Quantum communication networks involving the preparation, sharing, and measurement of quantum states are ubiquitous in quantum information. Of particular relevance within this context is to understand under which conditions a given quantum resource can give rise to correlations incompatible with a classical explanation. Here we consider the so-called prepare-and-measure scenario, in which a quantum or classical message with bounded dimension is transmitted between two parties. In this scenario we derive criteria witnessing whether a set of quantum states can lead or not to nonclassical correlations. Based on that, we show that quantum resources that can only give rise to classical correlations in the simplest prepare-and-measure scenario can have their nonclassicality witnessed if we increase the number of preparations or measurements.

We calculate the electronic structure of the narrow gap semiconductors PbTe, SnTe, and GeTe in the cubic phase using density functional theory (DFT) and the $G_0{W}_0$ method. Within DFT, we show that the band ordering obtained with a conventional semilocal exchange-correlation approximation is correct for SnTe and GeTe but wrong for PbTe. The correct band ordering at the high-symmetry point $L$ is recovered adding $G_0{W}_0$ quasiparticle corrections. However, one-shot $G_0{W}_0$ produces artifacts in the band structure due to the wrong orbital character of the DFT single-particle states at the band edges close to $L$. We show that in order to correct these artifacts it is enough to consider the off-diagonal elements of the $G_0{W}_0$ self-energy corresponding to these states. We also investigate the pressure dependence of the band gap for these materials and the possibility of a transition from a trivial to a nontrivial topology of the band structure. For PbTe, we predict the band crossover and topological transition to occur at around 4.8 GPa. For GeTe, we estimate the topological transition to occur at 1.9 GPa in the constrained cubic phase, a pressure lower than that of the structural phase transition from rombohedral to cubic. SnTe is a crystalline topological insulator at ambient pressure, and the transition into a trivial topology would take place under a volume expansion of approximately $10\%$.

We study the effect of the boundary on a system of weakly interacting bosons in one dimension. It strongly influences the boson density, which is completely suppressed at the boundary position. Away from it, the density is depleted over the distances on the order of the healing length at the mean-field level. Quantum fluctuations modify the density profile considerably. The local density approaches the average one as an inverse square of the distance from the boundary. We calculate an analytic expression for the density profile at arbitrary separations from the boundary. We then consider the problem of localization of a foreign quantum particle (impurity) in the potential created by the inhomogeneous boson density. At the mean-field level, we find exact results for the energy spectrum of the bound states, the corresponding wave functions, and the condition for interaction-induced localization. The quantum contribution to the boson density gives rise to small corrections of the bound-state energy levels. However, it is fundamentally important for the existence of a long-range Casimir-like interaction between the impurity and the boundary.

Coherent feedback is the use of feedback loops not involving any measurements for the purpose of quantum control. This can be contrasted to measurement-based feedback where the feedback signal is directed by the result of measurements upon the system. Here, we develop a general framework to describe interferometric coherent feedback loops and prove that, under any such scheme, the steady-state squeezing of a bosonic mode subject to a rotating wave coupling with a white-noise environment and to any quadratic Hamiltonian must abide by a noise-dependent bound that reduces to the 3-dB limit at zero temperature. Such a finding is compared, at fixed dynamical parameters, with the performance of homodyne continuous monitoring of the output modes, which allows one to beat coherent feedback and the 3-dB limit under certain dynamical conditions.

Resistance to slip across hydrogel surfaces is typically minimal, and sliding is smooth. However, recent surface friction experiments under high applied pressures caused stick-slip behavior between a glass probe and polyacrylamide hydrogel. In this paper we analyze the surface-based interface and its behavior similar to the more internal slip avalanches that occur in plastic deformations of metallic glasses and crystals using statistical descriptions. We find that the stick-slip surface friction satisfies universal power laws and scaling functions predicted by a simple mean field theory (MFT). We provide the rescaled average avalanche slip rate profiles, or average avalanche ‘‘shapes,’’ defined as the average slip rate versus time, averaged over all avalanches with similar durations or sizes. We show that the avalanche shapes obtained in this friction experiment are consistent with MFT. This suggests the similar character of force buildup and release events in internal slip and surface friction for this unique data set, and the suitability of surface microfriction experiments to explore this space.

