We study the effects of temperature on the topological nature of ${\mathrm{ZrTe}}_5$, which sits near the phase boundary between strong and weak topological insulating orders. Using first-principles calculations, we show that the band gap and the topological indices of ${\mathrm{ZrTe}}_5$ are extremely sensitive to thermal expansion and that the electron-phonon interaction accounts for more than a third of the total temperature-dependent effects in ${\mathrm{ZrTe}}_5$. We find that the temperature dependence of the band gap has an opposite sign in the strong and weak topological insulator phases. Based on this insight, we propose a robust and unambiguous method to determine the topological nature of any given sample of ${\mathrm{ZrTe}}_5$: If the band gap decreases with temperature it is a strong topological insulator, and if it increases with temperature it is a weak topological insulator. An analogous strategy is expected to be generally applicable to other materials and to become particularly important in the vicinity of topological phase boundaries where other methods provide ambiguous results.

We show that a strained wire of a Weyl semimetal supports another type of gapless excitation, the chiral sound wave (CSW). It is a longitudinal charge density wave analog to the chiral magnetic wave predicted in the quark-gluon plasma but driven by an elastic axial pseudomagnetic field. It involves the axial-axial-axial contribution to the chiral anomaly which couples the chiral charge density to the elastic axial gauge field. The chiral sound is unidirectional: It propagates along the elastic magnetic field and not in the opposite direction. The CSW may propagate for long distances as it does not couple directly to quickly dissipating electromagnetic plasmons, while its damping is controlled by the slow chirality flip rate. We propose an experimental setup to directly detect the chiral sound, which is excited by mechanical vibrations of the crystal lattice in the GHz frequency range. Our findings contribute to the trend, the chiral acoustics, in strained Weyl semimetals.

The many-body localization transition in quasiperiodic systems has been extensively studied in recent ultracold atom experiments. At intermediate quasiperiodic potential strength, a surprising Griffiths-like regime with slow dynamics appears in the absence of random disorder and mobility edges. In this Rapid Communication, we study the interacting Aubry-Andre model, a prototype quasiperiodic system, as a function of incommensurate potential strength using a dynamical measure, information scrambling, in a large system of 200 lattice sites. Between the thermal phase and the many-body localized phase, we find an intermediate dynamical phase where the butterfly velocity is zero and information spreads in space as a power law in time. This is in contrast to the ballistic spreading in the thermal phase and logarithmic spreading in the localized phase. We further investigate the entanglement structure of the many-body eigenstates in the intermediate phase and find strong fluctuations in eigenstate entanglement entropy within a given energy window, which is inconsistent with the eigenstate thermalization hypothesis. Machine learning on the entanglement spectrum also reaches the same conclusion. Our large-scale simulations suggest that the intermediate phase with a vanishing butterfly velocity could be responsible for the slow dynamics seen in recent experiments.

This paper considers the dynamics in the Charney-Hasegawa-Mima equation, basic to several different phenomena. In each of them, the generation of poloidal/zonal flow is important. The paper suggests a possibility to generate such flows (which can serve as transport barriers). Namely, one needs to create significant increments and decrements in the neighborhoods of some wave vectors ${\mathbf{k}}_1$ and ${\mathbf{k}}_2$ (respectively) such that (1) $R_{{\mathbf{k}}_1}<R_{{\mathbf{k}}_2}$, where $R_{\mathbf{k}}$ is the spectral density of the extra invariant kernel ($I=\int R_{\mathbf{k}}{E}_{\mathbf{k}}d\mathbf{k}$ is the extra invariant, with $E_{\mathbf{k}}$ being the energy spectrum), (2) $|{\mathbf{k}}_1|<|{\mathbf{k}}_2|$, and (3) ${\mathbf{k}}_1+{\mathbf{k}}_2$ is a poloidal/zonal wave vector. These three conditions define a quite narrow region.

In a fertile patch of the string landscape which includes the minimal supersymmetric standard model (MSSM) as the low-energy effective theory, rather general arguments from Douglas suggest a power-law statistical selection of soft breaking terms $[m_{\mathrm{soft}}^n$ where $n=2n_F+n_D-1$ with $n_F$ the number of hidden sector $F$-supersymmetry (SUSY) breaking fields and $n_D$ the number of $D$-term SUSY breaking fields]. The statistical draw towards large soft terms must be tempered by requiring an appropriate breakdown of electroweak (EW) symmetry with no contributions to the weak scale larger than a factor 2–5 of its measured value, lest one violates the (anthropic) atomic principle. Such a simple picture of stringy naturalness generates a light Higgs boson with mass $m_h\simeq 125$ GeV with sparticles (other than higgsinos) typically beyond LHC reach. Then we expect first and second generation matter scalars to be drawn independently to the tens of TeV regime where the upper cutoff arises from two-loop renormalization group equation (RGE) terms that drive third generation soft masses towards tachyonic values. Since the upper bounds on $m_0(1,2)$ are the same for each generation, and flavor independent, then these will be drawn toward quasidegenerate values. This mechanism leads to a natural mixed decoupling/quasidegeneracy solution to the SUSY flavor problem and a decoupling solution to the SUSY $CP$ problem.

