The activity of neurons within brain circuits has been ubiquitously reported to be correlated. The impact of these correlations on brain function has been extensively investigated. Correlations can in principle increase or decrease the information that neural populations carry about sensory stimuli, but experiments in cortical areas have mostly reported information-limiting correlations, which decrease the information encoded in the population. However, a second stream of evidence suggests that temporal correlations between the spiking activity of different neurons may increase the impact of neural activity downstream, implying that temporal correlations affect both the encoding of information and its downstream readout. The principle of how encoding and readout combine are still unclear. Here, we consider a model of transmission of stimulus information encoded in presynaptic input spike trains with information-limiting time-correlations to the output firing of a postsynaptic biologically plausible leaky integrate and fire (LIF) readout neuron. We derive an analytical solution of the model in the diffusion approximation, in which the encoding spiking activity is treated as a continuous-time stochastic variable. An ansatz based on a separation of timescales allows us compute the stimulus information transmitted to the readout over a broad range of parameters. Our analytical results reveal that, for sufficiently low input firing rates, large enough difference in input stimulus-specific activity, and moderately large input temporal correlations, the stimulus discriminability of the firing of the LIF readout neuron can be enhanced by the presence of input time correlations, despite their decreasing the stimulus information encoded in its inputs.

A Fokker-Planck equation (FPE) approach is presented to calculate the magnetization probability densities in nanomagnetic systems characterized by complex magnetization dynamics. The formulation is based on an eigenstate expansion of the solutions of the linearized Landau-Lifshitz-Gilbert equation (LLGE), which allows considering statistical properties of nonuniform magnetization dynamics, such as finding write error rates in spin transfer torque magnetic random access memories induced by thermal noise. The formulation includes finding eigenstates and eigenfrequencies of the structure of interest, formulating an FPE describing the probability density dynamics of the excitation coefficients of the eigenstate solutions, and using this equation to find derived quantities. Compared to using the stochastic LLGE, the presented approach directly obtains statistical properties. Compared to generally used FPE approaches, the presented formulation allows handling nonuniform dynamics. The formulation of using the eigenstate-based FPE is applicable to many types of physics.

Eigenstate filters underpin near-optimal quantum algorithms for ground-state preparation. Their realization on current quantum computers, however, poses a challenge as the filters are typically represented by deep quantum circuits. Additionally, since the filters are created probabilistically, their circuits need to be rerun many times when the associated success probability is small. Here, we describe a strategy to implement a ground-state filter on quantum hardware in the presence of noise by prepending the filter with digitized adiabatic quantum computing. The adiabatically prepared input state increases the success probability of the filter and also reduces its circuit depth requirements. At the same time, the filter enhances the accuracy of the adiabatically prepared ground state. We compare the approach to the purely adiabatic protocol through numerical simulations and experiments on the Quantinuum H1-1 quantum computer. We demonstrate a significant improvement in ground-state accuracies for paradigmatic quantum spin models.

As has been known for almost 20 years, topological characteristics of polygonal proliferative epithelial cells of various animal and plant species are universal. Here, we address the question of whether this universality paradigm is applicable to the level of colonial organisms, i.e., at a significantly higher level of life organization than cell populations. By comparing the cellular structure of the epithelium with the arrangement of individual polyps in coral colonies, we discover common topological patterns at distinct levels of life structuring. We hypothesize that the mechanism behind this universality is a repulsion between structural units limited by a certain critical distance. By minimizing the corresponding repulsive energy, we produce packings, which are similar to the epithelial and colonial systems in the distribution of structural units by both the number of their neighbors and their area. The surprising analogies between the systems together with the developed theoretical approach could provide an efficient tool for further studies on morphogenesis, development, and evolution of living systems.

Quantum uncertainty relations impose fundamental limits on the joint knowledge that can be acquired from complementary observables: Perfect knowledge of a quantum state in one basis implies maximal indetermination in all other mutually unbiased bases (MUBs). Uncertainty relations derived from joint properties of the MUBs are generally assumed to be uniform, irrespective of the specific observables chosen within a set. In this work, we demonstrate instead that the uncertainty relations can depend on the choice of observables. Through both experimental observation and numerical methods, we show that selecting different sets of three MUBs in a five-dimensional quantum system results in distinct uncertainty bounds, i.e., in varying degrees of complementarity, in terms of both entropy and variance.