Quantum computers can be used to simulate nonlinear non-Hamiltonian classical dynamics on phase space by using the generalized Koopman–von Neumann formulation of classical mechanics. The Koopman–von Neumann formulation implies that the conservation of the probability distribution function on phase space, as expressed by the Liouville equation, can be recast as an equivalent Schrödinger equation on Hilbert space with a Hermitian Hamiltonian operator and a unitary propagator. This Schrödinger equation is linear in the momenta because it derives from a constrained Hamiltonian system with twice the classical phase-space dimension. A quantum computer with finite resources can be used to simulate a finite-dimensional approximation of this unitary evolution operator. Quantum simulation of classical dynamics is exponentially more efficient than a deterministic Eulerian discretization of the Liouville equation if the Koopman–von Neumann Hamiltonian is sparse. Utilizing quantum walk techniques for state preparation and amplitude estimation for the calculation of observables leads to a quadratic improvement over classical probabilistic Monte Carlo algorithms.

Overlapping communities are commonly observed in real-world networks. This is a motivation to develop overlapping community detection methods, because methods for nonoverlapping communities may not perform well. However, deterioration mechanism of the detection methods used for nonoverlapping communities have rarely been investigated theoretically. Here, we analyze accuracy of spectral clustering, which does not consider overlapping structures, by using the replica method from statistical physics. Our analysis on an overlapping stochastic block model reveals how the structural information is lost from the leading eigenvector because of the overlapping structure.

In recently emerging correlated topological materials, such as magnetic Dirac/Weyl semimetals, additional tunabilities of their transport and magnetic properties may be achieved by utilizing possible interaction between the exotic relativistic fermions and magnetic degree of freedom. The two-dimensional antiferromagnetic (AFM) Dirac semimetal ${\mathrm{EuMnBi}}_2$, in which an intricate interplay between multiple magnetic sublattices and Dirac fermions was suggested, provides an ideal platform to test this scenario. We report here a comprehensive study of the AFM structures of the Eu and Mn magnetic sublattices as well as the interplay between Eu and Mn magnetism in this compound by using both polarized and nonpolarized single-crystal neutron diffraction. Magnetic susceptibility, specific heat capacity measurements, and the temperature dependence of magnetic diffractions suggest that the AFM ordering temperatures of the Eu and Mn moments are at 22 and 337 K, respectively. The magnetic moments of both Eu and Mn ions are oriented along the crystallographic $c$ axis, and the respective magnetic propagation vectors are ${\mathbf{k}}_{\mathrm{Eu}}=(0,0,1)$ and ${\mathbf{k}}_{\mathrm{Mn}}=(0,0,0)$. With proper neutron absorption correction, the ordered moments are refined at 3 K as 7.7(1) and 4.1(1) ${\mu }_B$ for the Eu and Mn ions, respectively. In addition, a spin-flop (SF) phase transition of the Eu moments in an applied magnetic field along the $c$ axis was confirmed to take place at a critical field of $H_c\approx$ 5.3 T. The AFM exchange interaction and magnetic anisotropy parameters ($J=0.81$ meV, $K_u=0.18$ meV, $K_e=-0.11$ meV) are determined based on a subsequent quantitative analysis of the SF transition. The evolution of the Eu magnetic moment direction as a function of the applied magnetic field in the SF phase was also determined. Clear kinks in both field and temperature dependences of the magnetic reflections ($\pm 1$, 0, 1) of Mn were observed at the onset of the SF phase transition and the AFM order of the Eu moments, respectively. This unambiguously indicates the existence of a strong coupling between Eu and Mn magnetism. The interplay between two magnetic sublattices could bring new possibilities to tune Dirac fermions via changing magnetic structures by applied fields in this class of magnetic topological semimetals.

Transport and elastic properties of freestanding two-dimensional materials are determined by competition between dynamical and quenched out-of-plane deformations, i.e., between flexural phonons and ripples, respectively. They both tend to crumple the system by overcoming the strong anharmonicity which stabilizes the flat phases. Despite active research, it still remains unclear whether the rippled phase exists in the thermodynamic limit or is destroyed by thermal out-of-plane fluctuations. We demonstrate that a sufficiently strong short-range disorder stabilizes ripples, whereas in the case of a weak disorder the thermal flexural fluctuations dominate in the thermodynamic limit. Therefore the phase diagram of a flexible two-dimensional material with a quenched short-range disorder has four distinct phases. These phases have drastically different elastic and transport properties that are of crucial importance for the emergent field of flexible nanoelectronics.

