We present a systematic density-functional theory study of the effects of strain on the structural and electronic properties in vanadium dioxide (${\mathrm{VO}}_2$), with particular emphasis on its effect on the relative stability of the metallic rutile and the insulating monoclinic M1 phases. We consider various strain conditions that can be related to epitaxial strain present in ${\mathrm{VO}}_2$ films grown on different lattice planes. Our calculations confirm the dominant role of $c$-axis strain, i.e., along the direction of the V-V dimerization in the M1 phase. Our analysis suggests that this effect stems primarily from the weakening of the lattice stiffness, with the hopping amplitude along the $c$ axis playing a minor role. We also confirm that, in strain scenarios that deform the basal plane, the $c$-axis strain still has a dominant effect on the phase stability.

We introduce a general-purpose numerical algorithm for simulating single-shot realizations of partially coherent electromagnetic radiation, based solely on its known statistical correlations. The algorithm operates by shaping complex Gaussian noise using an arbitrary Wigner distribution function, without assuming factorability in the corresponding Fourier domains. This enables the generation of field realizations for a broad class of sources, including thermal light, synchrotron radiation, and, notably, free-electron laser (FEL) pulses in the linear regime. Compared to Monte Carlo particle-based methods, our approach circumvents the need for detailed modeling of emission mechanisms and offers a substantially more efficient means of generating large ensembles of stochastic fields. As a case study, we simulate FEL pulses with spectrally nontrivial coherence properties (e.g., frequency chirps), thereby demonstrating the versatility of the algorithm. We analytically confirm that ensemble-averaged intensity and correlation properties are correctly reproduced. The algorithm provides a computationally lightweight framework for modeling partially coherent radiation with an arbitrary Wigner distribution function and can serve both as a theoretical tool and as a practical simulation resource for users’ experiments at FELs.

Recent ${}^{91,92,93}\mathrm{Tc}$ activation for deuterons incident on ${}^{\text{nat}}\mathrm{Mo}$ has become a challenge for the nuclear level density (NLD) angular-momentum dependence. Actually, replacement of the moment of inertia rigid-body value $I_r$ by half of it, within a given NLD parameter set, demands a change of the rest of NLD parameters significantly beyond their fitted limits. The corresponding uncertainty of calculated cross sections versus the NLD parameter accuracy is also higher, while use of either the same or distinct compound-nucleus and preequilibrium emission spin distributions becomes significant at higher incident energies. Nevertheless, the current way to describe experimental isomeric cross sections by using at most half of $I_r$ values provides agreement of the measured and calculated data at the price of less and less correct NLDs. The moment of inertia relevance for the NLD correctness also emphasizes the value of a direct method to endorse it. Further measurements of average resonance spacings of $s$-wave neutrons and protons, corresponding to different spins of the same nucleus, are therefore highly demanded.

We report a random singlet ground state in the $S=\frac{1}{2}$ Heisenberg pyrochlore antiferromagnet ${\mathrm{NaCdCu}}_2{\mathrm{F}}_7$. Cationic ${\mathrm{Na}}^{+}$/${\mathrm{Cd}}^{2+}$ disorder on the pyrochlore $A$ site generates a broad distribution of ${\mathrm{Cu}}^{2+}\text{–}{\mathrm{F}}^{-}\text{–}{\mathrm{Cu}}^{2+}$ exchange couplings, introducing intrinsic magnetic bond disorder. Despite strong antiferromagnetic interactions $({\theta }_{\mathrm{CW}}=-72\mathrm{K})$, no magnetic order or global spin freezing is observed in dc and ac susceptibility, specific heat, or ${}^{23}\mathrm{Na}$ nuclear magnetic resonance to 120 mK, with muon spin relaxation experiments confirming persistent spin dynamics to 58 mK. $T$-linear specific heat, a Curie-like susceptibility tail, and power-law scaling with data collapse in $\chi \left(T\right)$, $M\left(H\right)$, $C_{\mathrm{mag}}/T$, ${}^{23}\mathrm{Na}$ $(1/T_{1}T)$, and the muon spin polarization $P\left(t\right)$ reveal a disorder-driven network of random singlets and orphan spins. Scaling across multiple bulk and local probes is consistent with a broad distribution of exchange energies, $P\left[\mathcal{J}\right]\sim {\mathcal{J}}^{-\alpha }$. This behavior contrasts with previously studied $\mathrm{Na}{A}^{″}{B}_2{\mathrm{F}}_7$ pyrochlore fluorides, where magnetic bond disorder precipitates spin-glass freezing, underscoring the crucial role of strong $S=\frac{1}{2}$ quantum fluctuations in ${\mathrm{NaCdCu}}_2{\mathrm{F}}_7$.

