Metalorganic chemical vapor deposition (MOCVD) has not been often used for studying exciton-polariton condensation and developing polaritonic devices, although it is a powerful mass productive method for practical applications. Here, we demonstrate that the MOCVD-grown GaAs-based microcavity can realize the nonequilibrium condensation of exciton-polariton. We obtained the Rabi splitting values of 10.1 meV in the angle-resolved reflectance spectrum from the MOCVD-grown microcavity. And, we also measured the angle-resolved photoluminescence depending on pumping power to observe condensation behaviors. Furthermore, we found that the occupancy distributions of polaritons are in a nonequilibrium state by fitting Bose-Einstein distributions. To realize the strong coupling and exciton-polariton condensation, we reduced the linewidth broadening of photoluminescence from quantum wells by optimizing growth conditions and designs.

The relaxor ferroelectric transition in ${\mathrm{Cd}}_2{\mathrm{Nb}}_2{\mathrm{O}}_7$ is thought to be described by the unusual condensation of two $\mathrm{\Gamma }$-centered phonon modes, ${\mathrm{\Gamma }}_4^{-}$ and ${\mathrm{\Gamma }}_5^{-}$. However, their respective roles have proven to be ambiguous, with disagreement between ab initio studies, which favor ${\mathrm{\Gamma }}_4^{-}$ as the primary mode, and global crystal refinements, which point to ${\mathrm{\Gamma }}_5^{-}$ instead. Here, we resolve this issue by demonstrating from x-ray pair distribution function measurements that locally, ${\mathrm{\Gamma }}_4^{-}$ dominates, but globally, ${\mathrm{\Gamma }}_5^{-}$ dominates. This behavior is consistent with the near degeneracy of the energy surfaces associated with these two distortion modes found in our own ab initio simulations. Our first-principles calculations also show that these energy surfaces are almost isotropic, providing an explanation for the numerous structural transitions found in ${\mathrm{Cd}}_2{\mathrm{Nb}}_2{\mathrm{O}}_7$, as well as its relaxor behavior. Our results point to several candidate descriptions of the local structure, some of which demonstrate two-in/two-out behavior for Nb displacements within a given Nb tetrahedron. Although this suggests the possibility of a charge analog of spin ice in ${\mathrm{Cd}}_2{\mathrm{Nb}}_2{\mathrm{O}}_7$, our results are more consistent with a Heisenberg-like description for dipolar fluctuations rather than an Ising one. We hope this encourages future experimental investigations of the Nb and Cd dipolar fluctuations, along with their associated mode dynamics.

We initiate a systematic study of fracton physics within the geometric framework of double field theory. We ascribe the immobility and large degeneracy of the former to the non-Riemannian backgrounds of the latter, in terms of generalized geodesics and infinite-dimensional isometries. A doubled pure Yang-Mills or Maxwell theory reduces to an ordinary one coupled to a strain tensor of elasticity theory and thus rather remarkably provides a unifying description of photons and phonons. Upon a general double field theory background, which consists of Riemannian and non-Riemannian subspaces, the dual photon-phonon pair becomes fractonic over the non-Riemannian subspace. When the elasticity displacement vector condenses, minimally coupled charged particles acquire an effective mass even in the purely Riemannian case, yielding predictions for polaron physics and time crystals. Furthermore, the immobility of neutral particles along the non-Riemannian directions is lifted to a saturation velocity for charged particles. Utilizing the differential geometry of double field theory, we also present curved spacetime extensions which exhibit general covariance.

Real systems are characterized by complex patterns of interactions between their units, by dynamical processes on them, and by the interplay of the two. It is well known that particular structures affect dynamical processes at different scales. Sometimes richly connected units are connected by costly, long-range links. In the brain, hubs form rich clubs for integrating information between different brain regions, and many biological and social networks show this same structural organization. It remains, however, unclear whether this structural organization alone enables a rapid communication between highly connected nodes or whether a functional rich club may emerge as a combination of direct links and longer paths between rich nodes. Here, we identify functional rich clubs through the diffusion geometry, providing a perspective on rich-club phenomena in complex networks. We show that weak structural rich clubs may be functionally stronger, thanks to bridge nodes, while diffusion inside strong structural rich clubs may be damped in modular networks.

We present a joint experimental and theoretical study of the driven Su-Schrieffer-Heeger model implemented by arrays of evanescently coupled plasmonic waveguides. Floquet theory predicts that this system hosts for suitable driving frequencies a topologically protected edge state that has no counterpart in static systems, the so-called anomalous Floquet topological $\pi$ mode. By using real- and Fourier-space leakage radiation microscopy in combination with edge and bulk excitation, we unequivocally identify the anomalous Floquet topological $\pi$ mode and study its frequency dependence.