A topological mechanism is a zero-elastic-energy deformation of a mechanical structure that is robust against smooth changes in system parameters. Here, we map the nonlinear elasticity of a paradigmatic class of topological mechanisms onto a supersymmetric field theory introduced by Witten and Olive. Heuristically, this approach entails taking the square root of a nonlinear Hamiltonian. It generalizes the standard procedure of obtaining two copies of the Dirac equation by taking the square root of the linear Klein-Gordon equation. Our real-space formalism goes beyond topological band theory by incorporating nonlinearities and spatial inhomogeneities, such as domain walls (i.e., kinks), where topological states are typically localized. We interpret the two components of the real fermionic field as site and bond displacements, respectively. The constraint of zero elastic energy insures that kinks in the mechanical system saturate the Bogomolny-Prasad-Sommerfield bound, while forbidding antikinks. This mechanism can be viewed as a manifestation of the underlying supersymmetry being half broken.

Heteroclinic dynamics is a suitable framework to describe transient dynamics that is characteristic for ecological as well as neural systems, in particular for cognitive processes. We consider different heteroclinic networks and zoom into the dynamics that emerges right at different bifurcation points. We identify features of criticality such as a proliferation of the dynamical repertoire and slowing down of the dynamics at the very bifurcations and in their immediate vicinity. It qualifies these bifurcation points as candidates for working points in systems which store and transfer information. We observe the emergence of a heteroclinic cycle between three manifolds that are densely covered by periodic orbits. Which periodic orbits are selected sensitively depends on the initial conditions due to hidden conservation laws. Our results add to the general observation that emergent features may be expected at borderlines, here between different dynamical regimes in heteroclinic dynamics.

Gravitational-wave observations of merging compact binaries hold the key to precision measurements of the objects' masses and spins. General relativistic precession, caused by spins misaligned with the orbital angular momentum, is considered a crucial tracer for determining the binary's formation history and environment and it also improves mass estimates; its measurement is therefore of particular interest with wide-ranging implications. Precession leaves a characteristic signature in the emitted gravitational-wave signal that is even more pronounced in binaries with highly unequal masses. The recent observations of GW190412 and GW190814 have confirmed the existence of such asymmetric compact binaries. Here we perform a systematic study to assess the confidence in measuring precession in gravitational-wave observations of high-mass-ratio binaries and our ability to measure the mass of the lighter companion in neutron-star–black-hole-type systems. Using Bayesian model selection, we show that precession can be decisively identified for low-mass binaries with mass ratios as low as 1:3 and mildly precessing spins with magnitudes $\lesssim 0.4$, even in the presence of systematic waveform errors.

In classical thermodynamics, heat cannot spontaneously pass from a colder system to a hotter system, which is called the thermodynamic arrow of time. However, if the initial states are entangled, the direction of the thermodynamic arrow of time may not be guaranteed. Here we take the thermofield double state at $0+1$ dimension as the initial state and assume its gravity duality to be the eternal black hole in ${\mathrm{AdS}}_2$ space. We make the temperature difference between the two sides by changing the Hamiltonian. We turn on proper interactions between the two sides and calculate the changes in energy and entropy. The energy transfer, as well as the thermodynamic arrow of time, are mainly determined by the competition between two channels: thermal diffusion and anomalous heat flow. The former is not related to the wormhole and obeys the thermodynamic arrow of time; the latter is related to the wormhole and reverses the thermodynamic arrow of time, i.e., transferring energy from the colder side to the hotter side at the cost of entanglement consumption. Finally, we find that the thermal diffusion wins the competition, and the whole thermodynamic arrow of time has not been reversed.

Laser-driven proton acceleration from ultrathin foils is investigated experimentally using $f/3$ and $f/1$ focusing. Higher energies achieved with $f/3$ are shown via simulations to result from self-focusing of the laser light in expanding foils that become relativistically transparent, enhancing the intensity. The increase in proton energy is maximized for an optimum initial target thickness, and thus expansion profile, with no enhancement occurring for targets that remain opaque, or with $f/1$ focusing to close to the laser wavelength. The effect is shown to depend on the drive laser pulse duration.

It is known that protocols based on weak measurements can be used to steer quantum systems into predesignated pure states. Here we show that weak-measurement-based steering protocols can be harnessed for on-demand engineering of mixed states. In particular, through a continuous variation of the protocol parameters, one can guide a classical target state to a discorded one, and further on, toward an entangled target state.

