We investigate compressive turbulence at sub-ion scales with measurements from the Magnetospheric MultiScale Mission. The tetrahedral configuration and high time resolution density data obtained by calibrating spacecraft potential allow an investigation of the turbulent density fluctuations in the solar wind and their three-dimensional structure in the sub-ion range. The wave-vector associated with the highest energy density at each spacecraft frequency is obtained by application of the multipoint signal resonator technique to the four-point density data. The fluctuations show a strong wave-vector anisotropy $k_{\perp }\gg k_{\parallel }$ where the parallel and perpendicular symbols are with respect to the mean magnetic-field direction. The plasma frame frequencies show two populations, one below the proton cyclotron frequency $\omega <{\mathrm{\Omega }}_{ci}$ consistent with kinetic Alfvén wave (KAW) turbulence. The second component has higher frequencies $\omega >{\mathrm{\Omega }}_{ci}$ consistent with ion Bernstein wave turbulence. Alternatively, these fluctuations may constitute KAWs that have undergone multiple wave-wave interactions, causing a broadening in the plasma frame frequencies. The scale-dependent kurtosis in this wave-vector region shows a reduction in intermittency at the small scales which can also be explained by the presence of wave activity. Our results suggest that small-scale turbulence exhibits linear-wave properties of kinetic Alfvén and possibly ion-Bernstein (magnetosonic) waves. Based on our results, we speculate that these waves may play a role in describing the observed reduction in intermittency at sub-ion scales.

We show that three-dimensional (3D) Dirac semimetals (DSMs) can achieve highly efficient terahertz high-order harmonic generation (HHG) up to the 31st harmonic with input intensities $\approx 10\mathrm{M}\mathrm{W}/{\mathrm{cm}}^2$—over ${10}^5$ times lower than required in conventional terahertz HHG systems. Our theory reveals that this extreme nonlinearity is made possible by the existence of an operation regime that differs from previous demonstrations of lower order harmonic generation. We also reveal an unexpected regime in which emitted harmonics abruptly become negligible beyond the third order. This unprecedented vanishing of higher order nonlinearity has a geometrical origin related to the combination of conical dispersion and extra dimensionality in 3D DSMs, breaking the common notion that 3D DSMs share the essential physics of two-dimensional DSMs. Our findings pave the way to unlocking the full potential of 3D DSMs as efficient platforms for terahertz light sources and optoelectronics at moderate intensities.

Quantum state-resolved scattering experiments for methane molecules colliding with a catalytically active nickel surface are compared to scattering from a nickel surface passivated by a single layer of graphene. The vibrational state distribution of the scattered methane is observed to differ dramatically for the two surfaces. Quantum-mechanical inelastic scattering calculations show that these differences are related to the catalytic activity of the surface impact site. Our results demonstrate how inelastic scattering can be used to probe the reactive potential-energy surfaces of molecule-metal systems important to heterogeneous catalysis.

We study the pedestrian dynamics with the consideration of two heuristic mechanisms of anticipation and attraction. The former enables the individuals to anticipate potential collisions in terms of the state of surrounding environment and slow down their moving speed accordingly, while the latter reflects the tendency of the individuals following (or being attracted by) those peer partners with high moving speeds, trying to reach their destinations as quickly as possible. By tuning the interaction ranges of anticipation and attraction, we explore the collection motion behavior of the pedestrians in three common movement scenarios, i.e., the bidirectional pedestrian flows in a straight corridor, the crossing pedestrian flows in a square and the crowd evacuation in a room. Interestingly, with appropriate combinations of the anticipation distance and attraction distance, we reproduce the typical collective motion patterns of the pedestrians observed in the empirical studies, such as the formation and separation of global pedestrian flow lanes, the stop-and-go waves, and the vortexlike patterns as well. Particularly, we find that a strong attraction effect by the fast-moving pedestrian flows benefits the efficient passing only in the corridor case, whereas in the case of crossing pedestrians the avoidance behavior due to anticipation will play a dominating role in guaranteeing an ordered collective motion.

