A strong ultraviolet pumping laser propagating through the atmosphere could activate the medium and produce the forward and backward coherent air lasing. In this work, we present the theoretical analyses of forward and backward lasing dynamics in the long gain medium consisting of oxygen atoms. By numerically solving the semiclassical Maxwell-Bloch equations, we demonstrate that the atom density, the gain medium length, and the collisional decoherence rate substantially regulate the forward and backward air laser radiation at 845 nm. Especially, our study points out that lower atom density and moderate collisional decoherence rate are favorable for the backward radiation amplification in a long medium, since such conditions balance the competing processes of forward and backward radiation fields. The simulation shows that long medium backward air lasing becomes challenging in the oxygen atom system via double-photon pump, because the amplification can only occur around $z=0$, and the amplified backward radiation does not carry the spectral information of atoms throughout the medium. On the other hand, certain mechanisms such as variation of backward-seeding delay could possibly alleviate the suppression of backward amplification.

Few-layer (FL) transition metal dichalcogenides have been found to exhibit discrete subbands, called van der Waals quantum well (vdWQW) states, resulting from out-of-plane quantum confinement. In this study, we reveal the twisted-resonant tunneling characteristics of a vdWQW device using a three-layer (3L) $\mathrm{W}{\mathrm{Se}}_2/h$-BN/3L-$\mathrm{W}{\mathrm{Se}}_2$ junction with different twist angles ${\theta }_{\mathrm{tunnel}}$ between the two 3L $\mathrm{W}{\mathrm{Se}}_2$ flakes. Using an ambipolar graphene contact to $\mathrm{W}{\mathrm{Se}}_2$, two primary vdWQW states of 3L $\mathrm{W}{\mathrm{Se}}_2$ located in the conduction band (CB) and valence band (VB) were investigated. We found that while the current peak positions due to the electron resonant tunneling between the CB-QW states significantly change with ${\theta }_{\mathrm{tunnel}}$ having a periodicity of ${60}^{\circ }$, the hole resonant tunneling between the VB-QW states exhibits no ${\theta }_{\mathrm{tunnel}}$ dependence; these are due to different angular dispersions of two vdWQWs. The results highlight the different twistronic properties of CB and VB vdWQWs in FL $\mathrm{W}{\mathrm{Se}}_2$.

We formulate a theoretical approach to describe photon vortex production in synchrotron/cyclotron radiation from a helical moving electron under a uniform magnetic field in the relativistic quantum framework. In quantum theory, electron orbitals in a magnetic field are under Landau states. The Landau level density increases with decreasing the magnetic field strength, and it is practically impossible to calculate exactly the synchrotron radiation in possible magnetic fields in the laboratory. We present a method to calculate it by using asymptotic formulations of the emission amplitudes under the condition of $L_i-L_f\ll L_i$, where $L_i$ and $L_f$ are the initial and final Landau numbers, respectively. The wave function of the generated photon is the eigenstate of the $z$ component of the total angular momentum (zTAM) when the magnetic field is parallel to the $z$ axis. The approximation is applicable for photon vortex production of several tens $\hslash$ of zTAM. We also calculate numerically the energy spectra and spatial distribution of photon vortices in magnetic field strengths of 10 and 20 T using electrons with energies of 150 MeV and 8 GeV and Larmor radii of 10 and 100 µm for the helical motion. Although the Landau number becomes up to ${10}^8$ in these conditions, the radiations could be calculated using the approximation. The results indicate that it is possible to produce predominantly photon vortices with a fixed zTAM when we control the energy and entrance trajectory of electrons and the structure of a magnetic field. The present formula contributes to the prediction and verification of photon vortex generation.

We explore the dynamics of quantum spin systems in two and three dimensions using an exact mapping to classical stochastic processes. In recent work, we explored the effectiveness of sampling around the mean-field evolution as determined by a stochastically averaged Weiss field. Here, we show that this approach can be significantly extended by sampling around the instantaneous Weiss field associated with each stochastic trajectory taken separately. This trajectory-resolved approach incorporates sample to sample fluctuations and allows for longer simulation times. We demonstrate the utility of this approach for quenches in the two-dimensional and three-dimensional quantum Ising model. We show that the method is particularly advantageous in situations where the average Weiss field vanishes, but the trajectory-resolved Weiss fields are nonzero. We discuss the connection to the gauge-P phase-space approach, where the trajectory-resolved Weiss field can be interpreted as a gauge degree of freedom.

