Characterizing the ultimate rates of entanglement distribution is essential for both foundational research and the practical deployment of quantum technologies. To investigate these limits, we introduce an erasure-Pauli channel model describing the distribution of polarization entanglement in optical fiber. For this channel, we derive bounds on the rates of entanglement distribution and related quantum resources under optimal local operations and two-way classical communication (two-way assisted capacities). This framework allows us to determine the optimal repeaterless performance achievable over realistic optical fibers affected by polarization mode dispersion, thereby providing a rigorous benchmark for long-distance polarization-based quantum communication. Finally, we show that both our model and capacity bounds remain robust under the inclusion of detector dark counts.

We explore the interplay between security and key rate of a quantum key distribution (QKD) protocol. This fundamental trade-off is investigated both at the link layer, i.e., for a QKD protocol implemented on a channel, and at the network layer, i.e., within the context of a multiuser multihop communication network. In the former scenario, we consider both discrete- and continuous-variable protocols in the composable finite-size framework, showing how their epsilon security restricts the key rate achievable on a single fiber link under various assumptions for the process of parameter estimation. In the latter scenario, we analyze the impact of composable security in the network setting, studying the interplay between end-to-end (ETE) epsilon security and ETE key rate for arbitrarily chosen remote end points. Starting from basic topologies to more complex ones, such as perturbed lattices, we analyze this trade-off under different types of routing strategies. In multipath routing, a greater number of paths can improve key rates, but this enhancement is accompanied by a reduction in the ETE security level. In the setting of single-path routing, we introduce a modified Dijkstra algorithm, which we call “lambda protocol,” able to optimize route construction depending on the joint ETE demand for security and rate. Our results are first derived for specific network instances (i.e., fixed topologies within a given network class) and subsequently extended to ensemble averages over entire network classes, where phase transitions are revealed with respect to node connectivity.

It is generally argued that Mermin-Wagner theorem excludes the possibility of long-range order in two-dimensional bosonic systems at nonzero temperatures. In contrast, we show here that generic bilayer semiconductors could demonstrate true Bose-Einstein condensation of interlayer excitons. We find that the key requirements include (1) reduction of the interlayer band gap using an applied electric field so that excitons spontaneously appear in the ground state, (2) band structure that allows for long-range electron-hole exchange interaction, and (3) a finite magnetic field. When these conditions are fulfilled, superfluidity and ferroelectric order can coexist in two-dimensional excitonic insulators. Our results provide an example where coupling to the vacuum electromagnetic near field leads to a finite temperature second-order phase transition.

Laser-driven proton acceleration provides a powerful route for generating ultrashort, high-charge proton beams. Many applications, including secondary neutron sources and inertial fusion, benefit from tight proton beam focusing. Concave targets offer a robust solution, yet the scaling of proton focusing with laser and target parameters remains poorly understood. Here, we present a numerical study of laser-driven proton acceleration and focusing from hemispherical targets using the fully kinetic, relativistic Particle-In-Cell code EPOCH. We focus on the subpicosecond laser-pulse regime (duration $\lesssim {10}^2$ fs), centering on the laser parameters of our recent experiment at the Colorado State University ALEPH laser facility. We investigate the proton acceleration mechanisms, characterize proton focusing, and assess how focal spot parameters scale with laser and target parameters. We identify Target Normal Sheath Acceleration as the dominant mechanism, supplemented by a secondary postacceleration stage near the geometrical center of the hemisphere. We demonstrate that both the proton focal spot size and focal plane position scale approximately linearly with the hemisphere radius, with the focal plane consistently located downstream of the geometrical center. The opening angle of the concave target mainly affects the proton beam waist. Energy-dependent proton focusing can be understood, in part, as arising from the evolving curvature of the accelerating structure, which departs from the target curvature. Evidence for self-similar proton focusing is found in the regime of nearly uniform target irradiation.

The van der Waals transition metal dichalcogenide ${\mathrm{NiTe}}_2$ is investigated through soft x-ray photoelectron and absorption spectroscopy combined with density functional plus dynamical mean-field theory. We derive an electronic configuration of $d^9$ for the Ni ions with some admixture of $d^8$ and $d^{10}$, yielding an average $3d$ occupation of about 8.9. Despite the $d$ shell being open with non-negligible hole concentrations, calculations reveal the absence of local moments, thereby explaining the Pauli paramagnetic behavior as observed in magnetic susceptibility measurements. This behavior is unusual, particularly given that many Ni-based compounds exhibit strong electron correlations. We ascribe the silencing of correlation effects in this material to the fact that without the Ni–Te hybridization, the Ni would have the nonmagnetic full $d^{10}$ configuration, and that the Ni bands that get partially unfilled due to hybridization are extremely broad, making the holes “bandlike” or “highly delocalized”. The large Hubbard $U$ interaction does not become operative in this material.