We consider the sensitivity of chemical reactions, able to undergo spontaneous mirror symmetry breaking, to chiral bias in the presence of nonequilibrium temperature fluctuations and derive a selectivity criterion. For this, we estimate the magnitude of fluctuations $\delta k$ in chemical reaction rate constants $k$ arising from the nonequilibrium temperature fluctuations $\delta T$ about a mean value $T$. To leading order, the relative rate constant fluctuations $\delta k_i/k_i$ for each reaction $i$ are given by the product of the activation enthalpy $\mathrm{\Delta }{H}_i^{‡}/RT$ for the $i\mathrm{th}$ reaction multiplied by the relative rms temperature fluctuations $\delta T_{\mathrm{rms}}/T$. The latter are determined by the system's specific heat at constant volume: $C_V$. We test this criterion with simulations carried out for an open-flow fully reversible Frank model, and for a range of parity-violating energy differences (PVED) within the theoretically estimated upper and lower bounds. Depending on the relative magnitudes of the deterministic PVED bias and the temperature fluctuations, the PVED bias can either (i) select the final stable chiral outcome deterministically or (ii) select one of two possible stable chiral outcomes with an asymmetric statistical weighting. For larger temperature fluctuations, the PVED bias loses its selectivity, and the final stable chiral outcomes are stochastic and equally probable. This paper points towards the possible design of small volume chemical flow reactors capable of detecting the elusive PVED bias in bulk systems, provided other sources of fluctuations can be sufficiently controlled and attenuated.

When the exchange interaction between the impurity spin and the spins of itinerant quasiparticles is strong or weak enough, the ground states for a magnetic impurity in a superconductor are the screened or free spins, respectively. In both cases, the lowest excited state is a bound state within the superconducting gap, known as the Yu-Shiba-Rusinov (YSR) state. The YSR state is spatially localized, energetically isolated, and fully spin polarized, leading to applications such as functional scanning probes. While any application demands identifying whether the impurity spin is screened or free, a suitable experimental technique has been elusive. Here we demonstrate an unambiguous way to determine the impurity ground state using the Zeeman effect. We performed ultralow-temperature scanning tunneling spectroscopy of junctions formed between a Cu(111) surface and superconducting Nb tips decorated by single magnetic Fe atoms. Depending on the condition of the Fe adsorbate, the YSR peak in the spectrum either splits or shifts in a magnetic field, signifying that the Fe spin is screened or free, respectively. Our observations provide renewed insights into the competition between magnetism and superconductivity and constitute a basis for the applications of the YSR state.

Non-Hermitian quantum systems can exhibit spectral degeneracies known as exceptional points, where two or more eigenvectors coalesce, leading to a nondiagonalizable Jordan block. It is known that symmetries can enhance the abundance of exceptional points in noninteracting systems. Here we investigate the fate of such symmetry protected exceptional points in the presence of a symmetry preserving interaction between fermions and find that (i) exceptional points are stable in the presence of the interaction. Their propagation through the parameter space leads to the formation of characteristic exceptional “fans.” In addition, (ii) we identify a new source for exceptional points which are only present due to the interaction. These points emerge from diagonalizable degeneracies in the noninteracting case. Beyond their creation and stability, (iii) we also find that exceptional points can annihilate each other if they meet in parameter space with compatible many-body states forming a third order exceptional point at the endpoint. These phenomena are well captured by an “exceptional perturbation theory” starting from a noninteracting Hamiltonian.

Spin-dependent electrical injection has found useful applications in storage devices, but fully operational spin-dependent semiconductor electronics remain a challenging task because of weak spin-orbit couplings and/or strong spin relaxations. These limitations are lifted considering atoms instead of electrons or holes as spin carriers. In this emerging field of atomtronics, we demonstrate the equivalent of a Datta-Das transistor using a degenerate Fermi gas of strontium atoms as spin carriers in interaction with a tripod laser-beams scheme. We explore the dependence of spin rotation, and we identify two key control parameters which we interpret as equivalent to the gate-source and drain-source voltages of a field effect transistor. Our finding broadens the spectrum of atomtronics devices for implementation of operational spin-sensitive circuits.

Exceptional points (EPs) are non-Hermitian degeneracies in open quantum and wave systems at which not only eigenenergies but also the corresponding eigenstates coalesce. This is in strong contrast to degeneracies known from conservative systems, so-called diabolic points (DPs), at which only eigenenergies degenerate. Here, we connect these two kinds of degeneracies by introducing the concept of the distance of a given EP in matrix space to the set of DPs. We prove that this distance determines an upper bound for the response strength of a non-Hermitian system with this EP. A small distance therefore implies a weak spectral response to perturbations and a weak intensity response to excitations. This finding has profound consequences for physical realizations of EPs that rely on perturbing a DP. Moreover, we exploit this concept to analyze the limitations of the spectral response strength in passive systems. Several optical systems are investigated to illustrate the theory.