Core-periphery is a key feature of large-scale networks underlying a wide range of social, biological, and transportation phenomena. Nodes in the core have an influential position in the network, and thus, the periphery can be under structural disadvantage if the groups are aligned with external attributes such as gender or economic status. Despite its prevalence in empirical data, it is unclear whether core-periphery is a consequence of fundamental network evolution processes. While preferential attachment can create degree heterogeneity indistinguishable from core-periphery, it does not explain why cores and peripheries are aligned with some external node attribute, i.e., why specific groups of nodes gain dominance and become cores. We show that even small amounts of assortative attachment, e.g., homophily in social networks, can break the symmetric effect of preferential attachment and that the interplay of the two mechanisms leads to one of the groups emerging as a prominent core. A systematic analysis of the phase space of the proposed model reveals the levels of assortative and preferential attachment necessary for a group to become either core or periphery. We find that relative group size is significant, with minority groups typically having a disadvantage on becoming the core for similar assortative attachment levels among groups. We also find that growing networks are less prone to develop core-periphery than dynamically evolving networks and that these two network evolution mechanisms lead to different types of core-periphery structures. Analyzing five empirical networks, our findings suggest that core nodes are highly assortative, illustrating the potential of our model as a tool for designing and analyzing interventions on evolving networks.

We investigate the properties of a two-dimensional spinor microcavity polariton system driven by a linearly polarized continuous pump. In particular, we establish the role of the elementary excitations, namely, the so-called half-vortices and full vortices; these objects carry a quantum rotation only in one of the two, or both, spin components, respectively. Our numerical analysis of the steady state shows that it is only the half-vortices that are present in the vortex-antivortex pairing and dissociation responsible for the Berezinskii-Kosterlitz-Thouless transition. These are the relevant elementary excitations close to the critical point. However, by exploring the phase-ordering dynamics following a sudden quench across the transition we prove that full vortices become the relevant excitations away from the critical point in a deep quasi-ordered state at late times. The time scales for half-vortices binding into full vortices are much faster than the vortex-antivortex annihilations.

Efficient preparation of spin-squeezed states is important for quantum-enhanced metrology. Current protocols for generating strong spin squeezing rely on either high dimensionality or long-range interactions. A key challenge is how to generate considerable spin squeezing in one-dimensional systems with only nearest-neighbor interactions. Here, we develop variational spin-squeezing algorithms to solve this problem. We consider both digital and analog quantum circuits for these variational algorithms. After the closed optimization loop of the variational spin-squeezing algorithms, the generated squeezing can be comparable to the strongest squeezing created from two-axis twisting. By analyzing the experimental imperfections, the variational spin-squeezing algorithms proposed in this paper are feasible in recently developed noisy intermediate-scale quantum computers.

Learning the properties of dynamical quantum systems underlies applications ranging from nuclear magnetic resonance spectroscopy to quantum device characterization. A central challenge in this pursuit is the learning of strongly interacting systems, where conventional observables decay quickly in time and space, limiting the information that can be learned from their measurement. In this work, we introduce a new class of observables into the context of quantum learning—the out-of-time-order correlator—which we show can substantially improve the learnability of strongly interacting systems by virtue of displaying informative physics at large times and distances. We identify two general scenarios in which out-of-time-order correlators provide a significant learning advantage: (i) when experimental access to the system is spatially restricted, for example, via a single “probe” degree of freedom, and (ii) when one desires to characterize weak interactions whose strength is much less than the typical interaction strength. We numerically characterize these advantages across a variety of learning problems, and find that they are robust to both read-out error and decoherence. Motivated by these physical scenarios, we introduce several learning tasks—including Clifford tomography, and learning the connectivity of an unknown unitary—in which out-of-time-order experiments have a provable exponential advantage over any learning protocol involving only time-ordered operations.

Unbound electrons can experience resonant scattering and transient trapping in a molecular potential. In molecular photoemission, these shape resonances manifest as peaks in the cross section with a width of several electron volts. They depend on the details of the molecular potential and the wavelength of the photoelectron. We report experimental results on photo double-ionization which show the phenomenon of a collective two-electron resonance which occurs when the de Broglie wavelength of the dielectron quasiparticle is similar to that of a single electron exhibiting a single-electron shape resonance.

