Abstract Quantum cryptography communication encompasses both quantum key distribution (QKD) and the encryption and decryption of plaintext. Continuous-variable QKD (CVQKD) leverages the fundamental principles of physics to enable legitimate parties to share secure keys, while the CVQKD-compatible quantum noise stream cipher (QNSC) exploits quantum fluctuations to further obscure the ciphertext encrypted with these keys. Integrating these two functionalities into a unified system represents a promising developmental trend. However, existing integrated architectures typically rely on multiplexing two separate optical paths and have been demonstrated exclusively over optical fibers. Here, a unified local local oscillator (LLO) quantum cryptography architecture operating over a free-space channel is proposed and experimentally demonstrated. Requiring only simple software-defined switching, our payload-efficient architecture executes both CVQKD and QNSC functionalities utilizing a single optical setup. Over an indoor free-space channel with emulated time-varying atmospheric disturbances, an asymptotic secret key rate of 35.4292 kbps is obtained at a maximum channel attenuation of 19.5054 dB for the CVQKD mode. For the QNSC mode, an encrypted image is successfully transmitted, yielding pixel accuracies of 99.58% and 96.39% under channel attenuations of 10.1690 dB and 12.0620 dB, respectively. This work validates the feasibility of integrated quantum cryptography architectures in complex environments, marking a meaningful step toward constructing payload-constrained quantum communication networks.
Abstract In recent years, strong-field terahertz (THz) pulses have achieved substantial improvements in single-pulse energy, peak field strength, and high average power. As electromagnetic radiation, THz pulses intrinsically contain both electric- and magnetic-field components. Because the electric and magnetic fields have different physical dimensions, their numerical amplitudes should not be compared directly; the relevant distinction is instead the strength and selection rules of electric-dipole and magnetic-dipole coupling to matter. Although THz electric-field detection and applications have become increasingly mature, research on THz magnetic fields has progressed more slowly because of weak magnetic responses, which have limited the development of strong-field THz magnetic detection and magnetic manipulation. This review focuses on representative solid-state strong-field THz sources, including lithium niobate sources and spintronic THz emitters. It summarizes recent progress in these two source platforms and discusses advances in spintronic detection and magneto-optical detection. For magnetization dynamics, we explicitly distinguish direct B THz -driven Zeeman excitation from E THz -driven current/spin-orbit-torque processes and from purely optical excitation mechanisms. These developments are expected to advance magnetic field applications and next-generation THz spintronics.
Abstract We develop an exact finite-perturbation response theory for the higher-order statistics of general state observables in nonequilibrium Markov networks. For local perturbations, the moment generating function obeys an exact nonlinear response identity in the perturbation strength, controlled by a single kinetic parameter α . This induces an exact nonlinear map for the cumulant generating function and closed expressions for finite responses of all cumulants through partial Bell polynomials. As a central application, we obtain a complete theory of variance control, including sharp saturation limits, a classification of monotone-suppression and sign-reversal regimes, and meanfirst-passage-time design principles for optimal noise suppression. The same framework yields exact formulas for operational and symmetrized response resolution along the accessible control branch. A four-state gene-regulation model illustrates how local kinetic control suppresses the occupancy noise of a transcriptionally competent promoter state.
Abstract We theoretically investigate the inertial dynamics of magnetic skyrmions driven by the microwave-induced inverse Faraday effect (MIFE) of circularly polarized microwaves (CPM). By incorporating an inertial mass and MIFE force, we demonstrate fundamentally distinct dynamical regimes under continuous-wave (CW) versus pulsed excitation. Skyrmion inertia transforms trajectories from smooth spirals to polygonal orbits under CW driving and enables sustained post-pulse gyration. Handedness is determined by the topological charge and CPM helicity: left-circularly polarized (LCP) CPM attracts skyrmions toward the beam center, whereas right-circularly polarized (RCP) CPM repels them. Systematic parameter analysis reveals how Gilbert damping, CPM intensity, frequency, and skyrmion mass control the oscillatory-to-overdamped transition. Our work identifies inertia, topological charge, and CPM helicity as essential factors for GHz-frequency skyrmion manipulation.
