Getting control over the valency of colloids is not trivial and has been a long-desired goal for the colloidal domain. Typically, tuning the preferred number of neighbors for colloidal particles requires directional bonding, as in the case of patchy particles, which is difficult to realize experimentally. Here, we demonstrate a general method for creating the colloidal analogs of molecules and other new regular colloidal clusters without using patchiness or complex bonding schemes (e.g., DNA coating) by using a combination of long-ranged attractive and repulsive interactions between oppositely charged particles that also enable regular clusters of particles not all in close contact. We show that, due to the interplay between their attractions and repulsions, oppositely charged particles dispersed in an intermediate dielectric constant ($4<ϵ<10$) provide a viable approach for the formation of binary colloidal clusters. Tuning the size ratio and interactions of the particles enables control of the type and shape of the resulting regular colloidal clusters. Finally, we present an example of clusters made up of negatively charged large and positively charged small satellite particles, for which the electrostatic properties and interactions can be changed with an electric field. It appears that for sufficiently strong fields the satellite particles can move over the surface of the host particles and polarize the clusters. For even stronger fields, the satellite particles can be completely pulled off, reversing the net charge on the cluster. With computer simulations, we investigate how charged particles distribute on an oppositely charged sphere to minimize their energy and compare the results with the solutions to the well-known Thomson problem. We also use the simulations to explore the dependence of such clusters on Debye screening length ${\kappa }^{-1}$ and the ratio of charges on the particles, showing good agreement with experimental observations.

The sensitivity of an atomic interferometer increases when the phase evolution of its quantum superposition state is measured over a longer interrogation interval. In practice, a limit is set by the measurement process, which returns not the phase but its projection in terms of population difference on two energetic levels. The phase interval over which the relation can be inverted is thus limited to the interval $[-\pi /2,\pi /2]$; going beyond it introduces an ambiguity in the readout, hence a sensitivity loss. Here, we extend the unambiguous interval to probe the phase evolution of an atomic ensemble using coherence-preserving measurements and phase corrections, and demonstrate the phase lock of the clock oscillator to an atomic superposition state. We propose a protocol based on the phase lock to improve atomic clocks limited by local oscillator noise, and foresee the application to other atomic interferometers such as inertial sensors.

Much of the anticipation accompanying the development of a quantum computer relates to its application to simulating dynamics of another quantum system of interest. Here, we study the building blocks for simulating quantum spin systems with linear optics. We experimentally generate the eigenstates of the $XY$ Hamiltonian under an external magnetic field. The implemented quantum circuit consists of two cnot gates, which are realized experimentally by harnessing entanglement from a photon source and applying a cphase gate. We tune the ratio of coupling constants and the magnetic field by changing local parameters. This implementation of the $XY$ model using linear quantum optics might open the door to future studies of quenching dynamics using linear optics.

Multipulse sequences based on Carr-Purcell decoupling are frequently used for narrow-band signal detection in single-spin magnetometry. We have analyzed the behavior of multipulse sensing sequences under real-world conditions, including finite pulse durations and the presence of detunings. We find that these nonidealities introduce harmonics to the filter function, allowing additional frequencies to pass the filter. In particular, we find that the $XY$ family of sequences can generate signals at the $2f_{\mathrm{ac}}$, $4f_{\mathrm{ac}}$, and $8f_{\mathrm{ac}}$ harmonics and their odd subharmonics, where $f_{\mathrm{ac}}$ is the ac signal frequency. Consideration of the harmonic response is especially important for diamond-based nuclear-spin sensing where the nuclear magnetic resonance frequency is used to identify the nuclear-spin species, as it leads to ambiguities when several isotopes are present.

Hybrid systems made of quantum emitters and plasmonic nanoresonators offer a unique platform to implement artificial atoms with completely novel optical responses that are not available otherwise. However, their theoretical analysis is difficult, and since many degrees of freedom have to be explored, engineering their optical properties remains challenging. Here, we propose a new formalism that removes most limitations encountered in previous analytical treatments and allows a flexible and efficient study of complex nanoresonators with arbitrary shapes in an almost fully analytically way. The formalism brings accurate closed-form expressions for the hybrid-system optical response and provides an intuitive description based on the coupling between the quantum emitters and the resonance modes of the nanoresonator. The ability to quickly predict light-scattering properties of hybrid systems paves the way to a deep exploration of their fascinating properties and may enable rapid optimization of quantum plasmonic metamaterials or quantum information devices.

