of electrodeposited iron nanowires. Values for the temperature dependent magnon stiffness D(T) are extracted., Comment: This is the pre-peer reviewed version of the following article: P. Sergelius, J. M. Montero Moreno, M. Waleczek, T. B\"ohnert, D. G\"orlitz, K. Nielsch, Phys. Status Solidi RRL, 2015, DOI: 10.1002/pssr.201510057 which has been published in final form at http://onlinelibrary.wiley.com/doi/10.1002/pssr.201510057/abstract

Rapid and reproducible assembly of aligned nanostructures on a wafer-scale is a crucial, yet one of the most challenging tasks in the incorporation of nanowires into integrated circuits. We present the synthesis of a periodic nanochannel template designed for electrochemical growth of perfectly aligned, rectangular nanowires over large areas. The nanowires can be electrically contacted and characterized in situ using a pre-patterned multi-point measurement platform. During the measurement the wires remain within a thick oxide matrix providing protection against breaking and oxidation. We use laser interference lithography, reactive ion etching and atomic layer deposition to create cm-long parallel nanochannels with characteristic dimensions as small as 40 nm. In a showcase study pulsed electrodeposition of iron is carried out creating rectangular shaped iron nanowires within the nanochannels. By design of the device, the grown wires are in contact with an integrated electrode system on both ends directly after the deposition. No further processing steps are required for electrical characterization, minimizing the risk of damage and oxidation. The developed nanowire measurement device allows for multi-probe resistance measurements and can easily be adopted for transistor applications. The guided, in-plane growth of electrodeposited nanowire arrays which are tunable in size and density paves the way for the incorporation of nanowires into a large variety of multifunctional devices., Comment: This is an author-created, un-copyedited version of an article accepted for publication/published in Nanotechnology. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at http://dx.doi.org/10.1088/0957-4484/25/50/504002