Elastic electron scattering is one of the primary means of investigating materials on the atomic scale. It is usually described in a one-particle approach (the probe electron's evolution in a perturbative static potential), whereas we are dealing here with a two-body interaction between the probe and the sample, both described by separate quantum states, inducing entanglement. In this work, we present a quantum treatment of elastic electron scattering. We find that the entanglement between probe and scatterer can have far-reaching consequences, particularly on coherence and image contrast. As a timely example, we discuss decoherence in Bragg scattering on nanoparticles. We find that conventional scattering theory is recovered in most cases. The situation changes dramatically for freely evolving nanoparticles as, e.g., levitated motional ground states, an active field of research.

We rigorously analyze the nonequilibrium thermodynamic behavior of various formulations of quantum Brownian motion using the framework of stochastic thermodynamics. While the widely used Caldeira-Leggett master equation exhibits desirable thermodynamic features—such as the fulfillment of a detailed balance—it fails to ensure complete positivity. In contrast, several completely positive and trace-preserving extensions turn out to be thermodynamically controversial. We show that such extensions introduce anomalous phase-space structures that violate detailed balance at the steady state, leading to nonvanishing entropy production and spurious nonequilibrium currents lacking a clear physical interpretation. Our results highlight a fundamental tension between quantum consistency and thermodynamic equilibration in open quantum systems.

Photon entanglement is a fundamental resource for quantum technologies, yet it is extremely fragile especially in large quantum circuitry, which is necessary for quantum information processing and computing. We propose an integrated scheme for direct topological protection of entanglement, enabling robust transport of entangled photonic states, even when the photon dynamically evolves during propagation. Our platform relies on a two-dimensional discrete-time quantum walks in synthetic dimensions, supporting topological edge states at the interface between distinct domains. Thus far, in all previous approaches for exploiting topology to protect entanglement, the protection was linked to the position of the photon in the actual structure. In contrast, our scheme relies on a synthetic temporal dimension (time bins) where the entangled state is always reproduced, despite fluctuations or variations that may emerge in the system. Thus, our scheme provides precise temporal control important for synchronization and timing in quantum computation. By suppressing phase-sensitive errors—the most critical noise source in photonic platforms—our approach can lower the physical error rate below fault-tolerance thresholds and reduce the overhead typically required for scalable quantum information processing. The robustness to errors arises from the fact that the entangled state is preserved through correlations in the synthetic (temporal) dimension. As a result, we ensure that at specific designated times, the emergent state faithfully reproduces the initial entangled state, despite the photon having taken any dynamically evolving paths. This approach establishes a robust framework for directly protecting entanglement.

We experimentally investigate trapped ion dynamics in the strong-driving regime, where the Rabi frequency $\mathrm{\Omega }$ is comparable to the vibrational mode frequency $\nu$. In the conventional weak-driving regime ($\mathrm{\Omega }\ll \nu$), the dynamics is well described by effective Hamiltonians for the carrier and motional sidebands, associated with detunings $\delta =n\nu$ ($n=0,\pm 1,...$). In the strong-driving regime ($\mathrm{\Omega }\sim \nu$), these interactions can no longer be treated independently. We characterize this physics through the reduced dynamics of the qubit, where non-Markovian behavior emerges as an operational probe of the spin-motion coupling. We observe a structured non-Markovian response, with well-defined maxima that follow the generalized resonance condition ${\delta }^2+{\mathrm{\Omega }}^2={\nu }^2$, reflecting the strong underlying spin-motion hybridization characteristic of the strong-driving regime.

Recent advancements in topological analysis offer a powerful way to classify how phases of nonlinear bosonic resonators organize. These tools are rooted in the Morse-Smale flow topology framework, where a graph-based construction describes how steady-state solutions are arranged in phase space. This construction remains incomplete as self-sustained oscillations in the form of limit cycles can impact the global connectivity of phase-space flows and the corresponding Morse-Smale topological classification. In this work, we extend the bosonic classification scheme to include limit cycles as fundamental topological elements. We illustrate the approach with a minimal nonlinear Van der Pol resonator model, where limit cycles coexist with steady-state solutions. Our results provide a unified topological description of stationary and time-periodic phases in nonlinear bosonic systems, with direct relevance to photonic, superconducting, and optomechanical platforms. Our findings raise fundamental questions regarding synchronization and the extension of flow topology to the quantum regime.