Active matter states defy many notions that have been established for systems in thermodynamic equilibrium. Nevertheless, the lack of detailed balance might be utilized to design nonequilibrium materials with unique properties. Recently we have shown, employing a model of ring polymers containing segments with a larger mobility than given by equilibrium thermal fluctuations, that making polymers with intrinsic topology active can result in states that relax extremely slowly, the so-called active topological glass. In this paper, we focus on the role of nonequilibrium phase separation in the vitrification process. In particular, we detail the polymer dynamics and show that such activity-driven glassy states arise from heterogeneity of segmental dynamics that emerges on all scales. Provided that the activity quench is strong enough, the rings feature an oriented reptationlike motion, with the active segment serving as an effective chain's end, resulting into a dramatic increase of inter-ring treading that vitrifies the system. The scaling properties of the ensuing steady-state ring conformations, which are significantly elongated and usually possess a doubly folded structure, are discussed and compared to equilibrium counterparts. We further examine the connection between the glass formation and the nonequilibrium phase separation and we find that both appear to be initiated by the contrasting dynamics of ring segments. Finally, we consider the effect of nonequilibrium phase separation in other active copolymer architectures.

We investigate the low-frequency spectrum in a three-dimensional model of structural glass focusing on small system sizes, and using different observables, i.e., the density of states $D\left(\omega \right)$, the cumulative of the density of states $F\left(\omega \right)$, and the dynamical structure factor $S(q,\omega )$ in the harmonic approximation. When the glass is obtained by an instantaneous quench from high temperatures, we show that extended “phonon-like” modes always populate the low-energy spectrum. Looking at the properties of the dynamical structure factor $S(q,\omega )$, we observe that in agreement with early studies of Lennard-Jones glasses [V. Mazzacurati, G. Ruocco, and M. Sampoli, Europhys. Lett. 34, 681 (1996)], there are still extended modes below the lowest resonant peak. These modes give rise to a plateau in the $S(q,\omega )$ for $\omega \to 0$. This result indicates that the low-energy spectrum of extended modes in glasses can be probed using small system sizes and performing instantaneous quench from high parental temperatures. As we recently observed [M. Paoluzzi, L. Angelani, G. Parisi, and G. Ruocco, Phys. Rev. Lett. 123, 155502 (2019)], the situation changes when the glassy configuration is obtained by an instantaneous quench from lower temperatures. The former protocol suppresses extended modes below the lowest resonant peak emphasizing the localized modes with $D\left(\omega \right)\sim {\omega }^4$.

We demonstrate the existence of universal features in the finite-time thermodynamics of quantum machines by considering a many-body quantum Otto cycle in which the working medium is driven across quantum critical points during the unitary strokes. Specifically, we consider a quantum engine powered by dissipative energizing and relaxing baths. We show that under very generic conditions, the output work is governed by the Kibble-Zurek mechanism; i.e., it exhibits a universal power-law scaling with the driving speed through the critical points. We also optimize the finite-time thermodynamics as a function of the driving speed. The maximum power and the corresponding efficiency take a universal form, and are reached for an optimal speed that is governed by the critical exponents. We exemplify our results by considering a transverse-field Ising spin chain as the working medium. For this model, we also show how the efficiency and power vary as the engine becomes critical.