The black-hole laser (BHL) effect is the self-amplification of Hawking radiation in the presence of a pair of horizons which act as a resonant cavity. In a flowing atomic condensate, the BHL effect arises in a finite supersonic region, where Bogoliubov-Cherenkov-Landau (BCL) radiation is coherently excited by any static perturbation. Thus, experimental attempts to produce a black-hole laser unavoidably deal with the presence of a strong BCL background, making the observation of the BHL effect still a major challenge in the analog gravity field. Here, we perform a theoretical study of the BHL-BCL crossover using an idealized model where both phenomena can be unambiguously isolated. By drawing an analogy with an unstable pendulum, we distinguish three main regimes according to the interplay between quantum fluctuations and classical stimulation: quantum BHL, classical BHL, and BCL. Based on quite general scaling arguments, the nonlinear amplification of the initial amplitude of the quantum fluctuations up to saturation is identified as the most robust trait of a quantum BHL. A classical BHL behaves instead as a linear quantum amplifier, where the output is proportional to the input. The BCL regime also acts as a linear quantum amplifier, but its gain is exponentially smaller as compared to a classical BHL. In addition, we find that the decrease in the amplification for increasing BCL amplitude or the nonmonotonic dependence of the growth rate with respect to the background parameters are complementary signatures of black-hole lasing. We also identify interesting analog phenomena such as Hawking-stimulated white-hole radiation or quantum BCL-stimulated Hawking radiation. The results of this work not only are of interest for analog gravity, where they help to distinguish each phenomenon and to design experimental setups leading to a clear observation of the BHL effect, but they also open the prospect of finding applications of analog concepts in quantum technologies.

Inert interactions between randomly moving entities and spatial disorder play a crucial role in quantifying the diffusive properties of a system, with examples ranging from molecules advancing along dendritic spines to antipredator displacements of animals due to sparse vegetation. Despite the ubiquity of such phenomena, a general framework to model the movement explicitly in the presence of spatial heterogeneities is missing. Here, we tackle this challenge and develop an analytic theory to model inert particle-environment interactions in domains of arbitrary shape and dimensions. We use a discrete space formulation, which allows us to model the interactions between an agent and the environment as perturbed dynamics between lattice sites. Interactions from spatial disorder, such as impenetrable and permeable obstacles or regions of increased or decreased diffusivity, as well as many others, can be modelled using our framework. We provide exact expressions for the generating function of the occupation probability of the diffusing particle and related transport quantities such as first-passage, return, and exit probabilities and their respective means. We uncover a surprising property, the disorder indifference phenomenon of the mean first-passage time in the presence of a permeable barrier in quasi-1D systems. We demonstrate the widespread applicability of our formalism by considering three examples that span across scales and disciplines. (1) We explore an enhancement strategy of transdermal drug delivery. (2) We represent the movement decisions of an animal undergoing thigomotaxis, the tendency to remain at the peripheries of its enclosure, using a spatially disordered environment. (3) We illustrate the use of spatial heterogeneities to model inert interactions between particles by modeling the search for a promoter region on the DNA by transcription factors during gene transcription.

We investigate fundamental limits on the performance of information processing systems from the perspective of information thermodynamics. We first extend the thermodynamic uncertainty relation (TUR) to a subsystem. Specifically, for a bipartite composite system consisting of a system of interest $X$ and an auxiliary system $Y$, we show that the relative fluctuation of an arbitrary current for $X$ is lower bounded not only by the entropy production associated with $X$ but also by the information flow between $X$ and $Y$. As a direct consequence of this bipartite TUR, we prove universal tradeoff relations between the output power and efficiency of an information-thermodynamic engine in the fast relaxation limit of the auxiliary system. In this limit, we further show that the Gallavotti-Cohen symmetry is satisfied even in the presence of information flow. This symmetry leads to universal relations between the fluctuations of information flow and entropy production in the linear response regime. We illustrate these results with simple examples: coupled quantum dots and coupled linear overdamped Langevin equations. Interestingly, in the latter case, the equality of the bipartite TUR is achieved even far from equilibrium, which is a very different property from the standard TUR. Our results will be applicable to a wide range of systems, including biological systems, and thus provide insight into the design principles of biological systems.

