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QuTiP, the Quantum Toolbox in Python, has been at the forefront of open-source quantum software for the last ten years. It is used as a research, teaching, and industrial tool, and has been downloaded millions of times by users around the world. Here we introduce the latest developments in QuTiP v5, which are set to have a large impact on the future of QuTiP and enable it to be a modern, continuously developed and popular tool for another decade and more. We summarize the code design and fundamental data layer changes as well as efficiency improvements, new solvers, applications to quantum circuits with QuTiP-QIP, and new quantum control tools with QuTiP-QOC. Additional flexibility in the data layer underlying all "quantum objects" in QuTiP allows us to harness the power of state-of-the-art data formats and packages like JAX, CuPy, and more. We explain these new features with a series of both well-known and new examples. The code for these examples is available in a static form on GitHub and will be available also in a continuously updated and documented notebook form in the qutip-tutorials package., Comment: 70 pages, 25 figures

Accurately determining neutrino masses is a main objective of contemporary cosmology. Since massive neutrinos affect structure formation and evolution, probes of large scale structure are sensitive to the sum of their masses. In this work, we explore future constraints on $\sum m_\nu$ utilizing line-intensity mapping (LIM) as a promising emerging probe of the density of our Universe, focusing on the fine-structure [CII] line as an example, and compare these constraints with those derived from traditional galaxy surveys. Additionally, we perform a multi-tracer analysis using velocity tomography via the kinetic Sunyaev-Zeldovich and moving lens effects to reconstruct the three-dimensional velocity field. Our forecasts indicate that the next-generation AtLAST detector by itself can achieve $\sigma_{\Sigma m_\nu} \sim 50$ meV sensitivity. Velocity tomography will further improve these constraints by 4%. Incorporating forecasts for CMB-S4 and DESI-BAO in a comprehensive multi-tracer analysis, while setting a prior on the optical depth to reionization $\tau$ derived using 21-cm forecasted observations, to break degeneracies, we find that a $\gtrsim5\sigma$ detection of $\sum m_\nu\!\sim\! 60$ meV, under the normal hierarchy, is within reach with LIM. Even without a $\tau$ prior, our combined forecast reaches $\sigma_{\Sigma m_\nu} \!\sim\! 18$ meV., Comment: 19 pages, 8 figures

We present~\emph{KV-Tandem}, a modular architecture for building LSM-based storage engines on top of simple, non-ordered persistent key-value stores (KVSs). KV-Tandem enables advanced functionalities such as range queries and snapshot reads, while maintaining the native KVS performance for random reads and writes. Its modular design offers better performance trade-offs compared to previous KV-separation solutions, which struggle to decompose the monolithic LSM structure. Central to KV-Tandem is~\emph{LSM bypass} -- a novel algorithm that offers a fast path to basic operations while ensuring the correctness of advanced APIs. We implement KV-Tandem in \emph{XDP-Rocks}, a RocksDB-compatible storage engine that leverages the XDP KVS and incorporates practical design optimizations for real-world deployment. Through extensive microbenchmark and system-level comparisons, we demonstrate that XDP-Rocks achieves 3x to 4x performance improvements over RocksDB across various workloads. XDP-Rocks is already deployed in production, delivering significant operator cost savings consistent with these performance gains.

Spin defects in atomically thin two-dimensional (2D) materials such as hexagonal boron nitride (hBN) attract significant attention for their potential quantum applications. The layered host materials not only facilitate seamless integration with optoelectronic devices but also enable the formation of heterostructures with on-demand functionality. Furthermore, their atomic thickness renders them particularly suitable for sensing applications. However, the short coherence times of the spin defects in hBN limit them in quantum applications that require extended coherence time. One primary reason is that both boron and nitrogen atoms have non-zero nuclear spins. Here, we present another 2D material germanium disulfide ($\beta$-GeS$_2$) characterized by a wide bandgap and potential nuclear-spin-free lattice. This makes it as a promising host material for spin defects that possess long-coherence time. Our findings reveal the presence of more than two distinct types of spin defects in single-crystal $\beta$-GeS$_2$. Coherent control of one type defect has been successfully demonstrated at both 5 K and room temperature, and the coherence time $T_2$ can achieve tens of microseconds, 100-folds of that of negatively charged boron vacancy (V$_{\text{B}}^-$) in hBN, satisfying the minimal threshold required for metropolitan quantum networks--one of the important applications of spins. We entatively assign the observed optical signals come from substitution defects. Together with previous theoretical prediction, we believe the coherence time can be further improved with optimized lattice quality, indicating $\beta$-GeS$_2$ as a promising host material for long-coherence-time spins., Comment: 8 pages, 5 figures