We compare the bfgs optimizer, adam and NatGrad in the context of vqes. We systematically analyze their performance on the qaoa ansatz for the transverse field Ising and the XXZ model as well as on overparametrized circuits with the ability to break the symmetry of the Hamiltonian. The bfgs algorithm is frequently unable to find a global minimum for systems beyond about 20 spins and adam easily gets trapped in local minima or exhibits infeasible optimization durations. NatGrad on the other hand shows stable performance on all considered system sizes, rewarding its higher cost per epoch with reliability and competitive total run times. In sharp contrast to most classical gradient-based learning, the performance of all optimizers decreases upon seemingly benign overparametrization of the ansatz class, with bfgs and adam failing more often and more severely than NatGrad. This does not only stress the necessity for good ansatz circuits but also means that overparametrization, an established remedy for avoiding local minima in machine learning, does not seem to be a viable option in the context of vqes. The behavior in both investigated spin chains is similar, in particular the problems of bfgs and adam surface in both systems, even though their effective Hilbert space dimensions differ significantly. Overall our observations stress the importance of avoiding redundant degrees of freedom in ansatz circuits and to put established optimization algorithms and attached heuristics to test on larger system sizes. Natural gradient descent emerges as a promising choice to optimize large vqes.

We have investigated electron tunneling through an atomically smooth square potential barrier for both the dice lattice and graphene under a linearly polarized off-resonant and high-frequency dressing field. We have demonstrated Klein tunneling for a nonzero angle of incidence which is due to a nonalignment of optically controllable elliptical energy dispersions for the dressed states of Dirac particles and the direction of incoming kinetic particles. This finite angle of incidence has been found to depend on the light-induced anisotropy of energy dispersion, which is a function of the electron-light coupling strength, as well as the misalignment between directions of the light polarization and the electron beam incident on the potential barrier. Additionally, we have discovered much larger off-peak transmission amplitudes for dice lattices in contrast to graphene. We anticipate that the theoretical predictions could be applied to a wide range of Dirac materials and exploited for controlling both coherent tunneling and ballistic transport of electrons in the construction of novel electronic, optical, and valleytronic nanoscale switching devices.

Water mediates electrostatic interactions via the orientation of its dipoles around ions, molecules, and interfaces. This induced water polarization consequently influences multiple phenomena. In particular, water polarization affects ion adsorption and transport, biomolecular self-assembly, and surface chemical reactions. Therefore, it is of paramount importance to understand water-mediated interactions modulated by nanoconfinement at the nanoscale. Here we investigate the effective interaction between two oppositely charged ions in different positions in water confined between two graphene surfaces. We find that the attraction between physisorbed ions is enhanced in the surface normal direction while the in-plane interaction is almost unaffected. The attraction in the surface normal direction is further enhanced by decreasing the confinement distance. Conversely, when one ion is intercalated into the graphene layers, the interaction becomes repulsive. Moreover, upon exchange of the ions' positions along the surface normal direction, the interaction energy changes by about $5k_{B}T$. The nonequivalent and directional properties found here, referred to as nonreciprocal interactions, cannot be explain by current water permittivity models in confinement. Our x-ray reflectivity experiments of the water structure near a graphene surface support our molecular dynamics simulation results. Our work shows that the water structure is not enough to infer electrostatic interactions near interfaces.

Operator square roots are ubiquitous in theoretical physics. They appear, for example, in the Holstein-Primakoff representation of spin operators and in the Klein-Gordon equation. Often the use of a perturbative expansion is the only recourse when dealing with them. In this paper, we show that under certain conditions, differential equations can be derived which can be used to find perturbatively inaccessible approximations to operator square roots. Specifically, for the number operator $\widehat{n}=a^{†}a$ we show that the square root $\sqrt{\widehat{n}}$ near $\widehat{n}=0$ can be approximated by a polynomial in $\widehat{n}$. This result is unexpected because a Taylor expansion fails. A polynomial expression in $\widehat{n}$ is possible because $\widehat{n}$ is an operator, and its constituents $a$ and $a^{†}$ have a non trivial commutator $[a,a^{†}]=1$ and do not behave as scalars. We apply our approach to the zero-mass Klein-Gordon Hamiltonian in a constant magnetic field and, as a main application, the Holstein-Primakoff representation of spin operators, where we are able to find new expressions that are polynomial in bosonic operators. We prove that these new expressions exactly reproduce spin operators. Our expressions are manifestly Hermitian, which offers an advantage over other methods, such as the Dyson-Maleev representation.

