Although stoquastic Hamiltonians are known to be simulable via sign-problem-free quantum Monte Carlo (QMC) techniques, the nonstoquasticity of a Hamiltonian does not necessarily imply the existence of a QMC sign problem. We give a sufficient and necessary condition for the QMC-simulability of Hamiltonians in a given basis: We prove that a QMC simulation will be sign-problem-free if and only if all the overall total phases along the chordless cycles of the weighted graph whose adjacency matrix is the Hamiltonian are zero (modulo $2\pi$). We use our findings to provide a construction for nonstoquastic, yet sign-problem-free and hence QMC-simulable, quantum many-body models. We also demonstrate why the simulation of truly sign-problematic models using the QMC weights of the stoquasticized Hamiltonian is generally suboptimal. We offer a superior alternative.

The interaction between quantized electromagnetic fields in cavities and natural or artificial atoms has played a crucial role in developing our understanding of light-matter interactions and quantum technologies. Recently, new regimes beyond the weak and strong light-matter coupling of cavity-QED have been explored in several settings, wherein the light-matter coupling rate becomes comparable to (ultrastrong coupling) or even exceeds (deep-strong coupling) the photon frequency. These ultrastrong coupling regimes can give rise to new physical effects and applications, and they challenge our understanding of cavity QED; fundamental issues like the proper definition of subsystems, their quantum measurements, the structure of light-matter ground states, and the analysis of time-dependent interactions are subject to gauge ambiguities that lead to even qualitatively distinct predictions. The resolution of these ambiguities is important for understanding and designing next-generation quantum devices that can operate in extreme coupling regimes. Here we discuss and provide solutions to these ambiguities by adopting an approach based on operational procedures involving measurements on the individual light and matter components of the interacting system.

We study a limit cycle of a quantum Otto engine whose every cycle consists of two finite-time quantum isochoric (heating or cooling) processes and two quantum adiabatic work-extracting processes. Considering a two-level system as a working substance that weakly interacts with two reservoirs comprising an infinite number of bosons, we investigate the non-Markovian effect [short-time behavior of the reduced dynamics in the quantum isochoric processes (QIPs)] on work extraction after infinite repetition of the cycles. We focus on the parameter region where energy transferred to the reservoir can come back to the system in a short-time regime, which we call energy backflow to show partial quantum-mechanical reversibility. As a situation completely different from macroscopic thermodynamics, we find that the interaction energy is finite and negative by evaluating the average energy change of the reservoir during the QIPs by means of the full-counting statistics, corresponding to the two-point measurements. This feature leads us to the following findings: (1) The Carnot theorem is consistent with a definition of work including the interaction energy, although the commonly used definition of work excluding the interaction leads to a serious conflict with the thermodynamic law, and (2) the energy backflow can increase the work extraction. Our findings show that we need to pay attention to the interaction energy in designing a quantum Otto engine operated in a finite time, which requires us to include the non-Markovian effect, even when the system-reservoir interaction is weak.

Heisenberg's uncertainty principle, which imposes intrinsic restrictions on our ability to predict the outcomes of incompatible quantum measurements to arbitrary precision, demonstrates one of the key differences between classical and quantum mechanics. The physical systems considered in the uncertainty principle are static in nature and described mathematically with a quantum state in a Hilbert space. However, many physical systems are dynamic in nature and described with the formalism of a quantum channel. In this paper, we show that the uncertainty principle can be reformulated to include process measurements that are performed on quantum channels. Since both the preparation of quantum states and the implementation of quantum measurements are themselves special cases of quantum channels, our formalism encapsulates the uncertainty principle in its utmost generality. More specifically, we obtain expressions that generalize the Maassen–Uffink uncertainty relation and the universal uncertainty relations from quantum states to quantum channels.

The concept of Fermi liquid lays a solid cornerstone to the understanding of electronic correlations in quantum matter. This ordered many-body state rigorously organizes electrons at zero temperature in progressively higher momentum states, up to the Fermi surface. As such, it displays rigidity against perturbations. Such rigidity generates Fermi-surface resonances which manifest as longitudinal and transverse collective modes. Although these Fermi-liquid collective modes have been analyzed and observed in electrically neutral liquid helium, they remain unexplored in charged solid-state systems up to date. In this paper I analyze the transverse shear response of charged three-dimensional Fermi liquids as a function of temperature, excitation frequency and momentum, for interactions expressed in terms of the first symmetric Landau parameter. I consider the effect of momentum-conserving quasiparticle collisions and momentum-relaxing scattering in relaxation-time approximation on the coupling between photons and Fermi-surface collective modes, thus deriving the Fermi-liquid optical conductivity and dielectric function. In the high-frequency, long-wavelength excitation regime the electrodynamic response entails two coherent and frequency-degenerate polaritons, and its spatial nonlocality is encoded by a frequency- and interaction-dependent generalized shear modulus; in the opposite high-momentum low-frequency regime anomalous skin effect takes place. I identify observable signatures of propagating shear collective modes in optical spectroscopy experiments, with applications to the surface impedance and the optical transmission of thin films.

