Near-term quantum simulators are mostly based on qubit-based architectures. However, their imperfect nature significantly limits their practical application. The situation is even worse for simulating fermionic systems, which underlie most of material science and chemistry, as one has to adopt fermion-to-qubit encodings which create significant additional resource overhead and trainability issues. Thanks to recent advances in trapping and manipulation of neutral atoms in optical tweezers, digital fermionic quantum simulators are becoming viable. A key question is whether these emerging fermionic simulators can outperform qubit-based simulators for characterizing strongly correlated electronic systems. Here we perform a comprehensive comparison of resource efficiency between qubit and fermionic simulators for variational ground-state emulation of fermionic systems in both condensed matter systems and quantum chemistry problems. We show that the fermionic simulators indeed outperform their qubit counterparts with respect to resources for quantum evolution (circuit depth) as well as classical optimization (number of required parameters and iterations). In addition, they show less sensitivity to the random initialization of the circuit. The relative advantage of fermionic simulators becomes even more pronounced as interaction becomes stronger, or tunneling is allowed in more than one dimension, as well as for spinful fermions. Importantly, this improvement is scalable, i.e., the performance gap between fermionic and qubit simulators only grows for bigger system sizes.

Hybrid ionic-electronic conductors have the potential to generate memory effects and neuronal behavior. The functionality of these mixed materials depends on ion motion through thin polarizable channels. Here, we explore different polarization models to show that the current and conductivity of electrolytes is higher when confined by conductors than by dielectrics. We find nonlinear currents in both dielectrics and conductors, and we recover the known linear (Ohmic) result only in the two-dimensional limit between conductors. We show that the polarization charge location impacts electrolyte structure and transport properties. This work suggests a mechanism to induce memristor hysteresis loops using conductor-dielectric switchable materials.

The ideal honeycomb lattice, featuring sublattice and SU(2) spin rotation symmetries, is a fundamental model for investigating quantum matter with topology and correlations. With the rise of the moiré-based design of model systems, realizing a tunable and symmetric honeycomb lattice system with a narrow bandwidth can open access to new phases and insights. We propose the ABBA-stacked twisted double bilayer ${\mathrm{WSe}}_2$ as a realistic and tunable platform for reaching this goal. Adjusting the twist angle allows the bandwidth and the ratio between hopping parameters of different ranges to be tuned. Moreover, the system's small bandwidth and spin rotation symmetry enable effective control of the electronic structure through an in-plane magnetic field. We construct an extended Hubbard model for the system to demonstrate this tunability and explore possible ordered phases using the Hartree-Fock approximation. We find that at a hole filling of $\nu =2$ (two holes per moiré unit cell), an in-plane magnetic field of a few tesla can “dope” the system from a semimetal to a metal. Interactions then drive an instability towards a canted antiferromagnetic insulator ground state. Additionally, we observe a competing insulating phase with sublattice charge polarization. Finally, we discuss the experimental signatures of these insulating phases.

Motivated by the recent realization of an artificial quantum spin ice in an array of superconducting qubits with tunable parameters [King et al., Science 373, 576 (2021)], we scrutinize a quantum six-vertex model on the square lattice that distinguishes type-I and type-II vertices. We map the zero-temperature phase diagram using numerical (exact diagonalization) and analytical (perturbation expansion, Gershgorin theorem) methods. Following a symmetry classification, we identify three crystalline phases alongside a subextensive manifold of isolated configurations. Monte Carlo simulations at the multicritical Rokhsar-Kivelson point provide evidence for a quantum phase exhibiting a cascade of transitions with increasing flux. By comparing structure factors, we find evidence for the emergence of the fully flippable and plaquette phases in the artificial quantum spin ice.