FeGe is the only known kagome metal hosting the coexistence of long-range magnetic order and charge-density-wave (CDW) order, making it an exceptional platform for studying spin-charge-lattice entanglement. Despite extensive experimental and theoretical efforts, the microscopic origin of the CDW in FeGe remains under active debate. In this work, we reveal how in-plane tensile strain reshapes the hierarchy of competing CDW orders in the antiferromagnetic kagome metal FeGe. First-principles phonon calculations uncover two Kohn-anomaly-induced incommensurate CDWs under strain, while slight electron correlations suppress these modes and instead lead to unconventional correlation-driven CDW phases. Notably, a previously unreported fully dimerized $\sqrt{3}\times \sqrt{3}\times 2$ CDW emerges from the softened H-point phonon mode. Total-energy analyses establish a strain-induced phase transition sequence: $2\times 2\times 2\to \sqrt{3}\times \sqrt{3}\times 2\to$ fully dimerized $\sqrt{3}\times \sqrt{3}\times 2$ at strains of $\sim 0.6\%$ and $\sim 2.4\%$, respectively, while a small U = 0.2 eV will reduce the required strain to $\sim 0.2\%$ and $\sim 1.8\%$, respectively. These results demonstrate the coexistence of electron-phonon-driven and correlation-driven CDW mechanisms, positioning strained FeGe as a powerful platform for probing strongly coupled spin-charge-lattice entanglement in correlated magnetic kagome systems.

Much attention has been devoted to the use of machine learning to approximate physical concepts. Yet, due to challenges in interpretability of machine learning techniques, the question of what physics machine learning models are able to learn remains open. Here, we bridge the concept of a physical quantity and its machine learning approximation in the context of the original application of neural networks in physics: topological phase classification. We construct a hybrid tensor-neural network object that exactly expresses the real-space topological invariant and rigorously assess its trainability and generalization. Specifically, we benchmark the accuracy and trainability of a tensor-neural network to multiple types of neural networks, thus exemplifying the differences in trainability and representational power. Our work highlights the challenges in learning topological invariants and constitutes a stepping stone toward more accurate and better generalizable machine learning representations in condensed matter physics.

We introduce and theoretically analyze a quantum interference phenomenon in which a two-photon interference pattern is created using four-photon quantum states generated by two independent sources and without detecting two of the photons. Contrary to the common perception, the interference pattern can be made fully independent of phases acquired by the photons detected to construct it. However, it still contains information about spatially dependent phases acquired by the two undetected photons. This phenomenon can be observed with both bosonic and fermionic particles. We show that the phenomenon can be harnessed to develop a quantum quantitative phase imaging technique that is immune to uncontrollable phase fluctuations in the interferometer and allows image acquisition without detecting the photons illuminating the object.

Plants navigate fluctuating fluid environments in both soil and hydroponic systems, yet the physical principles by which roots transduce directional hydrodynamic forcing into developmental responses remain poorly quantified. By combining a flow-configurable microfluidic platform (P-ROOT) with fluid-structure interaction modeling and multiscale phenotyping, we establish a dimensionless, direction-dependent hydraulic stress index, defined as the ratio of flow-induced stress to root stiffness, that predicts growth-permissive and stress-adaptive hydraulic loading regimes in Brassica juncea roots. Our results reveal a pronounced directional asymmetry: Under parallel flow, root elongation is sustained across a broad hydraulic loading range, whereas counterflow concentrates stress near the root apex, restricting the growth-permissive range and reducing elongation. Transitions across these hydraulic loading regimes coincide with coordinated cortical remodeling, auxin redistribution, reduced nitrogen uptake, and enhanced nitro-oxidative signatures near the root tip. These findings indicate that root developmental plasticity is governed more strongly by localized stress amplification than by the overall magnitude of hydraulic loading. By establishing hydraulic stress indices as predictive metrics linking mechanical loading to multiscale biological responses, this work provides a quantitative framework for understanding plant-fluid interactions and for designing flow-managed cultivation systems that optimize nutrient delivery.

We introduce the concept of quantum observables over time (QOOT), an operator that jointly describes two observables at two distinct time points, as a dual of the quantum state over time formalism. We provide a full characterization of the conditions under which a QOOT can be properly defined, via a no-go theorem. We use QOOTs to establish a notion of time reversal for generic quantum channels with respect to a reference observable, enabling the systematic construction of recovery maps that preserve the latter. These recovery maps, although generally nonphysical, can be decomposed into realizable channels, enabling their application in noiseless expectation value estimation tasks. We provide explicit examples and compare our protocol with other error mitigation methods. We show that our protocol retrieves the noiseless expectation value of the reference observable and can achieve optimal sampling overhead, outperforming probabilistic error cancellation.

Molecular nanomagnets are compounds characterized by a high-spin magnetic core that is protected by organic ligands. They have recently gained attention as potential quantum information carriers in solid-state quantum computing platforms, simultaneously exhibiting classical macroscopic properties and quantum features in light of their complex nature and configuration. Addressing the condition when they manifest unquestionable quantum behavior is key to guarantee their effectiveness as resources for quantum information processing. We address the quantumness of molecular nanomagnets using a recently formulated criterion [T. Krisnanda et al., Phys. Rev. Lett. 119, 120402 (2017)] demonstrating that these systems exhibit an intrinsic quantum nature, as evidenced by their ability to generate and enhance quantum correlations between two noninteracting probes. Our analysis, which is performed addressing various dynamical regimes, paves the way to the design of experimentally viable tests of nonclassicality in multipartite registers consisting of ensembles of molecular nanomagnets.