Motivated by the commonplace observation of Mott insulators away from integer filling, we construct a simple thermodynamic argument for phase separation in first-order doping-driven Mott transitions. We show how to compute the critical dopings required to drive the Mott transition using electronic structure calculations for the titanate family of perovskites, finding good agreement with experiment. The theory predicts that the transition is percolative and should exhibit Coulomb frustration.

A challenging problem in the study of complex systems is that of resolving, without prior information, the emergent, mesoscopic organization determined by groups of units whose dynamical activity is more strongly correlated internally than with the rest of the system. The existing techniques to filter correlations are not explicitly oriented towards identifying such modules and can suffer from an unavoidable information loss. A promising alternative is that of employing community detection techniques developed in network theory. Unfortunately, this approach has focused predominantly on replacing network data with correlation matrices, a procedure that we show to be intrinsically biased because of its inconsistency with the null hypotheses underlying the existing algorithms. Here, we introduce, via a consistent redefinition of null models based on random matrix theory, the appropriate correlation-based counterparts of the most popular community detection techniques. Our methods can filter out both unit-specific noise and system-wide dependencies, and the resulting communities are internally correlated and mutually anticorrelated. We also implement multiresolution and multifrequency approaches revealing hierarchically nested subcommunities with “hard” cores and “soft” peripheries. We apply our techniques to several financial time series and identify mesoscopic groups of stocks which are irreducible to a standard, sectorial taxonomy; detect “soft stocks” that alternate between communities; and discuss implications for portfolio optimization and risk management.

The time variation of contacts in a networked system may fundamentally alter the properties of spreading processes and affect the condition for large-scale propagation, as encoded in the epidemic threshold. Despite the great interest in the problem for the physics, applied mathematics, computer science, and epidemiology communities, a full theoretical understanding is still missing and currently limited to the cases where the time-scale separation holds between spreading and network dynamics or to specific temporal network models. We consider a Markov chain description of the susceptible-infectious-susceptible process on an arbitrary temporal network. By adopting a multilayer perspective, we develop a general analytical derivation of the epidemic threshold in terms of the spectral radius of a matrix that encodes both network structure and disease dynamics. The accuracy of the approach is confirmed on a set of temporal models and empirical networks and against numerical results. In addition, we explore how the threshold changes when varying the overall time of observation of the temporal network, so as to provide insights on the optimal time window for data collection of empirical temporal networked systems. Our framework is of both fundamental and practical interest, as it offers novel understanding of the interplay between temporal networks and spreading dynamics.

Interesting non-Abelian states, e.g., the Moore-Read Pfaffian and the anti-Pfaffian, offer candidate descriptions of the $\nu =5/2$ fractional quantum Hall state. But, the significant controversy surrounding the nature of the $\nu =5/2$ state has been hampered by the fact that the competition between these and other states is affected by small parameter changes. To study the phase diagram of the $\nu =5/2$ state, we numerically diagonalize a comprehensive effective Hamiltonian describing the fractional quantum Hall effect of electrons under realistic conditions in GaAs semiconductors. The effective Hamiltonian takes Landau-level mixing into account to lowest order perturbatively in $\kappa$, the ratio of the Coulomb energy scale to the cyclotron gap. We also incorporate the nonzero width $w$ of the quantum-well and subband mixing. We find the ground state in both the torus and spherical geometries as a function of $\kappa$ and $w$. To sort out the nontrivial competition between candidate ground states, we analyze the following four criteria: its overlap with trial wave functions, the magnitude of energy gaps, the sign of the expectation value of an order parameter for particle-hole symmetry breaking, and the entanglement spectrum. We conclude that the ground state is in the universality class of the Moore-Read Pfaffian state, rather than the anti-Pfaffian, for $\kappa <{\kappa }_c(w)$, where ${\kappa }_c(w)$ is a $w$-dependent critical value $0.6\lesssim {\kappa }_c(w)\lesssim 1$. We observe that both Landau-level mixing and nonzero width suppress the excitation gap, but Landau-level mixing has a larger effect in this regard. Our findings have important implications for the identification of non-Abelian fractional quantum Hall states.

We describe a universal scheme of quantum computation by state injection on rebits (states with real density matrices). For this scheme, we establish contextuality and Wigner function negativity as computational resources, extending results of M. Howard et al. [Nature (London) 510, 351 (2014)] to two-level systems. For this purpose, we define a Wigner function suited to systems of $n$ rebits and prove a corresponding discrete Hudson’s theorem. We introduce contextuality witnesses for rebit states and discuss the compatibility of our result with state-independent contextuality.

