Using multidimensional particle-in-cell simulations, we demonstrate the generation of $>30\mathrm{GV}{\mathrm{m}}^{-1}$ terahertz (THz) fields by relativistic, few-femtosecond laser pulses interacting with gas plasmas embedded in $>100\mathrm{T}$ magnetic fields parallel to the laser path. Parameter scans in the laser and magnetic field strengths predict laser-to-THz conversion efficiencies in excess of $2\times {10}^{-3}$ in the highly relativistic, strong-magnetization regime. The simulation results are interpreted in light of a perturbative quasistatic model, revealing that the dominant azimuthal THz field is driven by transverse gradients in the longitudinal laser wakefield. Generation of an additional, radially polarized THz field is also foreseen at high magnetization levels as confirmed by numerical simulations.

Recently, unconventional materials (or obstructed atomic insulators) have attracted much attention owing to the unconventional feature of mismatch between Wannier centers and atomic positions. In this paper, we demonstrate that the trigonal selenium and tellurium host an unconventional nature in both electronic and phonon spectra. In electronic band structures, the band representation (BR) decomposition for occupied bands has to contain the essential BR of $A@3b$, and the real-space invariant is ${\delta }_1@3b=-1$. The $\pi$ Zak phase suggests that the one-dimensional Se/Te chain is a chiral Su-Schrieffer-Heeger chain. The effective magnetism can be induced by $p$ states at ends. More importantly, a large shift current is obtained in the Se quantum well. In addition, in phonon spectra, three sets of phonon bands are well separated and assigned to $B@3b, B@3a$, and $A@3b$ BRs, respectively. Thus, the obstructed phonon states are predicted on the (0001) surface. As the prototypes of unconventional materials in both electronic and phonon spectra, our findings could create much interest in the study of obstructed surface electronic and phonon states in these novel materials.

Quantum walks underlie an important class of quantum computing algorithms, and represent promising approaches in various simulations and practical applications. Here we design stroboscopically monitored quantum walks and their subsequent graphs that can naturally boost target searches. We show how to construct walks with the property that all the eigenvalues of the non-Hermitian survival operator, describing the mixed effects of unitary dynamics and the back-action of measurement, coalesce to zero, corresponding to an exceptional point whose degree is the size of the system. Generally, the resulting search is guaranteed to succeed in a bounded time for any initial condition, which is faster than classical random walks or quantum walks on typical graphs. We then show how this efficient quantum search is related to a quantized topological winding number and further discuss the connection of the problem to an effective massless Dirac particle.

We consider the propagation of light in arbitrarily curved step-index optical fibers. Using a multiple-scales approximation scheme, set up in Fermi normal coordinates, the full vectorial Maxwell equations are solved in a perturbative manner. At leading order, this provides a rigorous derivation of Rytov's law. At next order, we obtain nontrivial dynamics of the electromagnetic field, characterized by two coupling constants, the phase and the polarization curvature moments, which describe the curvature response of the light's phase and its polarization vector, respectively. The latter can be viewed as an inverse spin Hall effect of light, where the direction of propagation is constrained along the optical fiber and the polarization evolves in a frequency-dependent way.

The temperature dependence of mesoscopic spin-model parameters is derived in two-sublattice antiferromagnetically aligned systems based on Green's function theory. It is found that transversal spin correlations decrease the anisotropy terms while increasing the Heisenberg and Dzyaloshinsky–Moriya exchange interactions and the latter's contribution to the anisotropy. The obtained temperature dependences show quantitative agreement with the results for ferromagnets, and they also agree well with numerical atomistic simulations which treat the spin correlations without approximations. Possible applications of the results in multiscale modeling are discussed.