The efficient certification of nonclassical effects of light forms the basis for applications in optical quantum technologies. We derive general correlation conditions for the verification of nonclassical light based on multiplexed detection. The obtained nonclassicality criteria are valid for imperfectly balanced multiplexing scenarios with on-off detectors and do not require any knowledge about the detector system. In this sense, they are fully independent of the detector system. In our experiment, we study light emitted by clusters of single-photon emitters, whose photon number may exceed the number of detection channels. Even under such conditions, our criteria certify nonclassicality with high statistical significance.

We investigate the ground-state energy of a one-dimensional Fermi gas with two bosonic impurities. We consider spinless fermions with no fermion-fermion interactions. The fermion-impurity and impurity-impurity interactions are modeled with Dirac delta functions. First, we study the case where impurity and fermion have equal masses, and the impurity-impurity two-body interaction is identical to the fermion-impurity interaction, such that the system is solvable with the Bethe ansatz. For attractive interactions, we find that the energy of the impurity-impurity subsystem is below the energy of the bound state that exists without the Fermi gas. We interpret this as a manifestation of attractive boson-boson interactions induced by the fermionic medium, and refer to the impurity-impurity subsystem as an in-medium bound state. For repulsive interactions, we find no in-medium bound states. Second, we construct an effective model to describe these interactions, and compare its predictions to the exact solution. We use this effective model to study nonintegrable systems with unequal masses and/or potentials. We discuss parameter regimes for which impurity-impurity attraction induced by the Fermi gas can lead to the formation of in-medium bound states made of bosons that repel each other in the absence of the Fermi gas.

Quantum computing technologies promise to revolutionize calculations in many areas of physics, chemistry, and data science. Their power is expected to be especially pronounced for problems where direct analogs of a quantum system under study can be encoded coherently within a quantum computer. A first step toward harnessing this power is to express the building blocks of known physical systems within the language of quantum gates and circuits. In this paper we present a quantum calculation of an archetypal quantum system: neutrino oscillations. We define gate arrangements that implement the neutral lepton mixing operation and neutrino time evolution in two-, three-, and four-flavor systems. We then calculate oscillation probabilities by coherently preparing quantum states within the processor, time evolving them unitarily, and performing measurements in the flavor basis, with close analogy to the physical processes realized in neutrino oscillation experiments. Evaluations on publicly available quantum processors obtain excellent agreement with classical calculations. We provide recipes for modeling oscillation in the standard three-flavor paradigm as well as beyond-standard-model scenarios, including systems with sterile neutrinos, nonstandard interactions, Lorentz symmetry violation, and anomalous decoherence.

Usually the calculation of work distributions in an arbitrary nonequilibrium process in a quantum system, especially in a quantum many-body system, is extremely cumbersome. For all quantum systems described by quadratic Hamiltonians, we propose a universal method for solving the work distribution of quantum systems in an arbitrary driving process by utilizing the group-representation theory. This method enables us to efficiently calculate work distributions where previous methods fail. In some specific models, such as the time-dependent harmonic oscillator, the dynamical Casimir effect, and the transverse XY model, the exact and analytical solutions of work distributions in an arbitrary nonequilibrium process are obtained. Our work initiates the study of quantum stochastic thermodynamics based on group-representation theory.

Causal nonseparability refers to processes where events take place in a coherent superposition of different causal orders. These may be the key resource for experimental violations of causal inequalities and have been recently identified as resources for concrete information-theoretic tasks. Here, we take a step forward by deriving a complete operational framework for causal nonseparability as a resource. Our first contribution is a formal definition for the specific notion of quantum control of causal orders, a stronger form of causal nonseparability—with the celebrated quantum switch as best-known example—where the causal orders of events for a target system are coherently controlled by a control system. We then build a resource theory—for both generic causal nonseparability as well as quantum control of causal orders—with a physically motivated class of free operations, based on process-matrix concatenations. We present the framework explicitly in the mindset with a control register. However, our machinery is totally versatile, being directly applicable also to scenarios with a target register alone. Moreover, an important subclass of our operations is free not only with respect to causal nonseparability and quantum control of causal orders but it also preserves the very causal structure of causal processes. Hence, our treatment contains, as a built-in feature, the basis of a resource theory of quantum causal networks too. As applications, first, we establish a simple sufficient condition for pure-process free convertibility. This imposes a hierarchy of quantum control of causal orders with the quantum switch at the top. Second, we prove that causal-nonseparability distillation exists. More precisely, we show how to convert multiple copies of a process with arbitrarily little causal nonseparability into fewer copies of a quantum switch. Our findings reveal conceptually new, unexpected phenomena, with both fundamental and practical implications.