We propose an effective field theory (EFT) of fractional quantum Hall systems near the filling fraction $\nu =5/2$ that flows to pertinent IR candidate phases, including non-Abelian Pfaffian, anti-Pfaffian, and particle-hole Pfaffian states (Pf, APf, and PHPf). Our EFT has a $(2+1)\text{D} \mathrm{O}{\left(2\right)}_{2,L}$ Chern-Simons gauge theory coupled to four Majorana fermions by a discrete charge-conjugation gauge field, with Gross-Neveu-Yukawa-Higgs terms. Including deformations via a Higgs condensate and fermion mass terms, we can map out a phase diagram with tunable parameters, reproducing the prediction of the recently proposed percolation picture and its gapless topological quantum phase transitions. Our EFT captures known features of both gapless and gapped sectors of time-reversal-breaking domain walls between Pf and APf phases. Moreover, we find that $\mathrm{Pf}\mid \mathrm{APf}$ domain walls have higher tension than domain walls in the PHPf phase. Then the former, if formed, may transition to the energetically favored PHPf domain walls; this could, in turn, help further induce a bulk transition to PHPf.

We present a large-scale simulation of the three-dimensional Ising spin glass with Gaussian disorder to low temperatures and large sizes using optimized population annealing Monte Carlo. Our primary focus is investigating the number of pure states regarding a controversial statistic, characterizing the fraction of centrally peaked disorder instances, of the overlap function order parameter. We observe that this statistic is subtly and sensitively influenced by the slight fluctuations of the integrated central weight of the disorder-averaged overlap function, making the asymptotic growth behavior very difficult to identify. Modified statistics effectively reducing this correlation are studied, and essentially monotonic growth trends are obtained. The effect of temperature is also studied, finding a larger growth rate at a higher temperature. Our state-of-the-art simulation and variance reduction data analysis suggest that the many pure states picture is most likely and coherent.

Introducing internal-state transitions simultaneously on each branch of a light-pulse atom interferometer, we propose a scheme that is concurrently sensitive to both violations of the universality of free fall and gravitational redshift, two premises of general relativity. In contrast to redshift tests with quantum clocks, a superposition of internal states is not necessary but merely transitions between them, leading to a generalized concept of clocks in this context. The experimental realization seems feasible with already demonstrated techniques.

In this work, we report a ubiquitous presence of topological Floquet time crystal ($\mathrm{TFTC}$) in one-dimensional periodically driven systems. The rigidity and realization of spontaneous discrete time-translation symmetry ($\mathrm{DTS}$) breaking in our $\mathrm{TFTC}$ model require necessarily coexistence of anomalous topological invariants (0 modes and $\pi$ modes), instead of the presence of disorders or many-body localization. We found that in a particular frequency range of the underlying drive, the anomalous Floquet phase coexistence between 0 and $\pi$ modes can produce the period doubling ($2T$, two cycles of the drive) that breaks the $\mathrm{DTS}$ spontaneously, leading to the subharmonic response ($\omega /2$, half the drive frequency). The rigid period-$2T$ oscillation is topologically protected against perturbations due to both nontrivially opening of 0 and $\pi$ gaps in the quasienergy spectrum, thus, as a result, can be viewed as a specific “Rabi oscillation” between two Floquet eigenstates with certain quasienergy splitting $\pi /T$. Our modeling of the time-crystalline “ground state” can be easily realized in experimental platforms such as topological photonics and ultracold fields. Also, our work can bring significant interest to explore topological phase transition in Floquet systems and to bridge the gap between Floquet topological insulators and photonics, and period-doubled time crystals.