Water is a unique solvent with strong, yet highly dynamic, intermolecular interactions. Many insights into this distinctive liquid have been obtained using ultrafast vibrational spectroscopy of water’s O-H stretch vibration. However, it has been challenging to separate the different contributions to the dynamics of the O-H stretch vibration in ${\mathrm{H}}_2\mathrm{O}$. Here, we present a novel nonequilibrium molecular dynamics (NEMD) algorithm that allows for a detailed picture of water vibrational dynamics by generating nonequilibrium vibrationally excited states at targeted vibrational frequencies. Our ab initio NEMD simulations reproduce the experimentally observed time scales of vibrational dynamics in ${\mathrm{H}}_2\mathrm{O}$. The approach presented in this work uniquely disentangles the effects on the vibrational dynamics of four contributions: the delocalization of the O-H stretch mode, structural dynamics of the hydrogen bonded network, intramolecular coupling within water molecules, and intermolecular coupling between water molecules (near-resonant energy transfer between O-H groups). Our results illustrate that intermolecular energy transfer and the delocalization of the O-H stretch mode are particularly important for the spectral diffusion in ${\mathrm{H}}_2\mathrm{O}$.

The first law of thermodynamics imposes not just a constraint on the energy content of systems in extreme quantum regimes but also symmetry constraints related to the thermodynamic processing of quantum coherence. We show that this thermodynamic symmetry decomposes any quantum state into mode operators that quantify the coherence present in the state. We then establish general upper and lower bounds for the evolution of quantum coherence under arbitrary thermal operations, valid for any temperature. We identify primitive coherence manipulations and show that the transfer of coherence between energy levels manifests irreversibility not captured by free energy. Moreover, the recently developed thermomajorization relations on block-diagonal quantum states are observed to be special cases of this symmetry analysis.

High-entropy alloys constitute a new class of materials whose very existence poses fundamental questions regarding the physical principles underlying their unusual phase stability. Originally thought to be stabilized by the large entropy of mixing associated with their large number of components (five or more), these alloys have attracted attention for their potential applications. Yet, no model capable of robustly predicting which combinations of elements will form a single phase currently exists. Here, we propose a model that, through the use of high-throughput computation of the enthalpies of formation of binary compounds, predicts specific combinations of elements most likely to form single-phase, high-entropy alloys. The model correctly identifies all known single-phase alloys while rejecting similar elemental combinations that are known to form an alloy comprising multiple phases. In addition, we predict numerous potential single-phase alloy compositions and provide three tables with the ten most likely five-, six-, and seven-component single-phase alloys to guide experimental searches.

We use pseudo-quantum electrodynamics in order to describe the full electromagnetic interaction of the $p$ electrons in graphene in a consistent 2D formulation. We first consider the effect of this interaction in the vacuum polarization tensor or, equivalently, in the current correlator. This allows us to obtain the $T\to 0$ conductivity after a smooth zero-frequency limit is taken in Kubo’s formula. Thereby, we obtain the usual expression for the minimal conductivity plus corrections due to the interaction that bring it closer to the experimental value. We then predict the onset of an interaction-driven spontaneous quantum valley Hall effect below an activation temperature of the order of 2 K. The transverse (Hall) valley conductivity is evaluated exactly and shown to coincide with the one in the usual quantum Hall effect. Finally, by considering the effects of pseudo-quantum electrodynamics, we show that the electron self-energy is such that a set of $P$- and $T$-symmetric gapped electron energy eigenstates are dynamically generated, in association with the quantum valley Hall effect.

We analyze the interference field formed by two electromagnetic plane waves (with the same frequency but different wave vectors), and find that such a field reveals a rich and highly nontrivial structure of the local momentum and spin densities. Despite the seemingly planar and extensively studied character of the two-wave system, we find that it possesses a transverse (out-of-plane) helicity-independent spin density and also a transverse polarization-dependent momentum density with unusual physical properties. The polarization-dependent transverse momentum represents the so-called Belinfante spin momentum, which does not exert the usual optical pressure and is considered as “virtual” in field theory. We perform analytical estimations and exact numerical simulations of the interaction of the two-wave field with probe Mie particles. The results of these calculations clearly indicate the straightforward detectability of the unusual spin and momentum properties in the two-wave field and strongly motivate their future experimental verifications.