We theoretically investigate superconductivity under a spin-split band structure owing to a collinear-type antiferromagnetic order in quasi-two-dimensional organic compounds $\kappa$-(BEDT-${\mathrm{TTF})}_{2}X$. We find that the magnetic order can induce a Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state, where the Cooper pair possesses a finite center-of-mass momentum. We show this from two types of analyses: (1) an effective model where simple intraband attractive interactions are assumed and (2) many-body calculations of the repulsive Hubbard model based on the fluctuation-exchange approximation and the linearized Eliashberg equation. Our results show the possibility of realizing the FFLO state without applying an external magnetic field.

To test the quantum nature of gravity in a laboratory requires witnessing the entanglement between the two test masses (nanocrystals) solely due to the gravitational interaction kept at a distance in a spatial superposition. The protocol is known as the quantum-gravity-induced entanglement of masses (QGEM). One of the main backgrounds in the QGEM experiment is electromagnetic (EM) -induced entanglement and decoherence. The EM interactions can entangle the two neutral masses via dipole-dipole vacuum-induced interactions, such as the Casimir-Polder interaction. To mitigate the EM-induced interactions between the two nanocrystals, we enclose the two interferometers in a Faraday cage and separate them by a conducting plate. However, any imperfection on the surface of a nanocrystal, such as a permanent dipole moment, will also create an EM background interacting with the conducting plate in the experimental box. These interactions will further generate EM-induced dephasing, which we wish to mitigate. In this paper, we will consider a parallel configuration of the QGEM experiment, where we will estimate the EM-induced dephasing rate and run-by-run systematic errors which will induce dephasing, and also provide constraints on the size of the superposition in a model-independent way of creating the spatial superposition.

Correlated sampling has wide-ranging applications in Monte Carlo calculations. When branching random walks are involved, as commonly found in many algorithms in quantum physics and electronic structure, population control is typically not applied with correlated sampling due to technical difficulties. This hinders the stability and efficiency of correlated sampling. In this paper, we study schemes for allowing birth/death in correlated sampling and propose an algorithm for population control. The algorithm can be realized in several variants depending on the application. One variant is a static method that creates a reference run and allows other correlated calculations to be added a posteriori. Another optimizes the population control for a set of correlated, concurrent runs dynamically. These approaches are tested in different applications in quantum systems, including both the Hubbard model and electronic structure calculations in real materials.

A quantum kicked rotor model is one of the promising systems to realize various Floquet topological phases. We consider a double-kicked rotor model for a one-dimensional quasi-spin-1/2 Bose-Einstein condensate with spin-dependent and spin-independent kicks which are implementable for cold atomic experiments. We theoretically show that the model can realize all the Altland-Zirnbauer classes with nontrivial topology in one dimension. In the case of class CII, we show that a pair of winding numbers $(w_0,w_{\pi })\in 2\mathbb{Z}\times 2\mathbb{Z}$ featuring the edge states at zero and $\pi$ quasienergy, respectively, takes various values depending on the strengths of the kicks. We also find that the winding numbers change to $\mathbb{Z}$ when we break the time-reversal and particle-hole symmetries by changing the phase of a kicking lattice. We numerically confirm that the winding numbers can be obtained by measuring the mean chiral displacement in the long-time limit in the present case with four internal degrees of freedom. We further propose two feasible methods to experimentally realize the spin-dependent and spin-independent kicks required for various topological phases.

Raman spectra from molecular systems are typically regarded as comprising a set of sharp peaks. These arise from both single-molecule modes and extended $\mathrm{\Gamma }$ point phonons being, in theory, harmonic oscillators. Broadening is typically assigned to thermal, linewidth, and anharmonic effects. Here we measure significant broadening in the Raman signal from partially deuterated water ices VII and VIII, which is absent in the pure systems. This isotopic broadening is much greater than any anharmonic effects and is reproduced by lattice dynamics calculations assuming only harmonic bonding. Instead, it arises from mass disorder induced phonon localization. This localization leads to many overlapping modes with a range of frequencies dependant on local isotopic environment and Raman activity due to lack of distinct molecular or crystalline symmetry.