We propose a method to realize a broad class of tunable fermionic Hamiltonians in graphene bilayer. For that matter, we consider graphene bilayer functionalized with ${\mathrm{sp}}^3$ defects that induce zero-energy resonances hosting an individual electron each. The application of an off-plane electric field opens up a gap, so that the zero-energy resonance becomes an in-gap bound state whose confinement scales inversely with the gap. Controlling both the distance among the defects and the applied electric field, we can define fermionic models, even lattices, whose hoppings and Coulomb interactions can be tuned. We consider in detail the case of triangular and honeycomb artificial lattices and we show how, for a given arrangement of the ${\mathrm{sp}}^3$ centers, these lattices can undergo an electrically controlled transition between the weak and strong coupling regimes.

In a growing number of strongly disordered and dense systems, the dynamics of a particle pulled by an external force field exhibits superdiffusion. In the context of glass-forming systems, supercooled glasses, and contamination spreading in porous media, it was suggested that this behavior be modeled with a biased continuous-time random walk. Here we analyze the plume of particles lagging far behind the mean, with the single big jump principle. Revealing the mechanism of the anomaly, we show how a single trapping time, the largest one, is responsible for the rare fluctuations in the system. These nontypical fluctuations still control the behavior of the mean square displacement, which is the most basic quantifier of the dynamics in many experimental setups. We show how the initial conditions, describing either the stationary state or nonequilibrium case, persist forever in the sense that the rare fluctuations are sensitive to the initial preparation. To describe the fluctuations of the largest trapping time, we modify Fréchet's law from extreme value statistics, taking into consideration the fact that the large fluctuations are very different from those observed for independent and identically distributed random variables.

Optical microdisk cavities with certain asymmetric shapes are known to possess unidirectional far-field emission properties. Here, we investigate arrays of these dielectric microresonators with respect to their emission properties resulting from the coherent behavior of the coupled constituents. This approach is inspired by electronic mesoscopic physics where the additional interference effects are known to enhance the properties of the individual system. As an example, we study the linear arrangement of nominally identical Limaçon-shaped cavities and find mostly an increase of the portion of directional emitted light while its angular spread is largely diminished from 20 deg for the single cavity to about 3 deg for a linear array of 10 Limaçon resonators, in fair agreement with a simple array model. Moreover, by varying the intercavity distance, we observe windows of reversion of the emission directionality and superdirectionality that can be interesting for applications like optical sensing or interconnects. We introduce a generalized array factor model that takes the coupling into account.

Recent experiments have revealed that cytoplasms become glassy when their metabolism is suppressed, while they maintain fluidity in a living state. The mechanism of this active fluidization is not clear, especially for bacterial cytoplasms, since they lack traditional motor proteins, which can cause directed motions. We introduce a model of bacterial cytoplasm focusing on the impact of conformational change in proteins due to metabolism. In the model, proteins are treated as particles under thermal agitation and conformation changes are treated as changes in particle volume. Simulations reveal that a small change in volume fluidizes the glassy state, accompanied by a change in fragility, as observed experimentally.

Using position averaged convergent beam electron diffraction (PACBED) the Jahn-Teller distortion in $\mathrm{LaMn}{\mathrm{O}}_3$ is quantitatively measured using a straightforward pattern-matching approach. The fit between experimental patterns and PACBED patterns simulated using the quantum excitation of phonons model allows a three-dimensional measure of octahedral distortion and rotation information from the near transmitted disk region with picometer precision. The effects of plasmon and other inelastic components on quantification using this method are investigated and discussed. The results provide an avenue for accurate local studies of the crystal structure origins of emergent physics in parallel with high-resolution annular dark field scanning transmission electron microscopy imaging at interfaces and defects in quantum materials.

We investigate the spatial nonlinear localization of light on a quasi-plane-wave background with a harmonic perturbation induced by modulation instability in a quadratic nonlinear optical medium. In particular, we demonstrate experimentally the excitation of deterministic Akhmediev breathers and thus the growth-decay dynamics of modulation instability in a ${\mathrm{LiNbO}}_3$ slab waveguide. The results should stimulate new interest in modulation instability, extreme events, turbulence, recurrence, and supercontinuum generation in quadratic nonlinear optics.