Despite many efforts to rationalize the strongly correlated electronic ground states in doped Mott insulators, the nature of the doping-induced insulator-to-metal transition is still a subject under intensive investigation. Here, we probe the nanoscale electronic structure of the Mott insulator ${\mathrm{Sr}}_2{\mathrm{IrO}}_{4-\delta }$ with low-temperature scanning tunneling microscopy and find an enhanced local density of states (LDOS) inside the Mott gap at the location of individual defects which we interpret as defects at apical oxygen sites. A chiral behavior in the topography for those defects has been observed. We also visualize the local enhanced conductance arising from the overlapping of defect states which induces finite LDOS inside of the Mott gap. By combining these findings with the typical spatial extension of isolated defects of about 2 nm, our results indicate that the insulator-to-metal transition in ${\mathrm{Sr}}_2{\mathrm{IrO}}_{4-\delta }$ could be percolative in nature.

We investigate the bound on the speed of state transformation in the quantum systems that are coupled to general environments. We focus on the energy fluctuation of the target system and the coupling to the environment, and clarify the universal constraint on the state transformation deriving the general speed-limit inequality. This speed-limit inequality is applicable to any open systems including physical situations that have not been studied so far. We apply the inequality to the thermalization phenomena in isolated quantum systems, and the quantum cyclic engines. For the thermalization, we provide several inequalities to estimate the equilibration time. In the cyclic heat engines, we derive a rigorous constraint on the state change in the cycle where the Planck constant plays a fundamental role.

Belief propagation is a well-studied message-passing algorithm that runs over graphical models and can be used for approximate inference and approximation of local marginals. The resulting approximations are equivalent to the Bethe-Peierls approximation of statistical mechanics. Here, we show how this algorithm can be adapted to the world of projected-entangled-pair-state tensor networks and used as an approximate contraction scheme. We further show that the resultant approximation is equivalent to the “mean field” approximation that is used in the simple-update algorithm, thereby showing that the latter is essentially the Bethe-Peierls approximation. This shows that one of the simplest approximate contraction algorithms for tensor networks is equivalent to one of the simplest schemes for approximating marginals in graphical models in general and paves the way for using improvements of belief propagation as tensor networks algorithms.

Ultrafast spectroscopies can access the dynamics of electrons and nuclei at short timescales, shedding light on nonequilibrium phenomena in materials. However, development of accurate calculations to interpret these experiments has lagged behind as widely adopted simulation schemes are limited to subpicosecond timescales or employ simplified interactions lacking quantitative accuracy. Here we show a precise approach to obtain the time-dependent populations of nonequilibrium electrons and atomic vibrations (phonons) up to tens of picoseconds, with a femtosecond time resolution. Combining first-principles electron-phonon and phonon-phonon interactions with a parallel numerical scheme to time-step the coupled electron and phonon Boltzmann equations, our method provides microscopic insight into scattering mechanisms in excited materials. Focusing on graphene as a case study, we demonstrate calculations of ultrafast electron and phonon dynamics, transient optical absorption, structural snapshots, and diffuse x-ray scattering. Our first-principles approach paves the way for quantitative atomistic simulations of ultrafast dynamics in materials.

The strong light-matter optomechanical coupling offered by coherent scattering set-ups have allowed the experimental realization of quantum ground-state cavity cooling of the axial motion of a levitated nanoparticle [U. Delić et al., Science 367, 892 (2020)]. An appealing milestone is now quantum two-dimensional (2D) cooling of the full in-plane motion, in any direction in the transverse plane. By a simple adjustment of the trap polarization, one obtains two nearly equivalent modes, with similar frequencies ${\omega }_x\sim {\omega }_y$ and optomechanical couplings $g_x\simeq g_y$—in this experimental configuration we identify an optimal trap ellipticity, nanosphere size, and cavity linewidth which allows for efficient 2D cooling. Moreover, we find that 2D cooling to occupancies $n_x+n_y\lesssim 1$ at moderate vacuum (${10}^{-6}$ mbar) is possible in a “Goldilocks” zone bounded by $\sqrt{\kappa \mathrm{\Gamma }/4}\lesssim g_x,g_y\lesssim |{\omega }_x-{\omega }_y|\lesssim \kappa$, where one balances the need to suppress dark modes while avoiding far-detuning of either mode or low cooperativities, and $\kappa$ ($\mathrm{\Gamma }$) is the cavity decay rate (motional heating rate). With strong-coupling regimes $g_x,g_y\gtrsim \kappa$ in view one must consider the genuine three-way hybridization between $x,y$ and the cavity light mode resulting in hybridized bright/dark modes. Finally, we show that bright/dark modes in the levitated set-up have a simple geometrical interpretation, related by rotations in the transverse plane, with implications for directional sensing.