In this work, we compare two schemes for generating arbitrary qudit graph states using spin qudits in silicon. The first scheme proposes the creation of qudit linear graph states from a single emitter—a silicon spin qudit. By employing fusion—a destructive and nondeterministic measurement technique—these linear graphs can then be combined to form more complex resource states (multiphoton entangled states), such as ring or ladder structures, which are used to carry out the computation. The second scheme employs two spin qudits. Instead of relying on fusion, the two emitters are directly coupled via controlled-Z gate to generate the same resource states, thereby eliminating the need for fusion. We compare the two schemes in terms of their ability to produce equivalent resource states and discuss their respective advantages and limitations for building scalable architectures.

In this study, we estimate parameter values in stochastic oscillatory systems by developing a cost function. Our cost function incorporates power spectral density, analytic signal, and position crossings, each weighted to capture distinct characteristics of the oscillatory system such as amplitude, frequency, and shape. By minimizing this cost function, we estimate parameter values in a stochastic biophysical model for auditory mechanics. Furthermore, we develop a procedure to align the phases of two time series, allowing us to compare stochastic phase evolution of two time series. As a broader application of our procedure, we establish a framework for fitting stochastic oscillatory systems and comparing stochastic time series in other systems.

In this study, we have investigated the dynamic behavior of colloidal rods within their isotropic phase, employing multispeckle ultrasmall-angle x-ray photon correlation spectroscopy (UA-XPCS) focusing on the understanding of the interplay between structure and dynamics at the nearest-neighbor length scale. While prior research has primarily focused on the structural aspects, our work addresses the lesser-explored realm of dynamic behavior. Through ultrasmall-angle x-ray scattering and UA-XPCS, we have measured the key parameters, such as the effective structure factor $\left(S_{\mathrm{eff}}\left(q\right)\right)$ and the effective diffusion coefficient $\left(D_{\mathrm{eff}}\left(q\right)\right)$. A significant finding is the scaling relationship between $D_{\mathrm{eff}}\left(q\right)$ and $1/S_{\mathrm{eff}}\left(q\right)$, implying a connection between particle diffusion and structural arrangement. Additionally, the estimated scaling parameter (effective amplitude function, $A_{\mathrm{eff}}\left(q\right)$), which can be directly related to the effective hydrodynamic function $\left(H_{\mathrm{eff}}\left(q\right)\right)$ shows a similar dependence on wave vector $\left(q\right)$ as $S_{\mathrm{eff}}\left(q\right)$. Furthermore, our study precisely determines the volume fraction at which the arrest transition occurs. These findings shed light on the influence of structures on the intricate dynamics of anisotropic colloidal systems.

Understanding the light-driven responses of chiral molecular systems at a fundamental level—electronically—via photoelectron circular dichroism (PECD) provides a pathway to unlock the intricate mechanisms governing the functionality of chiral molecules. The specific role of oxygen atoms in chiral compounds remains under-explored, despite their pivotal role in numerous functional systems. We report a joint experimental and theoretical study of the PECD of O $1s$ electrons from the randomly oriented chiral molecule fenchone $\left({\mathrm{C}}_{10}{\mathrm{H}}_{16}\mathrm{O}\right)$ in the gas phase across a kinetic-energy range from 3 to 15 eV. We observe a substantial forward-backward asymmetry in the electron emission patterns across several eV, which is well reproduced by relaxed-core Hartree-Fock calculations. Our results provide a site-specific perspective on the chiral sensitivity of an oxygen atom directly connected to the stereocenter of a chiral molecule.

Quantum state tomography (QST) is a crucial tool for characterizing quantum states. However, performing full QST becomes impractical for reconstructing multiqubit density matrices since datasets and computational costs grow exponentially with qubit number. To bypass full QST, we introduce selective and efficient QST (SEEQST)—a method that enables efficient estimation of multiple selected elements of an arbitrary multiqubit density matrix. We show that any $N$-qubit density matrix can be partitioned into $2^N$ disjoint subsets, each containing $2^N$ elements. With SEEQST, any such subset can be accurately estimated from just two experiments with only single-qubit measurements. The complexity for estimating any subset remains constant regardless of Hilbert-space dimension, so, if desired, SEEQST can reconstruct the full density matrix, using $2^{N+1}-1$ experiments, where standard methods would use $3^N$ experiments. We provide a circuit decomposition for the SEEQST experiments, demonstrating that their maximum circuit depth scales logarithmically with $N$ assuming all-to-all connectivity.