Abstract Defective chalcopyrites have recently emerged as promising thermoelectric materials because their ordered intrinsic vacancies can profoundly reshape both lattice dynamics and electronic structure. Here, we present a comprehensive theoretical investigation of the lattice thermal and carrier transport properties of II-III 2 -VI 4 defective chalcopyrites by combining first-principles calculations with machine-learning interatomic potentials. We show that vacancy ordering enhances lattice distortion, leading to strong anharmonicity and metavalent bonding. The interplay of soft low-frequency phonons, strongly negative Grüneisen parameters, and a substantially enlarged four-phonon-scattering phase space strongly regulates heat transport, yielding an ultralow lattice thermal conductivity. Meanwhile, systematic anion substitution at the VI-site provides an effective route to tune the electronic structure: reduced anion electronegativity weakens metal-anion hybridization, shifts anion p states upward, narrows the band gap, and thereby improves electrical transport. Benefiting from this synergy between vacancy-induced phonon suppression and anion-regulated electronic optimization, CdGa 2 Te 4 exhibits an ultralow lattice thermal conductivity of 0.19 W·m −1 ·K −1 and a high room-temperature ZT of 0.689. This work not only predicts defective chalcopyrites as a promising platform for high-performance thermoelectrics but also provides a practical design strategy by integrating vacancy ordering, higher-order phonon scattering, and anion-dependent band engineering.
Abstract When a transformation generated by an operator is continuous, states are the eigenstates of the corresponding operator, such as translation symmetry for momentum eigenstates and rotational symmetry for total angular momentum eigenstates. Here, we show that the duality symmetry, the transformation generated by the helicity operator in the electric-magnetic representation, is insufficient to ensure that light forms a helicity eigenstate. As required by the symmetry of helicity, the eigenvectors of the helicity operator should be isomorphic to a two-dimensional irreducible representation, determined by the geometrical symmetry of the waveguides. When the symmetry of modes is met, but the duality symmetry of the waveguide is broken, helicity eigenstates only approximately exist in the paraxial limit of a guided mode. This reveals that transverse confinement is the underlying physical origin of duality symmetry breaking for generic guided modes.
Abstract The Bardeen-Cooper-Schrieffer theory of superconductivity is restricted to weak electron-phonon coupling. In the strong-coupling regime, charge carriers self-trap via local lattice distortions, forming polarons whose condensation provides an alternative mechanism for superconductivity. Titanium oxides serve as a model system for polaron physics, suggesting the possibility of polaronic superconductivity. Here we report Oxygen-Stoichiometry-Driven Crossover from Polaronic Insulator to Superconductor in epitaxial Ti 9 O 10 thin films. Temperature-dependent Raman spectroscopy supports the films as a potential polaronic insulator, in which oxygen stoichiometry control reveals an anticorrelation between the polaronic insulating state and superconductivity. Tuning the oxygen pressure suppresses the polaronic insulating state and induces superconductivity, resulting in two distinct superconducting regions in the oxygen-pressure phase diagram. Our results provide a pathway toward understanding superconductivity in polaronic materials and offer a route for searching high-temperature superconductors.
Abstract In passive viscous systems, the local flow field is governed by the prescribed hydraulic properties and the applied pressure boundary conditions. Hydrodynamic metamaterials based on spatially engineered permeability or viscosity distributions can redistribute, concentrate, conceal, and even redirect flow fields. Transformation-based rotators can, in principle, produce large rotations of the local velocity direction, but such transport states are encoded in the prescribed material architecture and generally require increasingly strong anisotropy as the rotation angle increases. This motivates an alternative strategy for achieving dynamically tunable localized reverse transport without redesigning the passive structure. Here we demonstrate a flow-reversal hydrodynamic metamaterial that creates a target region with velocity opposite to the imposed background flow while retaining an approximately uniform external flow. The device combines an inner annular array of active source-sink dipoles with a surrounding passive Hele-Shaw compensation shell. The active dipoles supply the hydraulic work required to overcome the background transport within the target region, whereas the passive shell suppresses the exterior disturbance by hydraulic-resistance matching. This active-passive coupled design enables complete internal flow reversal while maintaining an almost undisturbed external flow field. This work uses active transport control on hydrodynamic metamaterials, enabling programmable localized transport in pressure-driven viscous flows.