The in-plane (thermal) Hall effect is an unconventional transverse response when the applied magnetic field is in the (heat) current plane. In contrast to the normal Hall effect, the in-plane Hall effect requires the absence of certain crystal symmetries, and possibly manifests a nontrivial topology of quantum materials. An accurate estimation of the intrinsic in-plane (thermal) Hall conductivity is crucial to identify the underlying mechanisms as in the case of the Kitaev spin-liquid candidate $\alpha \text{−}{\mathrm{RuCl}}_3$. Here, we give the symmetry conditions for the in-plane Hall effect and discuss the implications that may impede the experimental evaluation of the in-plane Hall conductivity within the single-device measurement. First, the lack of symmetry in crystals can create merohedral twin domains that cancel the total Hall signal. Second, even in a twin-free crystal, the intrinsic response is potentially contaminated by the out-of-plane conduction in three-dimensional systems, which is systematically unavoidable in the in-plane Hall systems. Third, even in a quasi-two-dimensional system, the conversion of (thermal) resistivity $\widehat{\rho }$ ($\widehat{\lambda }$) to (thermal) conductivity $\widehat{\sigma }$ ($\widehat{\kappa }$) requires protocols beyond the widely-used simplified formula ${\sigma }_{xy}={\rho }_{yx}/({\rho }_{xx}^2+{\rho }_{yx}^2)$ (${\kappa }_{xy}={\lambda }_{yx}/({\lambda }_{xx}^2+{\lambda }_{yx}^2)$) due to the lack of in-plane-rotational symmetry. In principle, two independent sample devices are necessary to accurately estimate the ${\sigma }_{xy}$ (${\kappa }_{xy}$). As a case study, we discuss the half-integer quantization of the in-plane thermal Hall effect in the spin-disordered state of $\alpha \text{−}{\mathrm{RuCl}}_3$. For an accurate measurement of the thermal Hall effect, it is necessary to avoid crystals with the merohedral twins contributing oppositely to ${\kappa }_{xy}$, while the out-of-plane transport may have a negligible effect. To deal with the field-induced rotational-symmetry breaking, we propose two symmetry-based protocols, improved single-device and two-device methods. The considerations in the paper are generally applicable to a broad class of materials and provide a useful starting point for understanding the unconventional aspects of the in-plane Hall effect.

The type-II Weyl semimetal (WSM) $\beta \text{-W}{\mathrm{P}}_2$ was recently demonstrated as a supreme quantum material with various interestingtransport properties. Toward deciphering these peculiar transport phenomena for future topological device applications, it is highly necessary to understand the dynamical process of phonon scatterings under nonequilibrium conditions. By performing temperature-dependent coherent phonon spectroscopy in $\beta \text{-W}{\mathrm{P}}_2$, we provide here direct experimental evidence that phonons decaying into electron-hole pairs overwhelm the anharmonic phonon-phonon scattering process. Remarkably, the dominating phonon-electron coupling in $\beta \text{-W}{\mathrm{P}}_2$ is further supported by observations of the transient Fano resonance. First-principles calculations suggest that the anomalous phonon decaying results from the acoustic bunching and the limited phonon density of states in $\beta \text{-W}{\mathrm{P}}_2$. Our real-time observations provide dynamical insight into quasiparticle scatterings that are of vital importance for fundamental transport properties in $\beta \text{-W}{\mathrm{P}}_2$.

Given the recent developments in quantum techniques, modeling the physical Hamiltonian of a target quantum many-body system is becoming an increasingly practical and vital research direction. Here, we propose an efficient strategy combining maximum-likelihood estimation, gradient descent, and quantum many-body algorithms. Given the measurement outcomes, we optimize the target model Hamiltonian and density operator via a series of descents along the quantum likelihood gradient, which we prove is negative semidefinite with respect to the negative-log-likelihood function. In addition to such optimization efficiency, our maximum-likelihood-estimate Hamiltonian learning respects the locality of a given quantum system, therefore, extends readily to larger systems with available quantum many-body algorithms. Compared with previous approaches, it also exhibits better accuracy and overall stability toward noises, fluctuations, and temperature ranges, which we demonstrate with various examples.