Spontaneous self-assembly in molecular systems is a fundamental route to both biological and engineered soft matter. Simple micellization, emulsion formation, and polymer mixing are well understood. However, the principles behind emergence of structures with competing length scales in soft matter systems remain unknown. Examples include droplet-inside-droplet assembly in many biomacromolecular systems undergoing liquid-liquid phase separation, analogous multiple emulsion formation in oil-surfactant-water formulations, and polymer core-shell particles with internal structure. We develop here a microscopic theoretical model based on effective interactions between the constituents of a soft matter system to explain self-organization both at single and multiple length scales. The model identifies how spatial ordering at multiple length scales emerges due to competing interactions between the system components, e.g., molecules of different sizes and different chemical properties. As an example of single and multiple length scale assembly, we map out a generic phase diagram for a solution with two solute species differing in their mutual and solvent interactions. We further connect the phase diagram to a molecular system via molecular simulations of a block-copolymer system that has a transition from regular single-core polymer particles to multicore aggregates that exhibit multiple structural length scales. The findings provide guidelines to understanding the length scales rising spontaneously in biological self-assembly but also open venues to the development and engineering of biomolecular and polymeric functional materials and pharmaceutical formulations.

Integration of contextual sensors into vehicle-borne mobile radiation detection systems delivers a rich description of the environment to inform estimates of the complex and variable gamma-ray signals observed in urban areas. Models based on these data streams could provide realistic inputs to urban radiological search algorithms and potentially improve the system's sensitivity to detect illicit radiological and nuclear materials. In this work, LiDAR and inertial data are combined using simultaneous localization and mapping techniques to create a three-dimensional (3D) representation of the surrounding scenery. Semantic segmentation of concurrently collected video imagery enables the division of the 3D model into distinct material categories. The radioactive flux of surfaces associated with these categories are inferred through maximum likelihood estimation maximization and the activity of the three most common isotopes (K-40, U-238 series, and Th-232 series) in the respective materials is predicted. The results, found to be in agreement with ground truth measurements performed at the facility, suggest that it is possible to quickly infer the composition of naturally occurring materials in structures that comprise a radiological scene. Such a capability could be used to inform radiological search algorithms and enable data-driven modeling of radiological search problems, which could facilitate system testing and operator training activities.

Strange metals possess transport properties that are markedly different from those of a conventional Fermi liquid. Despite strong similarities in behavior exhibited by distinct families, a consistent description of strange metallic transport and, in particular, its evolution from low to high magnetic field strength $H$, is still lacking. The electron nematic $\mathrm{Fe}{\mathrm{Se}}_{1-x}{\mathrm{S}}_x$ is one such strange metal displaying anomalous $H/T$ scaling in its transverse magnetoresistance as well as a separation of transport and Hall lifetimes at low $H$ beyond its (nematic) quantum critical point at $x_c\sim 0.17$. Here we report a study of the Hall response of $\mathrm{Fe}{\mathrm{Se}}_{1-x}{\mathrm{S}}_x$ across $x_c$ in fields up to 33 T. Upon subtraction of a normal $H$-linear component from the total Hall response (imposed by perfect charge compensation), we find a second component, ascribable to strange metal physics, that grows as $1/T$ upon approach to the quantum critical point. Through this decomposition, we reveal that lifetime separation is indeed driven primarily by the presence of the strange metal component.