We develop the nonequilibrium extension of bosonic dynamical mean-field theory and a Nambu real-time strong-coupling perturbative impurity solver. In contrast to Gutzwiller mean-field theory and strong-coupling perturbative approaches, nonequilibrium bosonic dynamical mean-field theory captures not only dynamical transitions but also damping and thermalization effects at finite temperature. We apply the formalism to quenches in the Bose-Hubbard model, starting from both the normal and the Bose-condensed phases. Depending on the parameter regime, one observes qualitatively different dynamical properties, such as rapid thermalization, trapping in metastable superfluid or normal states, as well as long-lived or strongly damped amplitude oscillations. We summarize our results in nonequilibrium “phase diagrams” that map out the different dynamical regimes.

We investigate the changes in spin and orbital patterns induced by magnetic transition-metal ions without an orbital degree of freedom doped in a strongly correlated insulator with spin-orbital order. In this context, we study the $3d$ ion substitution in $4d$ transition-metal oxides in the case of $3d^3$ doping at either $3d^2$ or $4d^4$ sites, which realizes orbital dilution in a Mott insulator. Although we concentrate on this doping case as it is known experimentally and more challenging than other oxides due to finite spin-orbit coupling, the conclusions are more general. We derive the effective $3d-4d$ (or $3d-3d$) superexchange in a Mott insulator with different ionic valencies, underlining the emerging structure of the spin-orbital coupling between the impurity and the host sites, and demonstrate that it is qualitatively different from that encountered in the host itself. This derivation shows that the interaction between the host and the impurity depends in a crucial way on the type of doubly occupied $t_{2g}$ orbital. One finds that in some cases, due to the quench of the orbital degree of freedom at the $3d$ impurity, the spin and orbital order within the host is drastically modified by doping. The impurity either acts as a spin defect accompanied by an orbital vacancy in the spin-orbital structure when the host-impurity coupling is weak or favors doubly occupied active orbitals (orbital polarons) along the $3d-4d$ bond leading to antiferromagnetic or ferromagnetic spin coupling. This competition between different magnetic couplings leads to quite different ground states. In particular, for the case of a finite and periodic $3d$ atom substitution, it leads to striped patterns either with alternating ferromagnetic or antiferromagnetic domains or with islands of saturated ferromagnetic order. We find that magnetic frustration and spin degeneracy can be lifted by the quantum orbital flips of the host, but they are robust in special regions of the incommensurate phase diagram. Orbital quantum fluctuations modify quantitatively spin-orbital order imposed by superexchange. In contrast, the spin-orbit coupling can lead to anisotropic spin and orbital patterns along the symmetry directions and cause a radical modification of the order imposed by the spin-orbital superexchange. Our findings are expected to be of importance for future theoretical understanding of experimental results for $4d$ transition-metal oxides doped with $3d^3$ ions. We suggest how the local or global changes of the spin-orbital order induced by such impurities could be detected experimentally.

Toroidal multipoles are the terms missing in the standard multipole expansion; they are usually overlooked due to their relatively weak coupling to the electromagnetic fields. Here, we propose and theoretically study all-dielectric metamaterials of a special class that represent a simple electromagnetic system supporting toroidal dipolar excitations in the THz part of the spectrum. We show that resonant transmission and reflection of such metamaterials is dominated by toroidal dipole scattering, the neglect of which would result in a misunderstanding interpretation of the metamaterials’ macroscopic response. Because of the unique field configuration of the toroidal mode, the proposed metamaterials could serve as a platform for sensing or enhancement of light absorption and optical nonlinearities.

There is a variety of situations in which Newton’s third law is violated. Generally, the action-reaction symmetry can be broken for mesoscopic particles, when their effective interactions are mediated by a nonequilibrium environment. Here, we investigate different classes of nonreciprocal interactions relevant to real experimental situations and present their basic statistical mechanics analysis. We show that in mixtures of particles with such interactions, distinct species acquire distinct kinetic temperatures. In certain cases, the nonreciprocal systems are exactly characterized by a pseudo-Hamiltonian; i.e., being intrinsically nonequilibrium, they can nevertheless be described in terms of equilibrium statistical mechanics. Our results have profound implications, in particular, demonstrating the possibility to generate extreme temperature gradients on the particle scale. We verify the principal theoretical predictions in experimental tests performed with two-dimensional binary complex plasmas.