Motivated by the recent experimental demonstration of stacking dependent interlayer magnetic interaction [T. Song et al., Nat. Mater. 18, 1298 (2019); T. Li et al., Nat. Mater. 18, 1303 (2019); W. Chen et al., Science 366, 983 (2019)], we investigate the magnetization textures and the control possibilities in the moiré pattern formed of twisted bilayer two-dimensional (2D) magnets $\mathrm{Cr}{\mathrm{X}}_3$ ($\mathrm{X}$=Br, I). We find that the stacking dependent interlayer magnetic interaction results in the formation of periodic magnetization domains in a long-period moiré pattern. Magnetization textures with various topological numbers can be constructed, depending on the winding of the textures around the domain walls. A uniform external magnetic field competes with the lateral modulated interlayer magnetic interaction and can be utilized to tune the magnetization textures.

We present a model for implementing fast entangling gates ($\sim 1\mu \mathrm{s}$) with ultrafast pulses in arbitrarily long ion chains, that requires low numbers of pulses and can be implemented with laser repetition rates well within experimental capability. We demonstrate that we are able to optimize pulse sequences that have theoretical fidelities above $99.99\%$ in arbitrarily long ion chains, for laser repetition rates on the order of $100–300$ MHz. Notably, we find higher repetition rates are not required for gates in longer ion chains, which is in contrast to scaling analyses with other gate schemes. When pulse imperfections are considered in our calculations, we find that achievable gate fidelity is independent of the number of ions in the chain. We also show that pulse control requirements do not scale up with the number of ions. We find that population transfer efficiencies of above $99.9\%$ from individual ultrafast pulses is the threshold for realizing high-fidelity gates, which may be achievable in near-future experiments.

Topological states of matter are particularly interesting for both fundamental research and practical applications. Simulating topological phases in a quantum system is of great interest due to the ability to explore a plethora of topologically nontrivial phenomena in a controllable fashion. We propose and analyze an efficient scheme for simulating topological Zak phase in two-dimensional spin-phononic crystal networks. We show that through a specially designed periodic driving, one can selectively control and enhance the bipartite silicon-vacancy center arrays, so as to obtain chiral-symmetry-protected spin-spin couplings. More importantly, the Floquet engineering spin-spin interactions support rich quantum phases associated with topological invariants. In momentum space, we analyze and simulate the topological nontrivial properties of the one- and two-dimensional systems. As an application in quantum information processing, we study the robust quantum state transfer via topologically protected edge states. This work opens up new prospects for studying quantum acoustics and offers an experimentally feasible platform for the study of topological phases of matter.

Several important proposals to use semiconductor quantum dots in quantum information technology rely on the control of the dark exciton ground states, such as dark exciton based qubits with a microsecond lifetime. In this paper, we present an efficient way to occupy the dark exciton ground state by a single short laser pulse. The scheme is based on an optical excitation with a longitudinal field component featured by, e.g., radially polarized beams or certain Laguerre-Gauss or Bessel beams. Utilizing this component, we show within a configuration interaction approach that high-energy exciton states composed of light-hole excitons and higher dark heavy-hole excitons can be addressed. When the higher exciton relaxes, a dark exciton in its ground state is created.

We present a fully analytical description of a many-body localization (MBL) transition in a microscopically defined model. Its Hamiltonian is the sum of one- and two-body operators, where both contributions obey a maximum-entropy principle and have no symmetries except Hermiticity (not even particle number conservation). These two criteria paraphrase that our system is a variant of the Sachdev-Ye-Kitaev model. We will demonstrate how this simple zero-dimensional system displays numerous features seen in more complex realizations of MBL. Specifically, it shows a transition between an ergodic and a localized phase, and nontrivial wave-function statistics indicating the presence of nonergodic extended states. We check our analytical description of these phenomena by a parameter-free comparison to high performance numerics for systems of up to $N=15$ fermions. In this way, our study becomes a test bed for concepts of high-dimensional quantum localization, previously applied to synthetic systems such as Cayley trees or random regular graphs. The minimal model describes a many-body system for which an effective theory is derived and solved from first principles. The hope is that the analytical concepts developed in this study may become a stepping stone for the description of MBL in more complex systems.