Pulse-based quantum machine learning (QML) has emerged as a promising paradigm in quantum artificial intelligence due to its exceptional hardware efficiency. For real-world applications, it is crucial that pulse-based models are both expressive and trainable. Previous studies have shown that pulse-based models operating under dynamical symmetry can be effectively trained, benefiting from a favorable loss landscape devoid of barren plateaus. However, this approach may lead to uncontrollability, which can limit expressivity if the model is poorly designed. In this paper, we explore the conditions necessary for pulse-based QML models to maintain both expressivity and trainability. We present a key requirement involving the system's initial state, the measurement observable, and the underlying dynamical symmetry Lie algebra, supported by numerical simulations. Our results provide a comprehensive framework for the design of practical pulse-based QML models that strike a good balance between expressivity and trainability.

Within a numerically exact simulation technique, the dissipative dynamics of a two-qubit architecture is considered in which each qubit couples to its individual noise source (reservoir). The goal is to reveal the role of subtle qubit-reservoir correlations including non-Markovian processes as a prerequisite to guide further improvements of quantum computing devices. This paper addresses the following three topics. First, we examine the validity of the rotating wave approximation imposed previously on the qubit-reservoir coupling with respect to the disentanglement dynamics. Second, generation of the entanglement as well as destruction are analyzed by monitoring the reduced dynamics during and after application of a $\sqrt{{\mathrm{iSWAP}}^{†}}$ gate, also focusing on memory effects caused by reservoirs. Finally, the performance of a $\text{Hadamard}+\mathrm{CNOT}$ sequence is analyzed for different gate decomposition schemes. In all three cases, various types of noise sources and qubit parameters are considered.

We present a theoretical study of quantum nonreciprocity induced via Dzyaloshinskii-Moriya interaction (DMI) in an otherwise achiral, waveguide quantum electrodynamics. Using the full quantum master equation and input-output formalism for two two-level systems coupled to a one-dimensional waveguide and driven by a coherent field, we show that an engineered DMI enables strong nonreciprocity in an otherwise reciprocal system, with tunable behavior governed by driving strength, detuning, and phase of the DMI. We not only demonstrate nonreciprocal transmission but also demonstrate nonreciprocal quantum entanglement and photon bunching. The system can end up in a pure state as certain decohering channels do not participate. The pure state leads to power-independent perfect transparency. Conditions are derived and depend on the propagation phase, the relative detuning of the two qubits, and the exchange interaction. At these pure-state points, the steady-state entanglement is reciprocal and admits a closed-form expression; away from them, phase control generates strong entanglement nonreciprocity. The DMI also reshapes photon statistics, redistributing two-photon correlations and shifting superbunching from transmission (no DMI) to reflection at finite DMI. These results establish DMIs as a versatile resource for engineering nonreciprocity, transparency, entanglement, and photon correlations in waveguide QED, enabling isolators, routers, and superbunching light sources without requiring chiral waveguides.

Data for the shock equation of state of helium are obtained up to $360\mathrm{G}\mathrm{Pa}$, nearly doubling the pressure of previous experimental measurements. The helium samples are first precompressed to 0.35–0.42 $\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$ in a diamond anvil cell prior to laser-driven shock compression at the Omega Laser Facility. Time-resolved Doppler velocimetry and pyrometry reveal significant reflectivity and greater compressibility compared to the predictions of existing broad-range tabular equation of state models, which may be caused by the onset of ionization. These experimental observations, however, are largely captured with molecular-dynamics simulations based on density functional theory, affirming the ability of first-principles techniques to capture complex physics, while enabling critical insight for the behavior of warm dense helium in Jovian interiors and white dwarf atmospheres.

The axion is a particularly well-motivated candidate for the dark matter comprising most of the mass of our visible Universe, leading to worldwide experimental and observational efforts toward its discovery. As is well known, resonant cavities are a primary tool in this search, where they are used to enhance the conversion rate of axions into photons in a background electromagnetic field. What is perhaps less well appreciated is that resonant cavities can also enhance the rate at which axions decay to two photons, a manifestation of the Purcell effect. We examine this possibility and show that it offers a previously unexplored method to search the axion parameter space in a way that is competitive and complementary to other approaches, with the capability to probe a well-motivated region for QCD axion dark matter. Furthermore, we establish that this method can be implemented via preexisting axion heterodyne detection schemes, enabling such experiments to perform these searches with minimal modification.