Nanoelectrical and photonic integration of quantum optical components is crucial for scalable solid-state quantum technologies. Silicon carbide stands out as a material with mature quantum defects and a wide variety of applications in semiconductor industry. Here, we study the behaviour of single silicon vacancy (V2) colour centres in a metal-semiconductor (Au/Ti/4H-SiC) epitaxial wafer device, operating in a Schottky diode configuration. We explore the depletion of free carriers in the vicinity of the defect, as well as electrical tuning of the defect optical transition lines. By detecting single charge traps, we investigate their impact on V2 optical line width. Additionally, we investigate the charge-photon-dynamics of the V2 centre and find its dominating photon-ionisation processes characteristic rate and wavelength dependence. Finally, we probe the spin coherence properties of the V2 system in the junction and demonstrate several key protocols for quantum network applications. Our work shows the first demonstration of low temperature integration of a Schottky device with optical microstructures for quantum applications and paves the way towards fundamentally scalable and reproducible optical spin defect centres in solids.

The neutral divacancy and the negatively charged nitrogen-vacancy defects in 4H-silicon carbide (SiC) are two of the most prominent candidates for functioning as room-temperature quantum bits (qubits) with telecommunication-wavelength emission. Nonetheless, the pivotal role of electron-phonon coupling in the spinpolarization loop is still unrevealed. In this work, we theoretically investigate the microscopic magneto-optical properties and spin-dependent optical loops utilizing the first-principles calculations. First, we quantitatively demonstrate the electronic level structure, assisted by symmetry analysis. Moreover, the fine interactions, including spin-orbit coupling and spin-spin interaction, are fully characterized to provide versatile qubit functional parameters. Subsequently, we explore the electron-phonon coupling, encompassing dynamics- and pseudo-Jahn--Teller effects in the intersystem crossing transition. In addition, we analyze the photoluminescence PL lifetime based on the major transition rates in the optical spinpolarization loop. We compare two promising qubits with similar electronic properties, but their respective rates differ substantially. Finally, we detail the threshold of ODMR contrast for further optimization of the qubit operation. This work not only reveals the mechanism underlying the optical spinpolarization but also proposes productive avenues for optimizing quantum information processing tasks based on the ODMR protocol., Comment: 17 pages, 7 figures, 7 tables

Silicon color centers (SiCCs) have recently emerged as potential building blocks for light emitters in Si photonics, quantum emitters with spin storage capabilities, and Si-based quantum repeaters. We have recently developed a non-invasive method to engineer carbon-related SiCCs confined to ultra-thin nanolayers within a pristine crystalline environment, which is of utmost importance for the photostability of SiCCs. Here, we demonstrate embedding these C-doping-based SiCCs into the only 9 nm wide intrinsic region of a p-i-n diode using the epitaxial self-assembly of color centers. We report electrically-pumped light emission with an exponential increase in the intensity as a function of the driving current until saturation. We associate this property with the shift of quasi-Fermi-level position upon electrical driving, which simultaneously improves the spectral homogeneity of the engineered SiCCs. Our study demonstrates the electrical control and driving of near-infrared emitters in high-quality silicon diodes, an essential milestone for advancing classical and quantum optoelectronics., Comment: Main text: 11 pages + 3 Figures & Supplementary material: 1 page and 1 Figure

Optically addressable defect qubits in wide band gap materials are favorable candidates for room temperature quantum information processing. The two-dimensional (2D) hexagonal boron nitride (hBN) is an attractive solid state platform with a great potential for hosting bright quantum emitters with quantum memories with leveraging the potential of 2D materials for realizing scalable preparation of defect qubits. Although, room temperature bright defect qubits have been recently reported in hBN but their microscopic origin, the nature of the optical transition as well as the optically detected magnetic resonance (ODMR) have been remained elusive. Here we connect the variance in the optical spectra, optical lifetimes and spectral stability of quantum emitters to donor-acceptor pairs (DAP) in hBN by means of ab initio calculations. We find that DAPs can exhibit ODMR signal for the acceptor counterpart of the defect pair with S=1/2 ground state at non-zero magnetic fields depending on the donor partner. The donor-acceptor pair model and its transition mechanisms provide a recipe towards defect qubit identification and performance optimization in hBN for quantum applications.