The wave functions of a disordered two-dimensional electron gas at the quantum-critical Anderson transition are predicted to exhibit multifractal scaling in their real space amplitude. We experimentally investigate the appearance of these characteristics in the spatially resolved local density of states of the two-dimensional mixed surface alloy ${\mathrm{Bi}}_{\mathrm{x}}{\mathrm{Pb}}_{(1-\mathrm{x})}/\mathrm{Ag}\left(111\right)$, by combining high-resolution scanning tunneling microscopy with spin- and angle-resolved inverse-photoemission experiments. Our detailed knowledge of the surface alloy's electronic band structure, the exact lattice structure, and the atomically resolved local density of states enables us to construct a realistic Anderson tight binding model, and to directly compare the measured local density of states characteristics with those from our model calculations. The statistical analyses of these two-dimensional local density of states maps reveal their log-normal distributions and multifractal scaling characteristics of the underlying wave functions with a finite anomalous scaling exponent. Finally, our experimental results confirm theoretical predictions of an exact scaling symmetry for Anderson quantum phase transitions in the Wigner-Dyson classes.

The low and weakly temperature-varying lattice thermal conductivity, ${\kappa }_L\left(\mathrm{T}\right)$, in crystals with a complex unit cell such as type-I clathrates is assumed to originate from a reduced momentum and energy space available for propagative lattice vibrations, which is caused by the occurrence of low-energy optical phonon modes. In the context of ab initio self-consistent phonon (SCP) theory, it has been shown that the cubic and quartic anharmonic interactions result in a temperature-induced energy renormalization of these low-lying optical branches which contributes to the anomalous behavior of ${\kappa }_L\left(\mathrm{T}\right)$ in structurally ordered type-I clathrates [T. Tadano and S. Tsuneyuki, Phys. Rev. Lett. 120, 105901 (2018)]. By means of inelastic neutron scattering, we provide evidence for this energy renormalization in temperature, which has been resolved for transversely and longitudinally polarized phonons in the single crystal type-I clathrate ${\mathrm{Ba}}_{7.81}{\mathrm{Ge}}_{40.67}{\mathrm{Au}}_{5.33}$. By mapping the neutron intensity in the momentum space, we demonstrate the coherent character of the low-lying optical phonons. The overall phonon spectrum and dynamical structure factors are satisfactorily reproduced by ab initio harmonic calculations using density functional theory with the meta-GGA SCAN functional and a fully ordered structure. However, a polarization-dependent cutoff energy with opposing temperature shifts for longitudinal and transverse acoustic dispersions is experimentally observed which is not reproduced by the simulations. Anharmonicity affects the energies of the low-lying optical phonons in the transverse polarization, which compares quantitatively well with available results from SCP theory, whereas differences are observed for the longitudinal polarization.

Eukarya and Bacteria are evolutionarily distant domains of life, which is reflected by differences in their cellular structure and physiology. For example, Eukarya feature membrane-bound organelles such as nuclei and mitochondria, whereas Bacteria have none. The greater complexity of Eukarya renders them difficult to study from both an experimental and theoretical perspective. However, encouraged by a recent experimental result showing that budding yeast (a unicellular eukaryote) obeys the same proportionality between ribosomal proteome fractions and cellular growth rates as Bacteria, we derive a set of relations describing eukaryotic growth from first principles of ribosome biogenesis. We recover the observed ribosomal protein proportionality, and assuming that rRNA synthesis is tightly coupled to ribosomal protein synthesis as in Bacteria, we continue to obtain two growth laws for the number of RNA polymerases synthesizing ribosomal RNA per ribosome in the cell. These growth laws, in turn, reveal two invariants of eukaryotic growth, i.e., quantities predicted to be conserved by Eukarya across growth conditions. The invariants clarify the coordination of transcription and translation kinetics as required by ribosome biogenesis, and link these kinetic parameters to cellular physiology. We demonstrate the application of the relations to the yeast S. cerevisiae and find several predictions from the growth laws to be in good agreement with currently available data. The remaining relations will require additional data for verification. We outline methods to quantitatively deduce several unknown kinetic and physiological parameters based on the invariants. The analysis is not specific to S. cerevisiae and can be extended to other lower (unicellular) Eukarya when data become available. The relations may also have relevance to certain cancer cells which, like bacteria and yeast, exhibit rapid cell proliferation and ribosome biogenesis.

