Ladder models of ultracold atoms offer a versatile platform for the experimental and theoretical study of different phenomena and phases of matter linked to the interplay between artificial gauge fields and interactions. Strongly correlated helical states are known to appear for specific ratios of the particle and magnetic flux densities, and they can often be interpreted as a one-dimensional limit of fractional quantum Hall states, thus being called pretopological. Their signatures, however, are typically hard to observe due to the small gaps characterizing these states. Here we investigate bosonic ladder models at filling factor $\nu =1$. Based on bosonization, renormalization group, and matrix product state simulations we pinpoint two strongly correlated helical phases appearing at this resonance. We show that one of them can be accessed in systems with two-species hardcore bosons and on-site repulsions only, thus amenable for optical lattice experiments. Its signatures are sizable and stable over a broad range of parameters for realistic system sizes.

Accurate information transfer is essential for biological, social, and technological networks. Computing transfer entropy (TE), a measure of information flow, usually relies on numerical methods even in small networks, which obscures the origin of accurate information transfer. In this study, we establish a diagrammatic formula that analytically computes TE for a stochastic Boolean network, where the intermediate network from signal to output can have arbitrary topology and nonidentical Boolean functions. By expressing TE in terms of network components, we elucidate the mechanism of information flow and provide optimal design principles of network architectures applicable to real networks.

For systems with spatial and nonspatial symmetries, the topological classification depends not only on these symmetries but also on the commutation/anticommutation relations between spatial and nonspatial symmetries. The coexistence of spatial and nonspatial symmetries together with appropriate commutation/anticommutation relations between them can give rise to crystalline and higher-order topological phases, which host gapless boundary modes. Alternatively, space-time symmetries in a Floquet system can take the role of spatial symmetries in deciding the topological classification. Promoting a spatial symmetry to a space-time symmetry can alter the commutation relations, which in turn can modify the topological properties of the system. We show how a coherently excited phonon mode can be used to promote a spatial symmetry with which the static system is always trivial to a space-time symmetry which supports a nontrivial Floquet topological phase. We demonstrate this effect by considering two systems: The first is a second-order topological superconductor, and the second is a first-order crystalline topological insulator. In both these cases, a coherently excited phonon mode is responsible for promoting the reflection symmetry to a time-glide symmetry. This newly introduced symmetry allows the previously trivial system to host gapless modes. In the first case, these are protected corner modes, while in the second case, these are gapless edge modes.

Modern electronic systems operate in complex electromagnetic environments and must handle noise and unwanted coupling. The capability to isolate or reject unwanted signals for mitigating vulnerabilities is critical in any practical application. In this work, we describe the use of a binary programmable metasurface to (i) control the spatial degrees of freedom for waves propagating inside an electromagnetic cavity and demonstrate the ability to create nulls in the transmission coefficient between selected ports, and (ii) create the conditions for coherent perfect absorption. Both objectives are performed at arbitrary frequencies. In the first case, an effective optimization algorithm is presented that selectively generates cold spots over a single-frequency band or simultaneously over multiple-frequency bands. We show that this algorithm is successful with multiple input port configurations and varying optimization bandwidths. In the second case, we establish how this technique can be used to establish a multiport coherent perfect absorption state for the cavity.

Majorana fermions act as their own antiparticle, and they have long been thought to be confined to the realm of pure theory. However, interest in them has recently resurfaced, as it was realized through the work of Kitaev that some experimentally accessible condensed matter systems can host these exotic excitations as bound states on the boundaries of one-dimensional chains, and that their topological and non-Abelian nature holds promise for quantum computation. Unambiguously detecting the experimental signatures of Majorana bound states has turned out to be challenging, as many other phenomena lead to similar experimental behavior. Here, we computationally study a ring comprised of two Kitaev model chains with tunnel coupling between them, where an applied magnetic field allows for Aharonov-Bohm interference in transport through the resulting ring structure. We use a nonequilibrium Green's function technique to analyze the transport properties of the ring in both the presence and absence of Majorana zero modes. Further, we show that these results are robust against weak disorder in the presence of an applied magnetic field. This computational model suggests another signature for the presence of these topologically protected bound states can be found in the magnetic field dependence of devices with loop geometries.