Spin squeezing can increase the sensitivity of interferometric measurements of small signals in large spin ensembles beyond the standard quantum limit. In many practical settings, the ideal metrological gain is limited by imperfect readout of the sensor. To overcome this issue, protocols based on time reversal of unitary one-axis-twist (OAT) spin-squeezing dynamics have been proposed. Such protocols mitigate readout noise and, when implemented using cavity feedback, have been argued to also be robust against dissipation as long as the collective cooperativity of the system is sufficiently large [Davis et al., Phys. Rev. Lett. 116, 053601 (2016)]. Here, we perform a careful systematic study of dissipative effects on three different implementations of a OAT twist-untwist sensing scheme (based on symmetric as well as asymmetric cavity feedback and on a Tavis-Cummings interaction). Our full treatment shows that the three approaches have markedly different properties and resilience when subject to dissipation. Moreover, the metrological gain for an implementation using symmetric cavity feedback is more sensitive to undesired dissipation than was previously appreciated.

We study a class of system composed of interacting unicyclic machines placed in contact with a hot and cold thermal baths subjected to a nonconservative driving worksource. Despite their simplicity, these models showcase an intricate array of phenomena, including pump and heat engine regimes as well as a discontinuous phase transition. We look at three distinctive topologies: a minimal and beyond minimal (homogeneous and heterogeneous interaction structures). The former case is represented by stark different networks (“all-to-all” interactions and only a central interacting to its neighbors) and present exact solutions, whereas homogeneous and heterogeneous structures have been analyzed by numerical simulations. We find that the topology plays a major role on the thermodynamic performance for smaller values of individual energies, in part due to the presence of first-order phase transitions. Contrariwise, the topology becomes less important as individual energies increases and results are well described by a system with all-to-all interactions.

Rydberg atom arrays are among the leading contenders for the demonstration of quantum speedups. Motivated by recent experiments with up to 289 qubits [Ebadi et al., Science 376, 1209 (2022)], we study the maximum-independent-set problem on unit-disk graphs with a broader range of classical solvers beyond the scope of the original paper. We carry out extensive numerical studies and assess problem hardness, using both exact and heuristic algorithms. We find that quasiplanar instances with Union-Jack-like connectivity can be solved to optimality for up to thousands of nodes within minutes, with both custom and generic commercial solvers on commodity hardware, without any instance-specific fine-tuning. We also perform a scaling analysis, showing that by relaxing the constraints on the classical simulated annealing algorithms considered in Ebadi et al., our implementation is competitive with the quantum algorithms. Conversely, instances with larger connectivity or less structure are shown to display a time-to-solution potentially orders of magnitudes larger. Based on these results, we propose protocols to systematically tune problem hardness, motivating experiments with Rydberg atom arrays on instances orders of magnitude harder (for established classical solvers) than previously studied.

The charge-density-wave (CDW) phase in the layered transition-metal dichalcogenide ${\mathrm{VTe}}_2$ is strongly coupled to the band inversion involving vanadium and tellurium orbitals. In particular, this coupling leads to a selective disappearance of the Dirac-type states that characterize the normal phase, when the CDW phase sets in. Here, we investigate the broadband time-resolved reflectivity variations caused by collective and single-particle excitations in the CDW ground state of ${\mathrm{VTe}}_2$. With the aid of density functional perturbation theory simulations we unveil the presence of two collective amplitude modes of the CDW ground state. By applying a double-pulse excitation scheme, we show the possibility to manipulate these modes, gaining insights into the coupling between these two collective excitations and demonstrating a more efficient way to perturb the CDW phase in ${\mathrm{VTe}}_2$.

An improved particle swarm optimization algorithm is proposed, and its superiority over the standard particle swarm optimization algorithm is tested on two typical benchmark functions. By employing this algorithm to search for the magnetic ground states of the Hubbard model on the real-space square lattice with finite size based on the mean-field approximation, two new magnetic states, namely, the double-striped-type antiferromagnetic state and the triple antiferromagnetic state, are found. We further perform mean-field calculations in the thermodynamical limit to confirm that these two new magnetic states are not a result of a finite-size effect, where the properties of the double-striped-type antiferromagnetic state are also presented.

We present a quantum heat engine based on a cavity with two oscillating mirrors that confine a quantum field. The engine performs an Otto cycle during which the walls and a field mode, together representing the working substance of the engine, interact via a nonlinear Hamiltonian. Resonances between the frequencies of the cavity mode and the walls allow one to transfer heat from the hot and the cold bath by exploiting the conversion between phononic and photonic excitations. We study the time evolution of the system and show that net work can be extracted after a full cycle. We evaluate the efficiency of the process.