We present a comprehensive investigation of the evolution of helimagnetic correlations in ${\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x\mathrm{Si}$ with increasing doping. By combining polarized neutron scattering and high resolution neutron spin echo spectroscopy we investigate three samples with $x$ = 0.09, 0.11, and 0.14, i.e., with compositions on both sides of the concentration $x^{*}\sim 0.11$ where the helimagnetic Bragg peaks disappear and between $x^{*}$ and the quantum critical concentration $x_C\sim 0.17$, where $T_C$ vanishes. We find that the abrupt disappearance of the long range helical periodicity at $x^{*}$ does not affect the precursor fluctuating correlations. These build up with decreasing temperature in a similar way as for the parent compound MnSi. Also the dynamics bears strong similarities to MnSi. The analysis of our results indicates that frustration, possibly due to achiral Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, increases with increasing Fe doping. We argue that this effect explains both the expansion of the precursor phase with increasing $x$ and the abrupt disappearance of long range helimagnetic periodicity at $x^{*}$.

All three motional modes of a charged dielectric nanoparticle in a Paul trap are cooled simultaneously by direct feedback to temperatures of a few millikelvins. We test two methods, one based on electrical forces and the other on optical forces; for both methods, we find similar cooling efficiencies. Cooling is characterized for both feedback forces as a function of feedback parameters, background pressure, and the particle's position.

We introduce a new technique for the simulation of dissipative quantum systems. This method is composed of an approximate decomposition of the Lindblad equation into a Kraus map, from which one can define an ensemble of wave functions. Using principal component analysis, this ensemble can be truncated to a manageable size without sacrificing numerical accuracy. We term this method ensemble rank truncation (ERT), and find that in the regime of weak coupling, this method is able to outperform existing wave-function Monte Carlo methods by an order of magnitude in both accuracy and speed. We also explore the possibility of combining ERT with approximate techniques for simulating large systems [such as matrix product states (MPS)], and show that in many cases this approach will be more efficient than directly expressing the density matrix in its MPS form. We expect the ERT technique to be of practical interest when simulating dissipative systems for quantum information, metrology, and thermodynamics.

We report the investigation on dipolar collisions in rotational state mixtures of ultracold bosonic ${}^{23}\mathrm{Na}{}^{87}\mathrm{Rb}$ molecules. The large resonant dipole-dipole interaction between molecules in rotational states of opposite parities brings about significant modifications to their collisions, even when an electric field is not present. In this work, this effect is revealed by measuring the dramatically enhanced two-body loss rate constants in the mixtures. In addition, the dipolar interaction strength can be tuned by preparing the NaRb mixture in different rotational levels with microwave spectroscopy. When the rotational level combination is not of the lowest energy, contributions from hyperfine changing collisions are also observed. Our measured loss rate constants are in good agreement with a quantum close-coupling calculation, which we also present in full detail.

The fidelity susceptibility has been used to detect quantum phase transitions in the Hermitian quantum many-body systems over a decade, where the fidelity susceptibility density approaches $+\infty$ in the thermodynamic limits. Here the fidelity susceptibility $\chi$ is generalized to non-Hermitian quantum systems by taking the geometric structure of the Hilbert space into consideration. Instead of solving the metric equation of motion from scratch, we chose a gauge where the fidelities are composed of biorthogonal eigenstates and can be worked out algebraically or numerically when not on the exceptional point (EP). Due to the properties of the Hilbert space geometry at the EP, we found that the EP can be found when $\chi$ approaches $-\infty$. As examples, we investigate the simplest $\mathcal{PT}$ symmetric $2\times 2$ Hamiltonian with a single tuning parameter and the non-Hermitian Su-Schriffer-Heeger model.

It is often assumed that adding losses to a resonant system necessarily broadens the width of the resonance. Here, the scattering spectrum of gold nanoparticles in P3HT (strongly absorbing) and PMMA (low loss) were measured using dark field microscopy and surprisingly, the linewidth was halved in the lossy medium. To gain physical insight into this unusual result, a Green function analysis was employed, and the roles of the permittivity, both magnitude and dispersion, in reducing the linewidth when material losses are increased were made clear. This surprising route to narrower linewidths is intriguing for nanoplasmonic applications including sensing, spectroscopy, imaging and nanolasing and also more generally for resonant systems where loss is conventionally avoided.