In recent years, the negatively charged group-IV--vacancy defects in diamond, labeled as G4V($-$) or G4V centers, have received a great attention in quantum information processing. In this study, we investigate the magneto-optical properties of the G4V centers under high compressive hydrostatic pressures up to 180~GPa. The spin-orbit splitting for the electronic ground and excited states and the hyperfine tensors are calculated by means of plane wave supercell density functional theory as unique fingerprints of these defects. To this end, we developed a theory for calculating the hyperfine tensors when the electronic states are subject to Jahn--Teller effect. We find that the zero-phonon-line energy increases with adding hydrostatic pressures where the coupling strength increases from SiV($-$) to PbV($-$). On the other hand, the calculated photoionization threshold energies indicate that the operation of PbV($-$) based quantum sensor is limited up to 30~GPa of hydrostatic pressure whereas SnV($-$), GeV($-$) and SiV($-$) remain photostable up to 180~GPa of hydrostatic pressure., Comment: 16 pages, 7 figures, 7 tables

Defects in semiconductors have in recent years been revealed to have interesting properties in the venture towards quantum technologies. In this regard, silicon carbide has shown great promise as a host for quantum defects. In particular, the ultra-bright AB photoluminescence lines in 4H-SiC are observable at room temperature and have been proposed as a single-photon quantum emitter. These lines have been previously studied and assigned to the carbon antisite-vacancy pair (CAV). In this paper, we report on new measurements of the AB-lines' temperature dependence, and carry out an in-depth computational study on the optical properties of the CAV defect. We find that the CAV defect has the potential to exhibit several different zero-phonon luminescences with emissions in the near-infrared telecom band, in its neutral and positive charge states. However, our measurements show that the AB-lines only consist of three non-thermally activated lines instead of the previously reported four lines, meanwhile our calculations on the CAV defect are unable to find optical transitions in full agreement with the AB-line assignment. In the light of our results, the identification of the AB-lines and the associated room temperature emission require further study., Comment: 14 main pages, 7 main figures, 2 tables, one appendix with software details, and supplementary with 2 figures and 2 tables

We investigate the effect of a waveguide on an emitter spontaneous emission in its vicinity. The impact of various possible orientations of an emitter with respect to the waveguide surface is studied through simulations and compared with experimental demonstration. Quantum emitters are dip coated on waveguides and Purcell enhancement and a decrease in the lifetime of the emitter are observed. This study serves as a proof of concept for the Purcell effect offered by waveguides and helps ineffectively estimate the efficiency of waveguide-based evanescent sensors and quantum photonic applications, Comment: 5 pages

Coherent light sources emitting in the terahertz range are highly sought after for fundamental research and applications. THz lasers rely on achieving population inversion. We demonstrate the generation of THz radiation using nitrogen-vacancy (NV) centers in a diamond single crystal. Population inversion is achieved through the Zeeman splitting of the $S=1$ state in $15\ \text{T}$, resulting in a splitting of $0.42\ \text{THz}$, where the middle $S_z=0$ sublevel is selectively pumped by visible light. To detect the THz radiation, we utilize a phase-sensitive THz setup, optimized for electron spin resonance measurements (ESR). We determine the spin-lattice relaxation time up to $15\ \text{T}$ using the light-induced ESR measurement, which shows the dominance of phonon-mediated relaxation and the high efficacy of the population inversion. The THz radiation is tunable by the magnetic field, thus these findings may lead to the next generation of tunable coherent THz sources., Comment: 45 pages, 14 figures

