Magnetic vortices hold great promise for advanced information storage applications due to their quartet degenerate states and high topological stability. The key to their application lies on meticulous control of its polarity and chirality, which traditionally relies on magnetic fields, currents, and spin waves. However, the vortex core’s intrinsic precession under these stimuli hampers fast switching of the polarity and chirality. Here, we demonstrate a fast and precise control of polarity and chirality in magnetic vortices using combined femtosecond (fs) laser and tiny magnetic fields via micromagnetic simulations on Permalloy nanodisks. The fs laser pulse induces an ultrafast quench effect to establish the initial paramagnetic state, while the simultaneously applied magnetic fields precisely target the final vortex structure. Intriguingly, a 110 mT out-of-plane field and a 7 mT in-plane circular field are sufficient to realize precise control of the polarity and chirality on sub-nanosecond time scale, respectively, which are much lower than that of the previous work. Our approach guarantees fast and reliable switching of magnetic vortex polarity and chirality, paving the groundwork for a high-speed quaternary data storage and contributing a novel perspective to the fundamentals of spintronics.

We consider a dipolar spin-1 Bose gas with SU(3) spin−orbit coupling trapped in a two-dimensional toroidal trap. Due to the combined effects of SU(3) spin−orbit coupling, dipole−dipole interaction, and spin−exchange interaction, the system exhibits a rich variety of ground-state phases and topological defects, including modified stripe, azimuthal distributed petal and triangular lattice, double-quantum spin vortices, and so on. In particular, by studying the spin texture of such a system, it is found that the formation and transformation between meron and skyrmion topological spin textures can be realized by a choice of dipole−dipole interaction, SU(3) spin−orbit coupling, and spin−exchange interaction. We also give an experimental protocol to observe such novel states within current experimental capacity.

Variational quantum algorithms have been widely demonstrated in both experimental and theoretical contexts to have extensive applications in quantum simulation, optimization, and machine learning. However, the exponential growth in the dimension of the Hilbert space results in the phenomenon of vanishing parameter gradients in the circuit as the number of qubits and circuit depth increase, known as the barren plateau phenomena. In recent years, research in non-equilibrium statistical physics has led to the discovery of the realization of many-body localization. As a type of floquet system, many-body localized floquet system has phase avoiding thermalization with an extensive parameter space coverage and has been experimentally demonstrated can produce time crystals. We applied this circuit to the variational quantum algorithms for the calculation of many-body ground states and studied the variance of gradient for parameter updates under this circuit. We found that this circuit structure can effectively avoid barren plateaus. We also analyzed the entropy growth, information scrambling, and optimizer dynamics of this circuit. Leveraging this characteristic, we designed a new type of variational ansatz, called the “many-body localization ansatz”. We applied it to solve quantum many-body ground states and examined its circuit properties. Our numerical results show that our ansatz significantly improved the variational quantum algorithm.

Based on new obtained analytical results, the main properties of photon echo quantum memory protocols are analysed and discussed together with recently achieved experimental results. The main attention is paid to studying the influence of spectral dispersion and nonlinear interaction of light pulses with resonant atoms. The distinctive features of the effect of spectral dispersion on the quantum storage of broadband signal pulses in the studied echo protocols are identified and discussed. Using photon echo area theorem, closed analytical solutions for echo protocols of quantum memory are obtained, describing the storage of weak and intense signal pulses, allowing us to find the conditions for the implementation of high efficiency in the echo protocols under strong nonlinear interaction of signal and control pulses with atoms. The key existing practical problems and the ways to solve them in realistic experimental conditions are outlined. We also briefly discuss the potential of using the considered photon echo quantum memory protocols in a quantum repeater.

In this paper, we present a novel method for the complete analysis of maximally hyperentangled state of photon system in two degrees of freedom (DOFs), resorting to the auxiliary high-dimensional entanglement in the third DOF. This method not only can be used for complete hyperentangled Bell state analysis of two-photon system, but also can be suitable for complete hyperentangled Greenberger−Horne−Zeilinger (GHZ) state analysis of three-photon system, and can be extended to the complete N-photon hyperentangled GHZ state analysis. In our approach, the parity information of hyperentanglement is determined via the measurement on evolved auxiliary high-dimensional entanglement, and the relative phase information of hyperentanglement is determined via the projective measurement. Moreover, this approach can be accomplished by just using linear optics, and is significant for the investigation of photonic hyperentangled state analysis.

Entanglement and quantum correlations between atoms are not usually considered key ingredients of the superradiant phase transition. Here we consider the Tavis−Cummings model, a solvable system of two-levels atoms, coupled with a single-mode quantized electromagnetic field. This system undergoes a superradiant phase transition, even in a finite-size framework, accompanied by a spontaneous symmetry breaking, and an infinite sequence of energy level crossings. We find approximated expressions for the ground state, its energy, and the position of the level crossings, valid in the limit of a very large number of photons with respect to that of the atoms. In that same limit, we find that the number of photons scales quadratically with the coupling strength, and linearly with the system size, providing a new insight into the superradiance phenomenon. Resorting to novel multipartite measures, we then demonstrate that this quantum phase transition is accompanied by a crossover in the quantum correlations and entanglement between the atoms (qubits). The latters therefore represent suited order parameters for this transition. Finally, we show that these properties of the quantum phase transition persist in the thermodynamic limit.