Numerical simulations of quantum spin models are crucial for a profound understanding of many-body phenomena in a variety of research areas in physics. An outstanding problem is the availability of methods to tackle systems that violate area laws of entanglement entropy. Such scenarios cover a wide range of compelling physical situations including disordered quantum spin systems among others. In this paper, we employ a numerical technique referred to as multilayer multiconfiguration time-dependent Hartree (ML-MCTDH) to evaluate the ground state of several disordered spin models. ML-MCTDH has previously been used to study problems of high-dimensional quantum dynamics in molecular and ultracold physics but is here applied to study spin systems. We exploit the inherent flexibility of the method to present results in one and two spatial dimensions and treat challenging setups that incorporate long-range interactions as well as disorder. Our results suggest that the hierarchical multilayering inherent to ML-MCTDH allows to tackle a wide range of quantum many-body problems such as spin dynamics of varying dimensionality.

${}^{229}\mathrm{Th}$ with a low-lying nuclear isomeric state is an essential candidate for a nuclear clock as well as many other applications. Laser excitation of the isomeric state has been a long-standing goal. With relativistic ${}^{229}\mathrm{Th}$ ions in storage rings, high-power lasers with wavelengths in the visible range or longer can be used to achieve high excitation rates of ${}^{229}\mathrm{Th}$ isomers. This can be realized through direct resonant excitation or excitation via an intermediate nuclear or electronic state, facilitated by the tunability of both the laser-beam and ion-bunch parameters. Unique opportunities are offered by highly charged ${}^{229}\mathrm{Th}$ ions due to the nuclear-state mixing. The significantly reduced isomeric-state lifetime corresponds to a much higher excitation rate for direct resonant excitation. Importantly, we propose electric dipole transitions changing both the electronic and nuclear states that are opened by the nuclear hyperfine mixing. We suggest using them for efficient isomer excitation in Li-like ${}^{229}\mathrm{Th}$ ions, via stimulated Raman adiabatic passage or single-laser excitation. We also propose schemes for probing the isomers, utilizing nuclear radiative decay or laser spectroscopy on electronic transitions, through which the isomeric-state energy can be determined with an orders-of-magnitude higher precision than the current value. The schemes proposed here for ${}^{229}\mathrm{Th}$ could also be adapted to low-energy nuclear states in other nuclei, such as ${}^{229}\mathrm{Pa}$.

We report the observation of the frequency shift of modulation instability sidelobes due to the synchronization mismatch between the pulsed pump period and the resonator's round-trip time. We show that a very small synchronization mismatch leads to a significant frequency shift of the modulation instability sidelobes. We develop a theory originating from absolute and convective instabilities to confirm experimental observations and numerical simulations. The temporal drifts of the Turing patterns induced by the synchronization mismatch are measured in real time using a time-lens system and confirm theoretical predictions.

Differential-phase-shift (DPS) quantum key distribution (QKD) is one of the major QKD protocols that can be implemented with a simple setup using a laser source and a passive detection unit. Recently, an information-theoretic security proof of this protocol was established by Mizutani et al. [npj Quantum Inf. 5, 87 (2019)], assuming an infinitely large number of emitted pulses. To implement the DPS protocol in a real-life world, it is indispensable to analyze the security with the finite number of emitted pulses. The extension of the security proof to the finite-size regime requires the accommodation of the statistical fluctuations to determine the amount of privacy amplification. In doing so, Azuma's inequality is often employed, but unfortunately we show that in the case of the DPS protocol, this results in a substantially low key rate. This low key rate is due to a loose estimation of the sum of probabilities regarding three-photon emission whose probability of occurrence is very small. The main contribution of our work is to show that this obstacle can be overcome by exploiting the recently found novel concentration inequality, Kato's inequality. As a result, the key rate of the DPS protocol is drastically improved. For instance, assuming typical experimental parameters, a 3-Mbit secret key can be generated over 77 km for 8.3 hours, which shows the feasibility of DPS QKD under a realistic setup.