This paper systematically develops the resource theory of asymmetric distinguishability, as initiated roughly a decade ago [Matsumoto, Reverse test and characterization of quantum relative entropy, arXiv:1010.1030]. The key constituents of this resource theory are quantum boxes, consisting of a pair of quantum states, which can be manipulated for free by means of an arbitrary quantum channel. We introduce bits of asymmetric distinguishability as the basic currency in this resource theory, and we prove that it is a reversible resource theory in the asymptotic limit, with the quantum relative entropy being the fundamental rate of resource interconversion. The distillable distinguishability is the optimal rate at which a quantum box consisting of independent and identically distributed (i.i.d.) states can be converted to bits of asymmetric distinguishability, and the distinguishability cost is the optimal rate for the reverse transformation. Both of these quantities are equal to the quantum relative entropy. The exact one-shot distillable distinguishability is equal to the min-relative entropy, and the exact one-shot distinguishability cost is equal to the max-relative entropy. Generalizing these results, the approximate one-shot distillable distinguishability is equal to the smooth min-relative entropy, and the approximate one-shot distinguishability cost is equal to the smooth max-relative entropy. As a notable application of the former results, we prove that the optimal rate of asymptotic conversion from a pair of i.i.d. quantum states to another pair of i.i.d. quantum states is fully characterized by the ratio of their quantum relative entropies.

This paper develops the resource theory of asymmetric distinguishability for quantum channels, generalizing the related resource theory for states [Matsumoto, arXiv:1010.1030, Wang and Wilde, Phys. Rev. Research 1, 033170 (2019)]. The key constituents of the channel resource theory are quantum channel boxes, consisting of a pair of quantum channels, which can be manipulated for free by means of an arbitrary quantum superchannel (the most general physical transformation of a quantum channel). One main question of the resource theory is the approximate channel box transformation problem, in which the goal is to transform an initial channel box (or boxes) to a final channel box (or boxes), while allowing for an asymmetric error in the transformation. The channel resource theory is richer than its counterpart for states because there is a wider variety of ways in which this question can be framed, either in the one-shot or $n$-shot regimes, with the latter having parallel and sequential variants. As in our prior work [Wang and Wilde, Phys. Rev. Research 1, 033170 (2019)], we consider two special cases of the general channel box transformation problem, known as distinguishability distillation and dilution. For the one-shot case, we find that the optimal values of the various tasks are equal to the nonsmooth or smooth channel min- or max-relative entropies, thus endowing all of these quantities with operational interpretations. In the asymptotic sequential setting, we prove that the exact distinguishability cost is equal to the channel max-relative entropy and the distillable distinguishability is equal to the amortized channel relative entropy of [Berta, Hirche, Kaur, and Wilde, arXiv:1808.01498]. This latter result can also be understood as a solution to Stein's lemma for quantum channels in the sequential setting. Finally, the theory simplifies significantly for environment-seizable and classical-quantum channel boxes.

The optical excitation of $\mathrm{Nb}{\mathrm{O}}_2$ is known to induce a transient metallicity that partially recovers on a subpicosecond speed and only fully recovers after several nanoseconds. Here, through low-fluence pump-probe spectroscopy, the short-lived and long-lived components of $\mathrm{Nb}{\mathrm{O}}_2$’s optically induced modulation are found to be linearly dependent on pump fluence in the low-fluence regime. Through an in-depth analysis, the short-lived and long-lived modulations are found to be caused by photocarrier-induced refractive index changes and photothermally induced changes in the lattice temperature of $\mathrm{Nb}{\mathrm{O}}_2$, respectively, and not an insulator-metal transition.

Natural gamma radiation (NGR), one of the major geophysical parameters directly connected with cloud electrification and lightning initiation, is highly enhanced during thunderstorms. At low energies below 3 MeV, the enhancement of NGR is due to natural isotope radiation, and for energies up to 50 MeV, it is due to the operation of the newly discovered electron accelerators in the thunderclouds. For the first time, we present a comprehensive model of the enhanced fluxes of radiation incident on the earth's surface during thunderstorms. In addition to the already explained minute-long fluxes of high-energy electrons and gamma rays from relativistic runaway electron avalanches (RREA), we clarify also the origin of hour-long isotropic fluxes of low-energy gamma rays from the Rn-222 progenies. Also, as a direct evidence of RREA, we present photographs of optical emission during the development of electron-gamma ray cascades in the atmosphere. Natural radioactivity is a source of continuous exposure of human beings to radiation. Radiation protection of living organisms requires an understanding of all sources and possible ways of enhancement of the radiation levels that can double for several hours in the energy domain of hundreds of keV. Therefore individual irradiation doses can be exceeded during thunderstorms. The models used for the forecasting of thunderstorms and other severe atmospheric phenomena need an accurate account of the ionizing radiation in the atmosphere. The airglows can influence the operation of optical, fluorescence, and atmospheric Cherenkov telescopes and fluorescence detectors.