We apply density functional theory to investigate interactions between electronic and vibrational states in crystal defects with multi-mode dynamical Jahn-Teller (JT) systems. Our focus is on transitions between orbital singlet and degenerate orbital doublet characterized by $E \otimes (e \oplus e \oplus \cdots)$ JT coupling, which frequently occurs in crystal defects that are investigated for applications in quantum information science. We utilize a recently developed methodology to model the photoluminescence (PL) spectrum of the negatively charged split nickel-vacancy center (NiV$^{-}$) in diamond, where JT-active modes significantly influence electron-phonon interactions. Our results validate the effectiveness of the methodology in accurately reproducing the observed 1.4 eV PL lineshape. The strong agreement between our theoretical predictions and experimental observations reinforces the identification of the 1.4 eV PL center with the NiV$^{-}$ complex. This study highlights the critical role of JT-active modes in affecting optical lineshapes and demonstrates the power of advanced techniques for modeling optical properties in complex systems with multiple JT-active frequencies., Comment: 13 pages, 7 figures

Defects in crystals can have a transformative effect on the properties and functionalities of solid-state systems. Dopants in semiconductors are core components in electronic and optoelectronic devices. The control of single color centers is at the basis of advanced applications for quantum technologies. Unintentional defects can also be detrimental to the crystalline structure and hinder the development of novel materials. Whatever the research perspective, the identification of defects is a key but complicated, and often long-standing issue. Here, we present a general methodology to identify point defects by combining isotope substitution and polytype control, with a systematic comparison between experiments and first-principles calculations. We apply this methodology to hexagonal boron nitride (hBN) and its ubiquitous color center emitting in the ultraviolet spectral range. From isotopic purification of the host hBN matrix, a local vibrational mode of the defect is uncovered, and isotope-selective carbon doping proves that this mode belongs to a carbon-based center. Then, by varying the stacking sequence of the host hBN matrix, we unveil different optical responses to hydrostatic pressure for the non-equivalent configurations of this ultraviolet color center. We conclude that this defect is a carbon dimer in the honeycomb lattice of hBN. Our results show that tuning the stacking sequence in different polytypes of a given crystal provides unique fingerprints contributing to the identification of defects in 2D materials.

Boron nitride exhibits diverse crystal structures, predominantly a layered arrangement with strong intraplanar covalent bonds and weak interplanar van der Waals bonds. While commonly referred to as hexagonal BN (hBN), the sp$^2$-bonded BN atomic planes can also arrange in other configurations like Bernal (bBN) or rhombohedral (rBN) stacking orders. Variations in the orientation and translation of successive atomic layers lead to changes in crystal symmetry, potentially resulting in piezoelectric, pyroelectric or ferroelectric effects. However, distinguishing between different polytypes using conventional methods like X-ray diffraction or Raman spectroscopy presents a significant challenge. In this work, we demonstrate that the optical response of the 4.1 eV defect can serve as an indicator of the polytype. To this end, we study BN samples grown by metalorganic vapor phase epitaxy (MOVPE), which contain different polytypes. The identification of the polytypes was achieved by X-ray diffraction and transmission electron microscopy. Photoluminescence and cathodoluminescence measurements with a high spatial resolution allowed for the deconvolution of the signal into two components from which we can extract a zero-phonon line (ZPL) at 4.096 eV (302.6 nm) for hBN and 4.143 eV (299.2 nm) for rBN. We performed calculations that enable us to identify the defect as a carbon dimer CBCN (C2) and show that the ZPL shift reflects differences in the crystal environment for different polytypes. Furthermore, we demonstrate that different polytypic composition ratios of hBN and rBN can be achieved by MOVPE, which could pave the way for future applications in large-area van der Waals heterostructures.

We consider the motion of a circularly-moving perturber in a self-gravitating, collisional system with spherically symmetric density profile. We concentrate on the singular isothermal sphere which, despite its pathological features, admits a simple polarization function in linear response theory. This allows us to solve for the acoustic wake trailing the perturber and the resulting dynamical friction, in the limit where the self-gravity of the response can be ignored. In steady-state and for subsonic velocities $v_p

Spin-to-photon interfaces from defects in silicon hold great promise towards realizing quantum repeaters with the combination of advanced semiconductor and photonics technologies. Recently, controlled creation and erasure of simple carbon interstitial defects have been successfully realised in silicon. This defect has a stable structure near room temperature and emits in the wave-length where the signal loss is minimal in optical fibres used in communication technologies. Our in-depth theoretical characterization confirms the assignment of the observed emission to the neutral charge state of this defect. We find that the emission is due to the recombination of a bound exciton. We also discovered a metastable triplet state that could be applied as a quantum memory. Based on the analysis of the electronic structure of the defect and its similarities to a known optically detected magnetic resonance centre in silicon, we propose that a carbon interstitial can act as a quantum bit and may realize a spin-to-photon interface in CMOS-compatible platforms., Comment: 2 figures and 2 tables