Abstract In crystalline solids, symmetry can constrain formal polarization to discrete values. We show that this quantized formal polarization (QFP) should be understood as a symmetry-protected bulk invariant that encodes intrinsic information about a material’s symmetry and electronic structure. From this viewpoint, mismatch between the QFPs of different materials or phases represents a bulk incompatibility with potentially broad physical consequences. In particular, when such materials form an interface, the QFP mismatch imposes a nontrivial compensation requirement, which may be realized through metallic states, bound charges, structural reconstructions, or other mechanisms. Through the examples of the ZnS/MgS heterostructure, the HfZnN 2 domain wall, and the GaAs domain wall, we demonstrate that bulk QFP provides a predictive criterion for determining whether nontrivial interfacial compensation is required, without explicitly constructing the interface or domain wall. The specific realization of this compensation, however, must be determined from explicit calculations for each material system. Our work thus elevates formal polarization from a quantity used mainly in interface electrostatics to a more general bulk principle relevant to phase compatibility and related phenomena, and suggests a route toward designing functional quantum materials through controlled quantized polarization mismatch.
Abstract Tellurium (Te) has recently emerged as a promising thermoelectric material, exhibiting intrinsically low lattice thermal conductivity and high performance despite its elemental and structural simplicity. In this work, we employ first-principles calculations combined with Boltzmann transport theory to systematically study the thermoelectric property and performance of helical Te. Our analysis shows that lone electron pairs and the helical chain strongly enhance lattice anharmonicity and suppress phonon group velocities, yielding markedly reduced lattice thermal conductivity. Including Born effective charges introduces long-range corrections that strongly renormalize electrical transport and qualitatively reshape the dominant phonon modes governing electron–phonon scattering, leading to a ~31.0% reduction of the maximum zT in agreement with experiment. These results provide a microscopic understanding of Te’s anomalously low lattice thermal conductivity and anisotropic transport, and suggest design principles for high-performance thermoelectrics in helical-chain materials and electrides.
Abstract 4H b -TaS 2 is a natural heterostructure of correlated (T) and superconducting (H) layers. Using time- and angle-resolved photoemission spectroscopy, we observe a narrow feature slightly above the Fermi level on the T-layer termination, corresponding to the band edge of a charge-density-wave (CDW)-gapped electron-like dispersion. Above a characteristic incident fluence, the fitted CDW gap becomes indistinguishable from zero, while enhanced occupied-state spectral weight persists for tens of picoseconds after hot carriers decay. Together, these spectral and dynamical signatures identify a transient metastable electronic state. Its much shorter lifetime than that of the persistent hidden state in bulk 1T-TaS 2 is consistent with a role for interrupted direct T–T stacking, although relaxation channels involving the H layers may also contribute. These results highlight the influence of the interlayer environment on nonequilibrium-state stability and establish 4H b -TaS 2 as a platform for ultrafast control of correlated states.
Abstract We investigate the quantized charge pumping in graphene with Rashba spin–orbit coupling (RSOC) under two time-periodic staggered potentials. Surprisingly, the quantized charge pumped per cycle exhibits a sharp dependence on the transverse boundary conditions: a zero quantized charge is obtained under periodic boundary conditions (consistent with bulk thermodynamic limits), whereas a quantized value of 1 per spin emerges under open boundary conditions. This topological transition induced by the transverse boundary conditions persists even for large system widths, revealing a discontinuous crossover from finite to infinite geometries without relying on topological edge states at the boundaries. We attribute this phenomenon to RSOC-induced intravalley reflection at transverse boundaries, which modifies the winding number of the pumping cycle. These results challenge conventional wisdom about finite-to-infinite transitions in condensed matter systems and advance the fundamental understanding of boundary-sensitive quantum transport in multivalley systems.