Ensemble inequivalence, where a system's thermodynamic properties depend on the statistical ensemble used to describe it, is a well-established phenomenon in classical systems with long-range interactions. Here, we present a detailed analysis of a quantum ferromagnet spin model that exhibits this same behavior. We find that the microcanonical and canonical phase diagrams are identical at zero temperature ($T=0$), but differ significantly at finite temperatures. Our results highlight a breakdown of ensemble equivalence in this quantum long-range model, contrasting with what is observed for short range. These findings have important implications for the study and manipulation of long-range interacting quantum platforms, such as those used in atomic, molecular, and optical physics.

We present a finite-size security proof for generic quantum key distribution protocols against independent and identically distributed collective attacks and extend it to coherent attacks using the postselection technique. This work introduces two significant improvements over previous results. First, we achieve tighter finite-size key rates by employing refined concentration inequalities in the acceptance testing phase. Second, we improve second-order correction terms in the key rate expression, by reducing them to scale with the number of sifted rounds rather than the total number of protocol rounds. We apply these advancements to compute finite-size key rates for a qubit and decoy-state BB84 protocol, accommodating arbitrary protocol parameters. Finally, we extend our finite-size security proof to coherent attacks and variable-length protocols and present our results for the decoy-state 4-6 protocol incorporating imperfections such as unequal intensity settings.

Ferrotoroidic materials, which break both time- and space-reversal symmetries to enhance magnetoelectric responses, are of great interest for applications. ${\mathrm{Ba}}_6{\mathrm{Cr}}_2{\mathrm{S}}_{10}$, a recently synthesized quasi-one-dimensional ferrotoroidic candidate, combines polarization, magnetization, and toroidal moment, yet its electronic band structure, magnetic evolution, and phase transitions remain incompletely understood to date. Here, we investigate ${\mathrm{Ba}}_6{\mathrm{Cr}}_2{\mathrm{S}}_{10}$, using electrical transport, direct current magnetization, alternating current susceptibility, specific heat, and muon spin rotation ($\mu \mathrm{SR}$) measurements, along with various first-principles calculations. Electrical transport measurements and density functional theory confirm a narrow band gap ($\sim 0.707\mathrm{eV}$, experimentally; $\sim 0.680\mathrm{eV}$, theoretically). $\mu \mathrm{SR}$ measurements reveal that static magnetism dominates down to 2 K, with an ordered volume fraction of $\sim 89\%$ at this temperature. Two distinct transitions are identified: one around the Néel temperature $[T_{\mathrm{N}}=(8\pm 1)\mathrm{K}]$ and another corresponding to a phase transition near 30 K, as consistently indicated by our complementary experimental techniques. The absence of dynamic critical behaviors and the lack of a $\lambda$-type specific heat anomaly are consistent with a first-order-like transition at $T_{\mathrm{N}}$. Based on various complementary experimental results and theoretical calculations, we propose a hypothesis that the $\sim 30\mathrm{K}$ broad feature arises from the complex interplay between crystal structure distortion and magnetic coupling, which is potentially directly linked to ferrotoroidicity. As the first study integrating $\mu \mathrm{SR}$ with complementary experiments and first-principles calculations to explore ferrotoroidicity, this work provides key insights into the underlying physics of ${\mathrm{Ba}}_6{\mathrm{Cr}}_2{\mathrm{S}}_{10}$ and related compounds in this area.

Dimension in physical systems determines universal properties at criticality. Yet, the impact of structural perturbations on dimensionality remains largely unexplored. Here, we characterize the attraction basins of structural fixed points in scale-invariant networks from a renormalization group perspective, demonstrating that basin stability connects to a structural phase transition. This topology-dependent effect, which we term geometric criticality, triggers a geometric breakdown hitherto unknown, which induces nontrivial fractal dimensions and unveils hidden Laplacian renormalization group flows toward unstable structural fixed points. Our systematic study of how networks and lattices respond to disorder paves the way for future analysis of nonergodic behavior induced by quenched disorder.

Interfacing light with quantum systems is an integral part of quantum technology, with the most essential building block being single-photon emitters. Although various platforms exist, each with its individual strengths, molecular emitters boast a unique advantage—namely, the flexibility to tailor their design to fit the requirements of a specific task. However, the characteristics of the vast space of possible molecular configurations are challenging to understand and explore. Here, we present a theoretical and computational framework to initiate exploration of this vast potential by integrating database analysis with microscopic predictions. Using a model system of dibenzoterrylene in an anthracene host as benchmark, our approach identifies promising new candidates, among them a chiral molecular emitter. Future extensions of our approach integrated with machine learning routines hold the promise of ultimately unlocking the full potential of molecular quantum light-matter interfaces.