In the quest for realizations of quantum spin liquids, the exploration of Kitaev materials (spin-orbit-entangled Mott insulators with strong bond-directional exchanges) has taken center stage. However, in these materials the local spin-orbital $j=1/2$ moments typically show long-range magnetic order at low temperature, thus defying the formation of a spin-liquid ground state. It is an intriguing question how far the properties of the Kitaev spin liquid survive in these materials despite the presence of long-range magnetic order. Using resonant inelastic x-ray scattering (RIXS), we here report on a proximate spin-liquid regime with distinctive fingerprints of Kitaev physics in the magnetic excitations of the honeycomb iridates $\alpha \text{−}{\mathrm{Li}}_2\mathrm{Ir}{\mathrm{O}}_3$ and ${\mathrm{Na}}_2\mathrm{Ir}{\mathrm{O}}_3$. We observe a broad continuum of magnetic excitations and prove its magnetic character via the resonance behavior. The continuum is insensitive to temperature up to about 90 K and persists up to at least 300 K, more than an order of magnitude larger than the magnetic ordering temperatures. This striking temperature dependence of the RIXS intensity agrees with the behavior of the nearest-neighbor spin-spin correlations of the Kitaev model. This comparison is motivated by momentum-dependent RIXS measurements of the dynamical structure factor for energies within the continuum which show that dynamical spin-spin correlations are restricted to nearest neighbors, which is a key property of the Kitaev model. Notably, these spectroscopic observations are also present in the magnetically ordered state for excitation energies above the conventional magnon excitations. Phenomenologically, our data agree with inelastic neutron scattering results on the related honeycomb compound $\alpha \text{−}\mathrm{Ru}{\mathrm{Cl}}_3$, establishing a common ground for a proximate Kitaev spin-liquid regime in these materials.

Performing measurements for high-weight operators has been a practical problem in quantum computation, especially for quantum codes in the stabilizer formalism. The conventional procedure of measuring a high-weight operator requires multiple pairwise unitary operations, which can be slow and prone to errors. We provide an alternative method to passively detect the value of a high-weight operator using only two-local interactions and single-qubit continuous measurements. This approach involves joint interactions between the system and continuously monitored ancillary qubits. The measurement outcomes from the monitor qubits reveal information about the value of the operator. This information can be retrieved by using a numerical estimator or by evaluating the time average of the signals. The interaction Hamiltonian can be effectively built using only two-local operators, based on techniques from perturbation theory. We apply this indirect detection scheme to the four-qubit Bacon-Shor code, where the two stabilizers are indirectly monitored using four ancillary qubits. Due to the fact that the four-qubit Bacon-Shor code is an error-detecting code and that the quantum Zeno effect can suppress errors, we also study the error suppression under the indirect measurement process. In this example, we show that various types of non-Markovian errors can be suppressed.

We develop an analytical framework to study the influence of a weakly intercoupled in-plane spherical metal nanoparticle (MNP) assembly on a coherently illuminated quantum emitter (QE). We reduce the analytical expressions derived for the aforementioned generic planar setup into simple and concise expressions representing a QE mediated by a symmetric MNP constellation, by exploiting the symmetry. We use the recently introduced generalized nonlocal optical response (GNOR) theory that has successfully explained plasmonic experiments to model the MNPs in our system. Due to the use of GNOR theory, and our analytical approach, the procedure we suggest is extremely computationally efficient. Using the derived model, we analyze the absorption rate, resultant Rabi frequency, effective excitonic energy shift, and dephasing rate shift spectra of an exciton bearing QE at the center of a symmetric MNP setup. We observe that the QE experiences plasmon-induced absorption rate spectral linewidth variations that increase in magnitude with decreasing MNP-QE center separation and increasing number of MNPs. Our results also suggest that parameter regions where the QE exhibits trends of decreasing linewidth against decreasing MNP-QE center separation are likely to be associated with plasmon-induced excitonic energy redshifts. Similarly, regions where the QE absorption rate linewidth tends to increase against decreasing MNP-QE center separation are likely to be accompanied by plasmon-induced excitonic energy blueshifts. In both these cases, the magnitude of the observed redshift and blueshift was seen to increase with the number of MNPs in the constellation, due to enhancement of the plasmonic influence. We show that even when the magnitude of the QE absorption rate spectrum is much smaller compared to the isolated (collective) MNP spectra, it is sufficient to dramatically modify the spectrum of the MNP-QE nanohybrid, causing sharp Fano-type interference patterns.