We derive two fundamental laws of chiral band crossings: (i) a local constraint relating the Chern number to phase jumps of rotation eigenvalues and (ii) a global constraint determining the number of chiral crossings on rotation axes. Together with the fermion doubling theorem, these laws describe all conditions that a network of chiral band crossing must satisfy. We apply the fundamental laws to prove the existence of enforced double Weyl points, nodal planes, and generic Weyl points, among others. In addition, we show that chiral space group symmetries can not stabilize nodal lines with finite Chern numbers. Combining the local constraint with explicit low-energy models, we determine the generic topological phase diagrams of all multifold crossings. Remarkably, we find a fourfold crossing with Chern number 5, which exceeds the previously conceived maximum Chern number of 4. We identify materials crystallizing in space group 198, such as B20 materials and BaAsPt, as suitable compounds with this Chern number 5 crossing.

We theoretically study the real-space distribution of the supercurrent that flows under a uniform vector potential in a two-dimensional quasiperiodic structure. This is done by considering the attractive Hubbard model on the quasiperiodic Ammann-Beenker structure and studying the superconducting phase within the Bogoliubov-de Gennes mean-field theory. Decomposing the local supercurrent into the paramagnetic and diamagnetic components, we numerically investigate their dependencies on average electron density, temperature, and the angle of the applied vector potential. We find that the diamagnetic current locally violates the current conservation law, necessitating compensation from the paramagnetic current, even at zero temperature. The paramagnetic current shows exotic behaviors in the quasiperiodic structure, such as local currents, which are oriented transversally or reversely to that of the applied vector potential.

The Lindblad master equation can always be transformed into a first-order linear ordinary differential equation (1ODE) for the coherence vector. We pose the inverse problem: given a finite-dimensional, nonhomogeneous 1ODE, does a corresponding Lindblad equation exist? If so, what are the corresponding Hamiltonian and Lindblad operators? We provide a general solution to this problem, including a complete positivity test in terms of the parameters of the 1ODE. We also derive a host of properties relating the two representations (master equation and 1ODE) of independent interest.

We investigate the electron transport through a serially coupled quantum-dot molecule in which each dot is side-coupled to one Majorana zero mode (MZM) at an end of the topological-superconducting nanowire. Calculation results show that with the increase of dot number, the zero-bias conductance does not display the conventional odd-even effect but always keeps equal to $\frac{e^2}{h}$. On the other hand, if different superconducting phases are added to the dot-MZM couplings with the left-right asymmetry mode, the zero-bias conductances display apparent odd-even effect. Namely, in the odd-dot cases, the zero-bias conductances are less determined by the superconducting phase, whereas they are more dependent on the superconducting phase in the cases of even dots. Regarding the shot-noise Fano factors, they also exhibit clear odd-even effects, mainly manifested as the dependence of the relation between zero-bias Fano factors and superconducting phase difference on the parity of dot number. We believe that these results are helpful to construct mesoscopic cells based on quantum dots and MZMs.

The known quantum error-correcting codes are typically built on approximative open-quantum-system models such as Born-Markov master equations. However, it is an open question how such codes perform in actual physical systems that, to some extent, necessarily exhibit phenomena beyond the limits of these models. To this end, we employ numerically exact open-quantum-system dynamics to analyze the performance of a five-qubit error-correction code where each qubit is coupled to its own bath. We first focus on the performance of a single error-correction cycle covering timescales and coupling strengths beyond those of Born-Markov models. We observe distinct power-law behavior of the error-corrected channel infidelity $\propto t^{2a}$: $a\lesssim 2$ in the ultrashort times $t<3/{\omega }_{\mathrm{c}}$ and $a\approx 1/2$ in the short-time range $3/{\omega }_{\mathrm{c}}<t<30/{\omega }_{\mathrm{c}}$, where ${\omega }_{\mathrm{c}}$ is the cutoff angular frequency of the bath. Furthermore, the observed scaling of the performance of error correction is found to be robust against various imperfections in physical qubit systems, such as perturbations in qubit-qubit coupling strengths and parametric disorder. For repeated error correction, we demonstrate the breaking of the five-qubit error-correction code and of the Born-Markov models if the repetition rate of the error-correction cycles exceeds $2\pi /\omega$ or the coupling strength $\kappa \gtrsim \omega /10$, where $\omega$ is the angular frequency of the qubit. Our results provide bounds of validity for the standard quantum error-correction codes and pave the way for applying numerically exact open-quantum-system methods for further studies of error correction beyond simple error models and for other strongly coupled many-body models.