Globally coupled ensembles of phase oscillators serve as useful tools for modeling synchronization and collective behavior in a variety of applications. As interest in the effects of simplicial interactions (i.e., nonadditive, higher-order interactions between three or more units) continues to grow, we study an extension of the Kuramoto model where oscillators are coupled via three-way interactions that exhibits novel dynamical properties including clustering, multistability, and abrupt desynchronization transitions. Here we provide a rigorous description of the stability of various multicluster states by studying their spectral properties in the thermodynamic limit. Not unlike the classical Kuramoto model, a natural frequency distribution with infinite support yields a population of drifting oscillators, which in turn guarantees that a portion of the spectrum is located on the imaginary axes, resulting in neutrally stable or unstable solutions. On the other hand, a natural frequency distribution with finite support allows for a fully phase-locked state, whose spectrum is real and may be linearly stable or unstable.

Parametric coupling is commonly used to prepare oscillators in a time-varying squeezed state in which the variance of the coordinate or momentum may dip below that of the equilibrium thermal or quantum ground state during a fraction of the period. We show here that pulses applied to drive parametrically an inhomogeneously broadened set of harmonic oscillators result in a spontaneous recovery of coherence, manifesting itself as an echo, similar to those exhibited by an ensemble of spins aligned by a magnetic field, when excited by properly designed electromagnetic pulses. Such echoes, of classical or quantum nature, are expected to arise in the squeezing of linear systems of various sorts and, in particular, light and vibrational modes. Unlike existing methods for the generation of spin echoes, squeezed echoes do not require the use of resonant excitation.

Recent theoretical and experimental research has explored magnetic solitons in binary Bose-Einstein condensates. Here we demonstrate that such solitons are part of an SO(3) soliton family when embedded within a full three-component spin-1 manifold with spin-rotational symmetry. To showcase this, we have experimentally created a type of domain wall magnetic soliton obtained by ${90}^{\circ }$ rotations, which consists of a boundary between easy-axis and easy-plane polar phases. Collisions between SO(3) solitons are investigated by numerically solving the Gross-Pitaevskii equations, which exhibit novel properties, including rotation and dissipation of soliton spin polarization.

We investigate how the kinetic energy acquired by a dense granular system driven by an external vibration depends on the input energy. Our focus is on the dependence of the granular behavior on two main parameters: frequency and vibration amplitude. We find that there exists an optimal forcing frequency at which the system reaches the maximal kinetic energy: if the input energy is increased beyond this threshold, the system dissipates more and more energy and recovers a colder and more viscous state. Quite surprisingly, the nonmonotonic behavior is found for vibration amplitudes which are sufficiently low to keep the system always in contact with the driving oscillating plate. Studying dissipative properties of the system, we unveil a striking difference between this nonmonotonic behavior and a standard resonance mechanism. This feature is also observed at the microscopic scale of the single-grain dynamics and can be interpreted as an instance of negative specific heat. An analytically solvable model based on a generalized forced-damped oscillator well reproduces the observed phenomenology, illustrating the role of the competing effects of forcing and dissipation.

We put forward a versatile, highly scalable, tunable electronic platform for the simulation of single-excitation quantum transport phenomena. Our system, comprising 10 state-of-the-art, fully reconfigurable electronic oscillators, is implemented by making use of functional blocks synthesized with operational amplifiers and passive linear electrical components. To test the robustness and precise control of our platform, we simulate different quantum transport protocols, such as the ballistic propagation of a single-excitation wave function in an ordered lattice, and its localization due to disorder. We implement the Su-Schrieffer-Heeger model to directly observe the emergence of topologically protected one-dimensional edge states. Furthermore, we present the realization of the so-called perfect transport protocol, a key milestone for the development of scalable quantum computing and communication. Finally, we show a simulation of the exciton dynamics in the B800 ring of the purple bacteria LH2 complex. The high fidelity of our simulations together with the low decoherence of our device make it a robust, versatile, and promising platform for the simulation of quantum transport phenomena.

Synchronization, cooperation, and chaos are ubiquitous phenomena in nature. In a population composed of many distinct groups of individuals playing the prisoner's dilemma game, there exists a migration dilemma: No cooperator would migrate to a group playing the prisoner's dilemma game lest it should be exploited by a defector; but unless the migration takes place, there is no chance of the entire population's cooperator fraction to increase. Employing a randomly rewired coupled map lattice of chaotic replicator maps, modeling replication-selection evolutionary game dynamics, we demonstrate that the cooperators—evolving in synchrony—overcome the migration dilemma to proliferate across the population when altruism is mildly incentivized making few of the demes play the leader game.