Although it is well documented that the vortex core in a micromagnet can switch polarization in response to magnetic field pulses, the temperature dependence of this process has not yet been addressed by experiments. Using scanning transmission x-ray microscopy, we investigate the magnetic vortex dynamics in a ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ microplatelet at temperatures ranging from $150\mathrm{K}$ up to $T_{\mathrm{C}}$ at $350\mathrm{K}$. The time-resolved images reveal qualitatively different dynamic regimes as a function of temperature and applied field, as the relative strengths of the micromagnetic energy terms strongly vary over this temperature range. By explicitly accounting for the temperature dependence of the magnetic parameters in our micromagnetic simulations, we found excellent agreement with the experiments over the full measurement range. In line with previous models, the simulations reveal that the vortex core switches polarization when it reaches a critical velocity that mainly depends on the strength of the exchange interaction. It is therefore strongly temperature dependent, thus explaining our observation of the different dynamical regimes.

In filled-skutterudite compounds, guest atoms are centered at highly symmetric cages essentially formed by pnictogens. The weak bonding of the guest atoms to the cage-forming elements is expected to give rise to local atomic vibrations in anharmonic potential which can influence electronic properties and may even promote superconductivity. However, the relation between the unusual low-energy phonon excitations and the electronic properties is still elusive, and evidence establishing a direct link between them is a long-standing issue. Here, we investigate the impact of artificial atomic defects introduced by electron irradiation in the filled-skutterudite superconductor ${\mathrm{LaRu}}_4{\mathrm{As}}_{12}$. Our high-resolution heat capacity measurements reveal that the electronic specific heat is substantially reduced with increasing concentration of atomic defects. Moreover, the irradiation suppresses the fully gapped $s$-wave superconductivity as well as the contribution of anharmonic phonons in the specific heat in a correlated fashion. Our findings imply that the anharmonic phonons play an important role in enhancing electronic specific heat and superconductivity.

Encapsulating graphene in hexagonal boron nitride (hBN) has several advantages: The highest mobilities reported to date are achieved in this way, and precise nanostructuring of graphene becomes feasible through the protective hBN layers. Nevertheless, subtle effects may arise due to the differing lattice constants of graphene and hBN, and due to the twist angle between the graphene and hBN lattices. Here, we use a recently developed model which allows us to perform band structure and magnetotransport calculations of such structures, and show that with a proper account of the moiré physics an excellent agreement with experiments can be achieved, even for complicated structures such as disordered graphene, or antidot lattices on a monolayer hBN with a relative twist angle. Calculations of this kind are essential to a quantitative modeling of twistronic devices.

Certain lattices with specific geometries have one or more spectral bands that are strictly flat, i.e., the electron energy is independent of the momentum. This can occur robustly irrespective of the specific couplings between the lattices sites due to the lattice symmetry, or it can result from fine-tuned couplings between the lattice sites. While the theoretical picture behind flat electronic bands is well developed, experimental realization of these lattices has proven challenging. Utilizing scanning tunneling microscopy (STM) and spectroscopy (STS), we manipulate individual vacancies in a chlorine monolayer on Cu(100) to construct various atomically precise one-dimensional (1D) lattices with engineered flat bands. We realize experimentally both gapped and gapless flat band systems with single or multiple flat bands. We also demonstrate tunability of the energy of the flat bands and how they can be switched on and off by breaking and restoring the symmetry of the lattice geometry. The experimental findings are corroborated by tight-binding calculations. Our results constitute the first experimental realizations of engineered flat bands in a 1D solid-state system and pave the way toward the construction of, e.g., topological flat band systems and experimental tests of flat-band-assisted superconductivity in a fully controlled system.