Quantum teleportation allows the transmission of quantum states over arbitrary distances and is an applied tool in quantum computation and communication. This paper theoretically addresses the feasibility of quantum teleportation based on a single semiconductor quantum dot influenced by pure dephasing through the biexciton cascade decay. We also investigate the idea of remote sensing in quantum teleportation affected by pure dephasing. In particular, we compare the quality of quantum teleportation in single- and two-qubit schemes and show that, within the present model, single-qubit quantum teleportation has a quantum advantage. Finally, to investigate the dynamics of the system, we introduce important witnesses of the non-Markovian dynamics of the system, so that our results may solve outstanding problems in the realization of faithful quantum teleportation over a long time.

Metagratings (MGs) have emerged as a promising platform for manipulating the anomalous propagation of electromagnetic waves. However, traditional methods for designing functional MG-based devices face significant challenges, including complex model structures, time-consuming optimization processes, and specific polarization requirements. In this work, we propose an inverse-design approach to engineer simple MG structures comprising periodic air grooves on a flat metal surface, which can control anomalous reflection without polarization limitations. Through rigorous analytical methods, we derive solutions that achieve perfect retroreflection and perfect specular reflection, thereby leading to functional control over the linearly-polarized electromagnetic waves. Such capabilities enable intriguing functionalities including polarization-dependent retroreflection and polarization-independent retroreflection, as confirmed through full-wave simulations. Our work offers a simple and effective method to control freely electromagnetic waves, with potential applications spanning wavefront engineering, polarization splitting, cloaking technologies, and remote sensing.

Despite extensive research, the achievement of tunable Chern numbers in quantum anomalous Hall (QAH) systems remains a challenge in the field of condensed matter physics. Here, we theoretically proposed that Ti₂X₂ (X = P, As, Sb, Bi) can realize tunable Chern numbers QAH effect by adjusting their magnetization orientations. In the case of Ti₂P₂ and Ti₂As₂, if the magnetization lies in the x−y plane, and all C₂ symmetries are broken, a low-Chern-number phase with C = 1 will manifest. Conversely, if the magnetization is aligned to the z-axis, the systems enter a high-Chern number phase with C = 3. As for Ti₂Sb₂ and Ti₂Bi₂, by manipulating the in-plane magnetization orientation, these systems can periodically enter topological phases (C = ±1) over a 60° interval. Adjusting the magnetization orientation from +z to −z will result in the systems’ Chern number alternating between ±1. The non-trivial gap in monolayer Ti₂X₂ (X = P, As, Sb, Bi) can reach values of 23.4, 54.4, 60.8, and 88.2 meV, respectively. All of these values are close to the room-temperature energy scale. Furthermore, our research has revealed that the application of biaxial strain can effectively modify the magnetocrystalline anisotropic energy, which is advantageous in the manipulation of magnetization orientation. This work provides a family of large-gap QAH insulators with tunable Chern numbers, demonstrating promising prospects for future electronic applications.

We study the topology and localization properties of a generalized Su−Schrieffer−Heeger (SSH) model with a quasi-periodic modulated hopping. It is found that the interplay of off-diagonal quasi-periodic modulations can induce topological Anderson insulator (TAI) phases and reentrant topological Anderson insulator (RTAI), and the topological phase boundaries can be uncovered by the divergence of the localization length of the zero-energy mode. In contrast to the conventional case that the TAI regime emerges in a finite range with the increase of disorder, the TAI and RTAI are robust against arbitrary modulation amplitude for our system. Furthermore, we find that the TAI and RTAI can induce the emergence of reentrant localization transitions. Such an interesting connection between the reentrant localization transition and the TAI/RTAI can be detected from the wave-packet dynamics in cold atom systems by adopting the technique of momentum-lattice engineering.

Topological flat bands have attracted significant interest across various branches of physics, where synthetic gauge fields are typically considered an essential prerequisite. Numerous mechanisms have been proposed for implementing these fields, including magnetic fields on electrons, differential optical paths for photons, and strain-induced effective magnetic fields, among others. In this work, we introduce a novel approach to generating synthetic gauge fields through quantum statistics and demonstrate their effectiveness in realizing anyonic topological flat bands. Notably, we discover that a pair of strongly interacting anyons can induce square-root topological flat bands within a lattice model that remains dispersive and topologically trivial for a single particle. To validate our theoretical predictions, we experimentally simulate the quantum statistics-induced topological flat bands and square-root topological boundary states by mapping the eigenstates of two anyons onto modes in electric circuits. Our findings not only open a new pathway for creating topological flat bands but also deepen our understanding of anyonic physics and the underlying principles of flat-band topology.