For laser direct-drive (LDD) fusion implosions, intense laser beams are used to directly illuminate the inertial confinement fusion (ICF) capsule. The laser beams' intensity nonuniformity (due to speckles) on the target can impose perturbation seeds which are subsequently amplified by Rayleigh-Taylor instability growth, thereby leading to degradation of ICF implosion performance. To devise methods to mitigate this issue, adequate understanding of the underlying so-called laser-imprinting process is required. Here, we report measurements and modeling of the initial plasma formation process which has been shown to affect the laser imprint. Specifically, we measured the transient transmission of a femtosecond probe pulse through a polystyrene target for different 100-picosecond pump pulse intensities pertaining to ICF conditions. The experimental data are used to benchmark a microphysics model of initial plasma formation that overall describes the observed dynamics, thus providing a validated solid-to-plasma modeling for laser-imprinting purposes in radiation-hydrodynamic codes to accurately simulate and design LDD targets.

Periodically driven oscillators are commonly described in a frame corotating with the drive and using the rotating-wave approximation (RWA). This description, however, is known to induce errors for off-resonant driving. Here, we show that the standard quantum description, using the creation and annihilation of particles with the oscillators' natural frequency, necessarily leads to incorrect results when combined with the RWA. We demonstrate this on the simple (quantum) harmonic oscillator and present an alternative operator basis that reconciles the RWA with off-resonant driving. The approach is also applicable to more complex models, where it accounts for known discrepancies. As an example, we demonstrate the advantage of our scheme on a driven quantum Duffing oscillator.

We simulate the nonlinear chaotic dynamics of Lorenz-type models for a classical two-dimensional thermal convection flow with three and eight degrees of freedom by a hybrid quantum-classical reservoir computing model. The high-dimensional quantum reservoir dynamics are established by universal quantum gates that rotate and entangle the individual qubits of the tensor product quantum state. A comparison of the quantum reservoir computing model with its classical counterpart shows that the same prediction and reconstruction capabilities of classical reservoirs with thousands of perceptrons can be obtained by a few strongly entangled qubits. We demonstrate that the mean squared error between model output and ground truth in the test phase of the quantum reservoir computing algorithm increases when the reservoir is decomposed into separable subsets of qubits. Furthermore, the quantum reservoir computing model is implemented on a real noisy IBM quantum computer for up to seven qubits. Our work thus opens the door to model the dynamics of classical complex systems in a high-dimensional phase space effectively with an algorithm that requires a small number of qubits.

A wellknown approach to describe the dynamics of an open quantum system is to compute the master equation evolving the reduced density matrix of the system. This approach plays an important role in describing excitation transfer through photosynthetic light harvesting complexes (LHCs). The hierarchical equations of motion (HEOM) was adapted by Ishizaki and Fleming [J. Chem. Phys.130, 234111 (2009)] to simulate open quantum dynamics in the biological regime. We generate a set of time-dependent observables that depict the coherent propagation of electronic excitations through the LHCs by solving the HEOM. The computationally intractable problem here is addressed using classical machine learning (ML). The ML architecture constructed here is of model character and it is used to solve the inverse problem for open quantum systems within the HEOM approach. The objective is to determine whether a trained ML model can perform Hamiltonian tomography by using the time dependence of the observables as inputs. We demonstrate the capability of convolutional neural networks to tackle this research problem. The models developed here can predict Hamiltonian parameters such as excited state energies and inter-site couplings of a system up to 99.28% accuracy.

Quantum link models have attracted a lot of attention in recent times as a generalization of Wilson's lattice gauge theories, and are particularly suitable for realization on quantum simulators and computers. These models are known to host new phases of matter and act as a bridge between particle and condensed matter physics. In this paper, we study the Abelian $\text{U}\left(1\right)$ lattice gauge theory in $(3+1)\mathrm{D}$ tubes using large-scale exact diagonalization. We are then able to motivate the phase diagram of the model with finite-size scaling techniques, and in particular propose the existence of a Coulomb phase. Furthermore, we introduce models involving fermionic quantum links, which generalize the gauge degrees of freedom to be of fermionic nature. We prove that while the spectra remain identical between the bosonic and the fermionic versions of the $\text{U}\left(1\right)$-symmetric quantum link models in $(2+1)\mathrm{D}$, they are different in $(3+1)\mathrm{D}$. We discuss the prospects of realizing the magnetic field interactions as correlated hopping in quantum simulator experiments.