Among the wealth of single fluorescent defects recently detected in silicon, the G center catches interest for its telecom single-photon emission that could be coupled to a metastable electron spin triplet. The G center is a unique defect where the standard Born-Oppenheimer approximation breaks down as one of its atoms can move between 6 lattice sites under optical excitation. The impact of this atomic reconfiguration on the photoluminescence properties of G centers is still largely unknown, especially in silicon-on-insulator (SOI) samples. Here, we investigate the displacement of the center-of-mass of the G center in silicon. We show that single G defects in SOI exhibit a multipolar emission and zero-phonon line fine structures with splittings up to $\sim1$ meV, both indicating a motion of the defect central atom over time. Combining polarization and spectral analysis at the single-photon level, we evidence that the reconfiguration dynamics are drastically different from the one of the unperturbed G center in bulk silicon. The SOI structure freezes the delocalization of the G defect center-of-mass and as a result, enables to isolate linearly polarized optical lines. Under above-bandgap optical excitation, the central atom of G centers in SOI behaves as if it were in a 6-slot roulette wheel, randomly alternating between localized crystal sites at each optical cycle. Comparative measurements in a bulk silicon sample and ab initio calculations highlight that strain is likely the dominant perturbation impacting the G center geometry. These results shed light on the importance of the atomic reconfiguration dynamics to understand and control the photoluminescence properties of the G center in silicon. More generally, these findings emphasize the impact of strain fluctuations inherent to SOI wafers for future quantum integrated photonics applications based on color centers in silicon.

Quantum communication promises unprecedented communication capabilities enabled by the transmission of quantum states of light. However, current implementations face severe limitations in communication distance due to photon loss. Silicon carbide (SiC) defects have emerged as a promising quantum device platform, offering strong optical transitions, long spin coherence lifetimes and the opportunity for integration with semiconductor devices. Some defects with optical transitions in the telecom range have been identified, allowing to interface with fiber networks without the need for wavelength conversion. These unique properties make SiC an attractive platform for the implementation of quantum nodes for quantum communication networks. We provide an overview of the most prominent defects in SiC and their implementation in spin-photon interfaces. Furthermore, we model a memory-enhanced quantum communication protocol in order to extract the parameters required to surpass a direct point-to-point link performance. Based on these insights, we summarize the key steps required towards the deployment of SiC devices in large-scale quantum communication networks., Comment: 20 pages, 8 figures

Optically accessible solid state defect spins are primary platform for quantum information processing where tight control of the electron spin and ancilla nuclear spins is pivotal for the operation. We demonstrate on the exemplary nitrogen-vacancy (NV) color center in diamond by means of a combined group theory and density functional theory study that spin-phonon relaxation rate of the nitrogen nuclear spin is with several orders of magnitude enhanced by the strong electron-phonon coupling in the optical excited state of the defect. The mechanism is common to other solid state defect spins sharing similar optical excited states with that of the NV center., Comment: 6 pages, 3 figures, 2 tables

Silicon-based color-centers (SiCCs) have recently emerged as quantum-light sources that can be combined with telecom-range Si Photonics platforms. Unfortunately, using current SiCC fabrication, deterministic control over the vertical emitter position is impossible due to ion-implantation's stochastic nature. To overcome this bottleneck towards high-yield integration, we demonstrate a radically innovative creation method for various SiCCs, solely relying on epitaxial growth of Si and C-doped Si at atypically-low temperatures in a ultra-clean growth environment. These telecom emitters can be confined within sub-1nm thick layers embedded at arbitrary vertical positions within a highly crystalline Si matrix. Tuning growth conditions and doping, different SiCC types, e.g., W-centers, T-centers, G-centers, or derivatives like G'-centers can be created, which are particularly promising as Si-based single-photon sources and spin-photon interfaces. The zero-phonon emission from G'-centers can be conveniently tuned by the C-concentration, leading to a systematic wavelength shift and linewidth narrowing towards low emitter densities.