We unravel the many-body dynamics of a harmonically trapped impurity colliding with a bosonic medium confined in a double well upon quenching the initially displaced harmonic trap to the center of the double well. We reveal that the emerging correlation dynamics crucially depends on the impurity-medium interaction strength allowing for a classification into different dynamical response regimes. For strong attractive impurity-medium couplings the impurity is bound to the bosonic bath, while for intermediate attractions it undergoes an effective tunneling. In the case of weak attractive or repulsive couplings the impurity penetrates the bosonic bath and performs a dissipative oscillatory motion. Further increasing the impurity-bath repulsion results in the pinning of the impurity between the density peaks of the bosonic medium, a phenomenon that is associated with a strong impurity-medium entanglement. For strong repulsions, the impurity is totally reflected by the bosonic medium. To unravel the underlying microscopic excitation processes accompanying the dynamics, we employ an effective potential picture. We extend our results to the case of two bosonic impurities and demonstrate the existence of a qualitatively similar impurity dynamics.

Ti substituting Ru in ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$ in small concentrations induces incommensurate spin density wave order with a wave vector $\mathit{Q}\simeq (2\pi /3,2\pi /3)$ corresponding to the nesting vector of two of three Fermi surface sheets. We consider a microscopic model for these two bands and analyze the correlation effects leading to magnetic order through nonmagnetic Ti doping. For this purpose we use a position-dependent mean-field approximation for the microscopic model and a phenomenological Ginzburg-Landau approach, which both deliver consistent results and allow us to examine the inhomogeneous magnetic order. Spin-orbit coupling additionally leads to spin currents around each impurity, which in combination with the magnetic polarization produce a charge current pattern. This is also discussed within a gauge-field theory in both charge and spin channel. This spin-orbit coupling effect causes an interesting modification of the magnetic structure if currents run through the system. Our findings allow a more detailed analysis of the experimental data for ${\mathrm{Sr}}_2{\mathrm{Ru}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_4$. In particular, we find that the available measurements are consistent with our theoretical predictions.

We report thermopower measurements under hydrostatic pressure on the cuprate superconductor ${\mathrm{La}}_{1.6-x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (Nd-LSCO), at low temperature in the normal state accessed by suppressing superconductivity with a magnetic field up to $H=31\mathrm{T}$. Using an ac thermopower measurement technique suitable for high pressure and high field, we track the pressure evolution of the Seebeck coefficient $S$. At ambient pressure and low temperature, $S/T$ in Nd-LSCO was recently found to suddenly increase below the pseudogap critical doping $p^{★}=0.23$, consistent with a drop in carrier density $n$ from $n=1+p$ above $p^{★}$ to $n=p$ below. Under a pressure of 2.0 GPa, we observe that the large $S/T$ value just below $p^{★}$ is suppressed. This confirms a previous pressure study based on electrical resistivity and Hall effect, which found that pressure lowers $p^{★}$, thereby reinforcing the interpretation that this effect is driven by the pressure-induced shift of the van Hove point. It implies that the pseudogap only exists when the Fermi surface is hole-like, which puts strong constraints on theories of the pseudogap phase. We also report thermopower measurements on Nd-LSCO and ${\mathrm{La}}_{1.8-x}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ in the charge density wave phase near $p\sim 1/8$, which reveals a weakening of this phase under pressure.

Quantum calculations of tunneling in strong-field ionization predict intricate momentum distributions due to sub-laser-cycle attosecond electron dynamics. These are obscured in most experiments by the dominance of intercycle interference patterns, which are the hallmark of above-threshold ionization (ATI). Highly controlled one- to two-cycle laser pulses produce less intercycle interference but cannot accurately recreate the subcycle features produced by uniform-cycle calculations due to the effect of the carrier envelope of the pulse. We present a simple and effective technique to recover these subcycle features in experimental multicycle spectra. We time-filter the momentum distribution to highlight features originating from the interference of electron trajectory pairs with ionization times less than one field cycle apart. This method removes the ATI patterns and reveals subcycle interference structures in detail. We can resolve modulations in holographic structures that provide a reference for comparing with calculations.

We study the saddle points of the $p$-spin model—the best understood example of a “complex” (rugged) landscape—when its $N$ variables are complex. These points are the solutions to a system of $N$ random equations of degree $p-1$. We solve for $\overline{\mathcal{N}}$, the number of solutions averaged over randomness in the $N\to \infty$ limit. We find that it saturates the Bézout bound $ln\overline{\mathcal{N}}\sim Nln(p-1)$ The Hessian of each saddle is given by a random matrix of the form $C^{†}C$, where $C$ is a complex symmetric Gaussian matrix with a shift to its diagonal. Its spectrum has a transition where a gap develops that generalizes the notion of “threshold level” well known in the real problem. The results from the real problem are recovered in the limit of real parameters. In this case, only the square root of the total number of solutions are real. In terms of the complex energy, the solutions are divided into sectors where the saddles have different topological properties.