One of the potential applications of a quantum computer is solving quantum chemical systems. It is known that one of the fastest ways to obtain somewhat accurate solutions classically is to use approximations of density functional theory. We demonstrate a general method for obtaining the exact functional as a machine learned model from a sufficiently powerful quantum computer. Only existing assumptions for the current feasibility of solutions on the quantum computer are used. Several known algorithms including quantum phase estimation, quantum amplitude estimation, and quantum gradient methods are used to train a machine learned model. One advantage of this combination of algorithms is that the quantum wavefunction does not need to be completely re-prepared at each step, lowering a sizable prefactor. Using the assumptions for solutions of the ground-state algorithms on a quantum computer, we demonstrate that finding the Kohn-Sham potential is not necessarily more difficult than the ground-state density. Once constructed, a classical user can use the resulting machine learned functional to solve for the ground state of a system self-consistently, provided the machine learned approximation is accurate enough for the input system. It is also demonstrated how the classical user can access commonly used time- and temperature-dependent approximations from the ground-state model. Minor modifications to the algorithm can learn other types of functional theories including exact time and temperature dependence. Several other algorithms—including quantum machine learning—are demonstrated to be impractical in the general case for this problem.

We study the validity of the fluctuation relation for a non-equilibrium quantum mechanical system, viz., electrons quantum mechanically tunneling across a periodically modulating barrier. Experimentally this system is realized by measuring the fluctuation in the tunneling current between an STM tip and a vibrating gold film deposited on a piezo crystal. The long time series of tunneling signal shows large positive and negative fluctuations. The analysis shows that over finite observation time intervals, the probability distribution of the average rate of work done has a positive mean. About this mean, the distribution is broad and it is spread over not only positive but also negative values. These positive and negative values correspond to work done either on the electron by the external drive or work done by the electron against the drive, as it tunnels across the modulating barrier, respectively. For different driving frequencies, we show that the probability distribution satisfies the non-equilibrium fluctuation relation (NEFR). Thus, we prove that the NEFR is valid for a driven quantum mechanical system. For this tunneling process, we determine the large deviation function (LDF), which is related to NEFR. We see changes in the shape of the LDF as a function of the drive frequency, although NEFR is valid at all these frequencies. Measuring dissipation associated with microscopic irreversible trajectories in non-equilibrium quantum mechanical systems is a challenging task. Here we use NEFR also to obtain a measure of the dissipation associated with the electron tunneling across the modulated barrier.

Link prediction algorithms can help to understand the structure and dynamics of complex systems, to reconstruct networks from incomplete data sets, and to forecast future interactions in evolving networks. Available algorithms based on similarity between nodes are bounded by the limited amount of links present in these networks. In this Rapid Communication, we reduce this latter intrinsic limitation and show that different kinds of relational data can be exploited to improve the prediction of new links. To this aim, we propose a link prediction algorithm by generalizing the Adamic-Adar method to multiplex networks composed by an arbitrary number of layers, that encode diverse forms of interactions. We show that this metric outperforms the classical single-layered Adamic-Adar score and other state-of-the-art methods, across several social, biological, and technological systems. As a by-product, the coefficients that maximize the multiplex Adamic-Adar metric indicate how the information structured in a multiplex network can be optimized for the link prediction task, revealing which layers are redundant. Interestingly, this effect can be asymmetric with respect to predictions in different layers. Our work paves the way for a deeper understanding of the role of different relational data in predicting new interactions and provides another algorithm for link prediction in multiplex networks that can be applied to a plethora of systems.

We improve the efficiency of the minimally entangled typical thermal states (METTS) algorithm without breaking the Abelian symmetries. By adding the operation of Trotter gates that respects the Abelian symmetries to the METTS algorithm, we find that a correlation between successive states in Markov-chain Monte Carlo sampling decreases by orders of magnitude. We measure the performance of the improved METTS algorithm through the simulations of the canonical ensemble of the Bose-Hubbard model and confirm that the reduction of the autocorrelation leads to the reduction of computation time. We show that our protocol using the operation of Trotter gates is effective also for the simulations of the grand canonical ensemble.