We study the fluorescence of nanodiamond ensembles as a function of static external magnetic field and observe characteristic dip features close to the zero field with potential for magnetometry applications. We analyze the dependence of the features width and contrast of the feature on the size of the diamond (in the range 30 nm to 3 um) and on the strength of a bias magnetic field applied transversely to the field being scanned. We also perform optically detected magnetic resonance (ODMR) measurements to quantify the strain splitting of the zero-field ODMR resonance across various nanodiamond sizes and compare it with the width and contrast measurements of the zero-field fluorescence features for both nanodiamonds and bulk samples. The observed properties provide compelling evidence of cross-relaxation effects in the NV system occurring close to zero magnetic fields. Finally, the potential of this technique for use in practical magnetometry is discussed., Comment: 9 pages, 5 figures

Emitters in high refractive index materials like 4H-SiC suffer from reduced detection of photons because of losses caused by total internal reflection. Thus, integration into efficient nanophotonic structures which couple the emission of photons to a well defined waveguide mode can significantly enhance the photon detection efficiency. In addition, interfacing this waveguide to a classical fiber network is of similar importance to detect the photons and perform experiments. Here, we show a waveguide fiber interface in SiC. By careful measurements we determine efficiencies exceeding 93 % for the transfer of photons from SiC nanobeams to fibers. We use this interface to create a bright single photon source based on waveguide integrated V2 defects in 4H-SiC and achieve an overall photon count rate of 181 kilo-counts per second. We observe and quantify the strain induced shift of the ground state spin states and demonstrate coherent control of the electron spin with a coherence time of T2=42.5 $\rm\mu$s.

The crystallographic restriction theorem constrains two-dimensional nematicity to display either Ising ($Z_{2}$) or three-state-Potts ($Z_{3}$) critical behaviors, both of which are dominated by amplitude fluctuations. Here, we use group theory and microscopic modeling to show that this constraint is circumvented in a $30^{\circ}$-twisted hexagonal bilayer due to its emergent quasicrystalline symmetries. We find a critical phase dominated by phase fluctuations of a $Z_{6}$ nematic order parameter and bounded by two Berezinskii-Kosterlitz-Thouless (BKT) transitions, which displays only quasi-long-range nematic order. The electronic spectrum in the critical phase displays a thermal pseudogap-like behavior, whose properties depend on the anomalous critical exponent. We also show that an out-of-plane magnetic field induces nematic phase fluctuations that suppress the two BKT transitions via a mechanism analogous to the Hall viscoelastic response of the lattice, giving rise to a putative nematic quantum critical point with emergent continuous symmetry. Finally, we demonstrate that even in the case of an untwisted bilayer, a critical phase emerges when the nematic order parameter changes sign between the two layers, establishing an odd-parity nematic state., Comment: Manuscript (7 pages, 3 figures) plus Supplementary Material (5 pages, 1 figure)

The last decade has seen an explosive growth in the use of color centers for metrology applications, the paradigm example arguably being the nitrogen-vacancy (NV) center in diamond. Here, we focus on the regime of cryogenic temperatures and examine the impact of spin-selective, narrow-band laser excitation on NV readout. Specifically, we demonstrate a more than four-fold improvement in sensitivity compared to that possible with non-resonant (green) illumination, largely due to a boost in readout contrast and integrated photon count. We also leverage nuclear spin relaxation under resonant excitation to polarize the 14N host, which we then prove beneficial for spin magnetometry. These results open opportunities in the application of NV sensing to the investigation of condensed matter systems, particularly those exhibiting superconducting, magnetic, or topological phases selectively present at low temperatures.

Transition metal (TM) defects in silicon carbide (SiC) are a promising platform for applications in quantum technology as some of these defects, e.g. vanadium (V), allow for optical emission in one of the telecom bands. For other defects it was shown that straining the crystal can lead to beneficial effects regarding the emission properties. Motivated by this, we theoretically study the main effects of strain on the electronic level structure and optical electric-dipole transitions of the V defect in SiC. In particular we show how strain can be used to engineer the g-tensor, electronic selection rules, and the hyperfine interaction. Based on these insights we discuss optical Lambda systems and a path forward to initializing the quantum state of strained TM defects in SiC., Comment: 14 pages, 6 figures