We numerically study the effects of nonmagnetic impurities (vacancies) in the spin-$S$ Heisenberg antiferromagnet on the kagome lattice. For a range of low but nonzero temperatures, and spin values that extend down to $S=2$, we find that the magnetization response to an external magnetic field is consistent with the response of emergent “half-orphan” degrees of freedom that are expected to dominate the response of the corresponding classical magnet in a similar temperature range whenever there are two vacancies on the same triangle. Specifically, for all spin values we have considered (from $S=1/2$ to 4), there is a large enhancement of the local susceptibility of the lone spin on such a triangle with two vacancies; in the presence of a uniform magnetic field $h$, this lone-spin behaves effectively as an almost free spin $S$ in an effective field $h/2$. Quite remarkably, in the zero-temperature limit, the ground state in the presence of a half-orphan has a nonzero total spin value $S_{\text{GS}}$ that shows a trend similar to $S/2$ when $S\ge 2$. These qualitative aspects of the response differ strikingly from the more conventional response of diluted samples without such half-orphan degrees of freedom. We discuss how these findings could be checked experimentally.

Motivated by recent experimental and numerical progress on various cluster Mott insulators, we study an extended Hubbard model on a breathing kagome lattice with a single electron orbital and $1/6$ electron filling. Two distinct types of cluster localization are found in the cluster Mott regime due to the presence of the electron repulsion between neighboring sites, rather than from the on-site Hubbard interaction in the conventional Mott insulators. We introduce a unified parton construction framework to accommodate both types of cluster Mott insulating phase as well as a trivial Ferm liquid metal and discuss the phase transitions in the phase diagram. It is shown that, in one of the cluster localization phases, the strong intersite repulsion results into locally metallic behavior within one of two triangular clusters on the breathing kagome lattice. We further comment on experimental relevance to existing Mo-based cluster magnets.

We show that belief propagation combined with ordered statistics post-processing is a general decoder for quantum low-density parity-check codes constructed from the hypergraph product. To this end, we run numerical simulations of the decoder applied to three families of hypergraph product code: topological codes, fixed-rate random codes, and a new class of codes that we call semitopological codes. Our new code families share properties of both topological and random hypergraph product codes, with a construction that allows for a finely controlled trade-off between code threshold and stabilizer locality. Our results indicate thresholds across all three families of hypergraph product code, and provide evidence of exponential suppression in the low error regime. For the toric code, we observe a threshold in the range $9.9\%\pm 0.2\%$. This result improves upon previous quantum decoders based on belief propagation, and approaches the performance of the minimum-weight perfect-matching algorithm. We expect semitopological codes to have the same threshold as toric codes, as they are identical in the bulk, and we present numerical evidence supporting this observation.

We study entanglement dynamics in a dispersive optomechanical system consisting of two optical modes and a mechanical oscillator inside an optical cavity. The two optical modes interact with the mechanical oscillator, but not directly with each other. The appearance of optical entanglement witnesses nonclassicality of the oscillator. We study the dependence of the entanglement dynamics on the optomechanical coupling, the mean photon number in the cavity, and the oscillator temperature. An experimental realization with ultracold atomic ensembles is proposed.

An understanding of the origins of giant magnetostructural coupling is developed for the compound MnAs, a magnetic material that has served as a prototype for many promising technologies including caloric refrigeration, magnetic actuation, and spintronics. We demonstrate that strong coupling between magnetism and crystal structure arises from an orbital-specific competition between exchange energies and kinetic (bonding) energies and that thermally activated spin fluctuations drive the unusual first-order phase transition from high to low symmetry upon heating. The underlying mechanism raises the prospect of an exotic paramagnetic state featuring local fluctuations in atomic positions and bonding on the timescale of the moment fluctuations. The results should inform the design of new materials with enhanced magnetostructural coupling, found at the border between structural and magnetic stability.

We study the quantum Zeno effect (QZE) induced by continuous partial measurement in the presence of short-correlated noise in the system Hamiltonian. We study the survival probability and the onset of the QZE as a function of the measurement strength and find that, depending on the noise parameters, the quantum Zeno effect can be enhanced or suppressed by the noise in different regions of parameter space. Notably, the conditions for the enhancement of the QZE are different when determined by the short-time or long-time behavior of the survival probability, or by the measurement strength marking the onset of the quantum Zeno regime.