The ability to efficiently produce and manipulate nonclassical states of light is a critical requirement for the development of quantum optical technologies. In recent years, experiments have demonstrated that cascaded spontaneous parametric down-conversion is a promising approach to implement photon precertification, providing a way to overcome photon transmission losses for quantum communication, as well as to directly produce entangled three-photon states and heralded Bell pairs. However, the low efficiency of this process has so far limited its applicability beyond basic experiments. Here, we propose a scheme to amplify triplet production rates by using a fast switch and a delay loop to reuse photons that fail to convert on the first pass through the cascade's second nonlinear crystal. We construct a theoretical model to predict amplification rates and verify them in an experimental implementation. Our proof-of-concept device increases the rate of detected photon triplets as predicted, demonstrating that the method has the potential to dramatically improve the usefulness of cascaded down-conversion for device-independent quantum communication and entangled state generation.

We propose to use a buckled plate as a qubit, where a double-well potential is mechanically produced by pushing the plate from both sides. The right and left positions of the plate are assigned to be quantum states $|0\rangle$ and $|1\rangle$. The not gate is executed by changing the buckling force acting on the plate, while the Pauli-$Z$ gate and the phase-shift gate are executed by applying an electric field. A two-qubit phase-shift gate is materialized with the use of an electrostatic potential. They constitute a set of universal quantum gates. We estimate the relaxation time and the coherence time. We also discuss the fidelity of each gate operation. An examination of sample parameters leads to a feasibility of a nano-electro-mechanical-system-based quantum computer.

We study the emergence of a giant component in a spatial network where the nodes form a fractal set, and the interaction between the nodes has a long-range power-law behavior. The nodes are positioned in the metric space using a Lèvy flight procedure, with an associated scale-invariant step probability density function, that is then followed by a process of connecting each pair of nodes with a probability that depends on the distance between them. Since the nodes are positioned sequentially, we are able to calculate the probability for an edge between any two nodes in terms of their indexes and to map the model to the problem of percolation in a one-dimensional lattice with long-range interactions. This allows the identification of the conditions for which a percolation transition is possible. The system is characterized by two control parameters which determine the fractal dimension of the nodes and the power law decrease of the probability of a bond with the distance between the nodes. The competition between these two parameters forms an intricate phase diagram, which describes when the system has a stable giant component, and when percolation transitions occur. Understanding the structure of this class of spatial networks is important when analyzing real systems, which are frequently heterogeneous and include long-range interactions.

We unveil a duality that extends the notions of both flux attachment and statistical transmutation in spacetime dimensions beyond ($2+1$)D. Thus, a quantum system in arbitrary dimensions can experience a modification of its statistical properties if coupled to a certain gauge field. For instance, a bosonic quantum fluid can feature composite fermionic (or anyonic) excitations when coupled to a statistical gauge field. We compute the explicit form of the aforementioned synthetic gauge fields in $\text{D}\le 3+1$. We introduce a bosonic liquid and its composite dual in ($1+1$)D as proof of principle. We recover well-known results, resolve old controversies, and suggest a microscopic mechanism for the emergence of such a gauge field. We also outline potential directions for experimental realizations in ultracold atom platforms.

We derived a new speed limit in population dynamics, which is a fundamental limit on the evolutionary rate. By splitting the contributions of selection and mutation to the evolutionary rate, we obtained the new bound on the speed of arbitrary observables, named the selection bound, that can be tighter than the conventional Cramér-Rao bound. Remarkably, the selection bound can be much tighter if the contribution of selection is more dominant than that of mutation. This tightness can be geometrically characterized by the correlation between the observable of interest and the growth rate. We also numerically illustrate the effectiveness of the selection bound in the transient dynamics of evolutionary processes and discuss how to test our speed limit experimentally.

Discovering the governing equations of evolving systems from available observations is essential and challenging. In this paper, we consider a different scenario: discovering governing equations from streaming data. Current methods struggle to discover governing differential equations with considering measurements as a whole, leading to failure to handle this task. We propose an online modeling method capable of handling samples one by one sequentially by modeling streaming data instead of processing the entire data set. The proposed method performs well in discovering ordinary differential equations and partial differential equations from streaming data. Evolving systems are changing over time, which invariably changes with system status. Thus finding the exact change points is critical. The measurement generated from a changed system is distributed dissimilarly to before; hence the difference can be identified by the proposed method. Our proposal is competitive in identifying the change points and discovering governing differential equations in three hybrid systems and two switching linear systems.