Abstract We present a refined strong-field approximation (SFA) theory incorporating the intermediate-state Coulomb interaction for nonsequential double ionization (NSDI). By introducing the Coulomb−Volkov wave function into the intense-field many-body S-matrix theory, we develop an intermediate-state Coulomb-corrected strong-field approximation (ICSFA) to address the long-standing deficiency of neglecting the Coulomb potential acting on the recolliding electron. Using the recollision excitation with subsequent ionization (RESI) mechanism in helium as a benchmark, we perform a systematic comparison between the conventional SFA and the ICSFA. The correlated electron momentum distributions (CEMDs) calculated via ICSFA show a significant redistribution of electron yield toward high-momentum regions and off-axis quadrants, which aligns quantitatively with experimental observations, whereas the SFA fails to reproduce these features. Concurrently, the joint energy distributions (JEDs) manifest a corresponding evolution. Analysis reveals that the Coulomb potential promotes large-angle scattering, thereby reshaping the CEMD and the JED. Furthermore, the ICSFA reveals a substantial enhancement of multiple-return recollision (MRR) trajectories, stemming from the combined effects of Coulomb focusing (manifested through the normalization factor) and the energy-dependent enhancement of the inelastic scattering cross-section. Our work establishes that the intermediate-state Coulomb interaction is indispensable for a quantitatively accurate description of strong-field correlated electron dynamics.
Abstract Terahertz (THz) pump–THz probe spectroscopy has emerged as a powerful platform for investigating ultrafast nonlinear light–matter interactions. However, the physical origin of recently reported THz signal enhancement remains controversial because contributions from THz generation and electro–optic detection are often coupled. Here, we use a Fe 4 GeTe 2 spintronic THz emitter together with a ZnTe electro–optic detector to demonstrate that, with fixed THz pump polarity, reversing the magnetic-field direction of the Fe 4 GeTe 2 emitter does not change the signal enhancement or induce suppression, indicating that the modulation originates from the nonlinear electro–optic response of the ZnTe detector rather than the THz emitter. Furthermore, controlling the polarity of the THz pump field reveals a pronounced asymmetric response: a positive THz field induces signal enhancement, whereas a negative field leads to suppression, revealing strong polarity-dependent THz pump–THz probe modulation. This behavior cannot be solely attributed to carrier redistribution in the conduction band and is consistently described by a model incorporating both linear Pockels and nonlinear Kerr effects in ZnTe. This polarity-dependent reversal of the linear Pockels contribution accounts for the observed polarity-dependent THz-field-induced absorption modulation. These results clarify the physical origin of THz pump–THz probe modulation, establish detection-induced nonlinearity as the dominant mechanism governing the measured response, and highlight the importance of treating electro–optic detection as an active nonlinear element in THz pump–THz probe spectroscopy.
Abstract Rydberg atom-based time-receiving systems enable high-sensitivity demodulation of long-wave signals for precise one-pulse-per-second (1PPS) output, surpassing the limitations of conventional long-wave receivers. Here, we first report the theoretical and experimental demonstration of a nanosecond-jitter Rydberg time-receiving system for eLoran signals. We establish a novel theoretical model that quantitatively reveals the relationship between timing jitter and system parameters, exemplified by the DC Stark shift and the coupling-beam Rabi frequency. Experimentally, we first demonstrate 1PPS timing output based on an atomic system, achieving a root-mean-square (RMS) timing jitter of 27.90 ± 0.39 ns. Furthermore, we demonstrate the timing performance of our system across high-noise conditions and at low input signal amplitudes, validating its robustness under practical weak eLoran signal conditions. This work validates the feasibility of Rydberg atom-based architectures for high-precision ground-based timing and establishes a performance benchmark for next-generation atomic-optical positioning, navigation, and timing (PNT) systems.