We employ reservoir engineering technique to build an artificial thermal bath at arbitrary (effective) negative and positive temperatures for a single spin system. The required interaction engineering is achieved by applying a specific sequence of radio-frequency pulses using nuclear magnetic resonance, while the temperature of the bath can be controlled by appropriate preparation of the initial ${}^1\mathrm{H}$ nuclear spin state. This artificial reservoir allowed us to implement a single qubit quantum engine that operates at a single reservoir at effective negative temperature and with maximum efficiency, independent of the unitary transformation performed on the qubit system, as long as it changes the qubit state. We measured the population of the carbon spin and the efficiency of our quantum engine, which are in very good agreement with the predicted results.

The absorption of traveling photons resonant with electric dipole transitions of an atomic gas naturally leads to electric dipole spin-wave excitations. For a number of applications, it would be highly desirable to shape and coherently control the spatial waveform of the spin waves before spontaneous emission can occur. This paper details a recently developed optical control technique to achieve this goal, where counterpropagating, shaped subnanosecond pulses impart subwavelength geometric phases to the spin waves by cyclically driving an auxiliary transition. In particular, we apply this technique to reversibly shift the wave vector of a spin wave on the $D2$ line of laser-cooled ${}^{87}\mathrm{Rb}$ atoms by driving an auxiliary $D1$ transition with shape-optimized pulses, so as to shut off and recall superradiance on demand. We investigate a spin-dependent momentum transfer during the spin-wave control process, which leads to a transient optical force as large as $\sim 1\hslash k$/ns, and study the limitations to the achieved $70\sim 75\%$ spin-wave control efficiency by jointly characterizing the spin-wave control and matter-wave acceleration. Aided by numerical modeling, we project potential future improvements of the control fidelity to the $99\%$ level when the atomic states are better prepared and by equipping a faster and more powerful pulse shaper. Our technique also enables a background-free measurement of the superradiant emission to unveil the precise scaling of the emission intensity and decay rate with optical depth.

Liquid-liquid phase separation has emerged as one of the important paradigms in the chemical physics as well as biophysics of charged macromolecular systems. We elucidate an equilibrium phase separation mechanism based on charge regulation, i.e., protonation-deprotonation equilibria controlled by pH, in an idealized macroion system which can serve as a proxy for simple coacervation. First, a low-density density functional calculation reveals the dominance of two-particle configurations coupled by ion adsorption on neighboring macroions. Then a binary cell model, solved on the Debye-Hückel as well as the full nonlinear Poisson-Boltzmann level, unveils the charge symmetry breaking as inducing the phase separation between low- and high-density phases as a function of pH. These results can be identified as a charge symmetry broken complex coacervation between chemically identical macroions.

When light is incident on a medium with a spatially disordered index of refraction, interference effects lead to near-perfect reflection when the number of dielectric interfaces is large, so that the medium becomes a “transparent mirror.” We investigate the analog of this effect for electrons in twisted bilayer graphene (TBG), for which local fluctuations of the twist angle give rise to a spatially random Fermi velocity. In a description that includes only spatial variation of Fermi velocity, we derive the incident-angle-dependent localization length for the case of quasi-one-dimensional disorder by mapping this problem onto a one-dimensional Anderson localization. The localization length diverges at normal incidence as a consequence of Klein tunneling, leading to a power-law decay of the transmission when averaged over incidence angle. In a minimal model of TBG, the modulation of twist angle also shifts the location of the Dirac cones in momentum space in a way that can be described by a random gauge field, and thus Klein tunneling is inexact. However, when the Dirac electron's incident momentum is large compared to these shifts, the primary effect of twist disorder is only to shift the incident angle associated with perfect transmission away from zero. These results suggest a mechanism for disorder-induced collimation, valley filtration, and energy filtration of Dirac electron beams, so that TBG offers a promising new platform for Dirac fermion optics.