Measuring expectation values of observables is an essential ingredient in variational quantum algorithms. A practical obstacle is the necessity of a large number of measurements for statistical convergence to meet requirements of precision, such as chemical accuracy in the application to quantum chemistry computations. Here we propose an algorithm to estimate the expectation value based on its approximate expression as a weighted sum of classically tractable matrix elements with some modulation, where the weight and modulation factors are evaluated by sampling appropriately prepared quantum states in the computational basis on quantum computers. Each of those states is prepared by applying a unitary transformation consisting of at most $N$ CNOT gates, where $N$ is the number of qubits, to a target quantum state whose expectation value is evaluated. Our algorithm is expected to require fewer measurements than conventional methods for a required statistical precision of the expectation value when the target quantum state is concentrated in particular computational basis states. We provide numerical comparisons of our method with existing ones for measuring electronic ground-state energies (expectation values of electronic Hamiltonians for the lowest-energy states) of various small molecules. Numerical results show that our method can reduce the numbers of measurements to obtain the ground-state energies for a targeted precision by several orders of magnitudes for molecules whose ground states are concentrated. Our results provide another route to measure expectation values of observables, which could accelerate the variational quantum algorithms.

A pyrite system ${\mathrm{NiS}}_{2-x}{\mathrm{Se}}_x$ exhibits a bandwidth controlled Mott transition via (S,Se) substitutions in a two-step process: the antiferromagnetic insulator (AFI) to antiferromagnetic metal (AFM) transition at $x\sim 0.45$ followed by the AFM to paramagnetic metal (PMM) transition at $x\sim 1.0$. Among a few other Mott systems which exhibit similar two-step transitions, $\mathrm{Ni}{(\mathrm{S},\mathrm{Se})}_2$ is of particular interest because a large intermediate AFM region in the phase diagram would provide unique opportunities to study the interplay between the spin and charge order. Muon spin relaxation ($\mu \mathrm{SR}$) measurements on ${\mathrm{NiS}}_{2-x}{\mathrm{Se}}_x$ have been carried out on seven different Se concentrations from $x=0$ to 1.0. The results on quantum evolution demonstrate significantly random spin correlations in the AFM region associated with a rapid reduction of the average local static Ni moment size with increasing $x$, yet without signatures of macroscopic phase separation as confirmed by nearly full volume fraction participating in the static muon relaxation process up to $x\sim$ 0.9 at low temperatures. The observed time spectra in the AFM region indicate Lorentzian distribution of static internal field expected for a spatially dilute spin structure. No signature of dynamic critical behavior was observed in thermal phase transitions. The previous neutron scattering studies found sharp magnetic Bragg peaks with a slower reduction of the average ordered moment size in the AFM region. By comparing and combining the muon and neutron results, here we propose a picture where the spin order is maintained by the percolation of “nonmetallic” localized and dangling Ni moments surrounded by S, while the charge transition from AFI to AFM is caused by the percolation of the conducting paths generated by the Ni-Se-Ni bonds. This model of interpenetrating charge and spin percolation captures the behavior of experimental results on (a) Se concentration for the insulator to metal transition, (b) Se concentration for the AFM to PMM transition, (c) variation of Hall effect in the AFM region due to conducting Ni charges on the backbone of the percolating charge network, (d) evolution of the neutron Bragg intensity, (e) evolution of the muon static local fields, and (f) spatial variation of the local conductance observed by STM.

It is well established that the ground-state electron configuration of the octahedral ${\mathrm{Co}}^{3+}$ ion is $t_{2\mathrm{g}}^6{e}_{\mathrm{g}}^0$, which corresponds to a low-spin (LS) state. However, we theoretically demonstrate that the octahedral ${\mathrm{Co}}^{3+}$ ion in ${\mathrm{Co}}_3{\mathrm{S}}_4$ prefers a high-spin (HS) state with the $t_{2\mathrm{g}}^4{e}_{\mathrm{g}}^2$ configuration, resulting in unusual magnetism. The density-functional theory plus $U$ calculation and ligand-field theory show that weak crystal-field splitting associated with ${\mathrm{S}}^{2-}$ induces a spin crossover from the LS to HS state of the octahedral ${\mathrm{Co}}^{3+}$ ion along with a weak Jahn-Teller-like elongation, and as a result, a ferromagnetic (FM) metal phase is energetically stabilized. Nevertheless, this phase is significantly more stable than the antiferromagnetic (AFM) phase experimentally reported at low temperature. Furthermore, phonon calculations suggest that the FM metal phase is possibly one of the ${\mathrm{Co}}_3{\mathrm{S}}_4$ polymorphs, appearing in the certain experimental growth environment, but it is not expected to appear due to a temperature-dependent transition from the AFM phase. In addition, the Lyons, Kaplan, Dwight, and Menyuk theory shows that this phase is further expected to undergo a phase transition to the frustrated magnetic state. We believe that our work provides insight into magnetism of cobalt compounds and also paves the way to achieve the HS ${\mathrm{Co}}^{3+}$ by design.