Abstract Recent experiments support that the magic-angle graphene family can be modeled by a periodic array of correlated quantum impurities immersed in a Dirac sea. This work analytically demonstrates that, with (anti-) Hund’s interactions that can originate from electron-phonon couplings, a single impurity can exhibit novel quantum phase transitions, and readily nurture the pairing potential and pseudogap phenomenon relevant to this moiré family. For broad applicability, we tackle a spin-valley Anderson impurity with general symmetry-allowed (anti-) Hund’s parameters ( J D , J S ), and derive its full phase diagram at half-filling. While the model reduces to well-known Kondo problems in certain limits, we uncover that in the large J D regime, the low-energy physics is controlled by a novel “pair-Kondo” coupling between the bath and an impurity valley-doublet. Using bosonization–refermionization mapping, we show there is a BKT transition from a Fermi liquid with pairKondo resonance, into an anisotropic doublet phase that exhibits a non-analytic zero-energy kink in the impurity spectral function, and non-universal power-law scaling in impurity susceptibilities. Importantly, by analyzing the pairing potentials across the phase diagram, we unveil ubiquitous existence of attractive channels under general (anti-) Hund’s interactions. More crucially, we analytically reveal the pseudogap shoulders represent multiplet excitations induced by the injected electron or hole, and derive ansätze of correlation self-energy that reproduce the pseudogap phenomenon in the lattice. All results are established analytically, with further verification by numerical renormalization group calculations.
Abstract Multiaxial magnetic states offer a route beyond binary spintronic functionalities and enable zero-magnetic-field spin orbit torque (SOT) switching via mirror symmetry breaking, yet remain unexplored. Here, we uncover field-free multistate SOT switching in a SrIrO 3 /SrRuO 3 bilayer with exceptionally large monoclinic distortion. Pulsed current excitations fully map all twelve deterministic transitions among the four states, through a systematic switching protocol defined by two characteristic current densities. In-situ scanning nitrogen-vacancy (NV) center magnetometry provides direct real-space evidence for previously unobserved in-plane canted states, and spin dynamics simulations uncover a two-step switching pathway, driven by the concerted action of spin torques and the effective anisotropy field within the fourfold anisotropy landscape. Our results demonstrate that precise lattice-symmetry engineering enables the creation and control of multiaxial magnetic states for field-free multistate spintronic devices.
Abstract Multiferroic materials offer a promising platform for ultrafast optical control of coupled magnetic and polar orders. However, a prerequisite for such control is to precisely identify how the magnetically induced polarization manifests itself on the ultrafast timescale, and then to probe its dynamics upon external perturbations. Here, we address this issue in the polar magnet Fe 2 Mo 3 O 8 by combining static and time-resolved second harmonic generation (SHG). Temperature-dependent static SHG reveals that, among the symmetry-allowed tensor elements, only χ ccc (2) exhibits a pronounced anomaly at the antiferromagnetic transition ( T N ≈ 60 K), identifying χ ccc (2) as the nonlinear susceptibility tensor element coupled to the magnetically induced polarization. Guided by this result, time-resolved SHG selectively tracks the dynamics of this tensor element following ultrafast photoexcitation. We observe a rapid enhancement of the χ ccc (2) -related SHG signal, followed by biexponential recovery. The response is independent of the pump polarization and reaches saturation at a remarkably low fluence, indicating a highly efficient coupling between optical excitation and the magnetically induced polarization. These results establish SHG as tensor-selective probe of ultrafast magnetoelectric dynamics and demonstrate the high sensitivity of the magnetically induced polarization in Fe 2 Mo 3 O 8 to optical excitation.
Abstract The reliability of simulations of thermal properties in micro-/nano-scale structures critically depends on the accuracy of potential function. While empirical force fields offer limited precision, machine learning potentials effectively bridge this gap by combining the computational accuracy of density functional theory with the efficiency of empirical force fields. This review provides a systematic investigation of various frameworks of machine learning potentials, including descriptors, training methodologies, and accuracy. Furthermore, a comparison of their predictive performance for thermal conductivity across different material systems is presented. Finally, key challenges and future research directions for machine learning potentials are discussed, aiming to promote their applications in thermal transport studies and enhance the understanding of heat conduction mechanisms.