We show that the energy statistics resulting from a two-point measurement of an isolated quantum system subject to a time-dependent driving protocol can be probed by subjecting the same system to a collision with a suitably prepared incoming particle. This means that the particle acts both as an external drive and as an energy measurement device for the quantum system and that energy fluctuations can be defined within a fully autonomous setting.

Hole-spin qubits in semiconductors represent a mature platform for quantum technological applications. Here we consider their use as quantum sensors, and specifically for inferring the presence and estimating the distance from the qubit of a remote charge. Different approaches are considered, based on the use of single or double quantum dots, ground and out-of-equilibrium states, Rabi and Ramsey measurements, and comparatively analyzed by means of the discrimination probability, and of the classical and quantum Fisher information. Detailed quantitative aspects result from the multiband character of the hole states, which we account for by means of the Luttinger-Kohn Hamiltonian. Furthermore, general conclusions can be drawn on the relative efficiency of the above options, and analytical expressions are derived for the Fisher information of a generic qubit within the Rabi and Ramsey schemes.

Nonlinear interferometers are promising tools for quantum metrology, as they are characterized by an improved phase sensitivity scaling compared to linear interferometers operating with classical light. However, the multimodeness of the light generated in these interferometers results in the destruction of their phase sensitivity, requiring advanced interferometric configurations for multimode light. Moreover, in contrast to the single-mode case, time-ordering effects play an important role for the high-gain regime in the multimode scenario and must be taken into account for a correct estimation of the phase sensitivity. In this paper, we present a theoretical description of spatially multimode $\mathrm{SU}(1,1)$ interferometers operating at low and high parametric gains. Our approach is based on a step-by-step solution of a system of integro-differential equations for each nonlinear interaction region. We focus on interferometers with diffraction compensation, where focusing elements such as a parabolic mirror are used to compensate for the divergence of the light. We investigate plane-wave and Gaussian pumping and show that for any parametric gain, there exists a region of phases for which the phase sensitivity surpasses the standard shot-noise scaling and discuss the regimes where it approaches the Heisenberg scale. Finally, we arrive at insightful analytical expressions for the phase sensitivity that are valid for both low and high parametric gain and demonstrate how it depends on the number of spatial modes of the system.

When measuring physiological data, the central limit theorem typically implies a consistent variance, resulting in data that closely follow a Gaussian distribution. However, physiological measurements often deviate from this expectation, increasing variance due to nonlinear correlations across various scales. The challenge lies in testing these tails, which comprise only rare and extreme values. We introduce multiscale probability density function (PDF) analysis, a method that estimates this non-Gaussianity parameter for physiological fluctuations in each of multiple timescales. We gain valuable insights into the observed distributions with heavier tails and nonlinear correlations by exploring the relationship between non-Gaussianity and logarithmic scale. To maintain the fidelity of the original data, we incorporate an adaptive detrending filter into our multiscale PDF analysis. This filter effectively eliminates trends without distorting the distribution in a way that might risk artifactual signatures of non-Gaussianity. Additionally, we explain why multiscale PDF analysis is especially well suited for examining data that follow lognormal distributions. In the final stretch, we demonstrate how multiscale PDF analysis can provide fresh perspectives on heart rate variability and postural control. This innovative approach can facilitate diagnoses in health and disease while also deepening our comprehension of how constraints influence human physiological performance.