A unique phenomenon in non-Hermitian systems is the non-Hermitian skin effect (NHSE), namely, the boundary localization of continuous-spectrum eigenstates. However, studies on the NHSE in systems without translational invariance are still limited. Here, we unveil a new class of NHSE, dubbed the imaginary-Stark skin effect (ISSE), in a one-dimensional lossy lattice with a spatially increasing loss rate. This ISSE is beyond the framework of non-Bloch band theory and exhibits intriguing properties significantly different from the conventional NHSE. Specifically, the energy spectrum of our model has a T-shaped feature, with approximately half of the eigenstates localized at the left boundary. Furthermore, each skin mode can be expressed as a single stable, exponentially decaying wave within the bulk region. Such peculiar behaviors are analyzed via the transfer-matrix method, whose eigendecomposition quantifies the formation of the ISSE. Our work provides new insights into the NHSE in systems without translational symmetry and contributes to the understanding of non-Hermitian systems.

Methods for creating endofullerenes have been steadily improving since their discovery, allowing for new types of endofullerenes to be created in larger numbers. When a molecule is trapped in a fullerene, the fullerene creates a harmonic trapping potential that leaves most of the fundamental properties of the internal molecule intact. The fullerene cage does create a preferred axis for the internal molecule, which we refer to as the encapsulation-induced alignment of the molecule. We explore the alignment of AlF and ${\mathrm{N}}_2$ inside of ${\mathrm{C}}_{60}$ by first computing the interaction between the internal molecule and the fullerene cage using ab initio electronic structure methods. Our results show that the internal molecules are found to be strongly aligned despite finding that all the calculated spectroscopic constants are relatively unaffected by the fullerene cage.

It is well known that the time-dependent response of a correlated system can be inferred from its spectral correlation functions. As a textbook example, the zero-sound collective modes of a Fermi liquid appear as poles of its particle-hole susceptibilities. However, the Fermi liquid’s interactions endow these response functions with a complex analytic structure, so that this time-frequency relationship is no longer straightforward. We study how the geometry of this structure is modified by a nontrivial band geometry, via a calculation of the zero-sound spectrum of a Dirac cone in two dimensions. We find that the chiral wave functions, which encode the band geometry, fundamentally change the analytic structure of the response functions, which encode its Riemannian geometry. As a result, isotropic interactions can give rise to a variety of unconventional zero-sound modes, which, due to the geometry of the functions in frequency space, can only be identified via time-resolved probes. These modes are absent in a conventional Fermi liquid with similar interactions, so that these modes can be used as a sensitive probe for the existence of Dirac points in a band structure.

We investigate fundamental bounds on the ability to determine photon number distribution and other related quantities from tomographically incomplete measurements with an array of $M$ detectors that can only distinguish the absence or presence of photons. We show that the lower and upper bounds on photon number probabilities can be determined by solving a linear program. We present and discuss numerical results for various input states, including thermal states, coherent states, squeezed states, and highly nonclassical single-photon subtracted squeezed vacuum states. Besides photon number probabilities, we also investigate bounds on the parity of photon number distribution that determines the value of Wigner function of the state at the origin of phase space. Moreover, we also discuss estimation of mean photon number as an example of a quantity described by an unbounded operator. Our approach and results can provide quantitative guidance on the number of detection channels required to determine the photon number distribution with a given precision.

In a previous work [K. A. Meissner and H. Nicolai, Eur. Phys. J. C 84, 269 (2024)], two of the present authors have suggested possible experimental ways to search for stable supermassive particles with electric charges of $O\left(1\right)$ in upcoming underground experiments, in particular the new Jiangmen Underground Neutrino Observatory (JUNO) experiment. In this paper, we present a detailed analysis of the specific signature of such gravitino-induced events for the JUNO detector and for upcoming liquid argon detectors like DUNE (Deep Underground Neutrino Experiment). The proposed method of detection relies on the “glow” produced by photons during the passage of such particles through the detector liquid, which would last for about a few to a few hundred microseconds depending on its velocity and the track. The cross sections for electronic excitation of the main component of the scintillator liquid, namely, linear alkylbenzene, by the passing gravitino are evaluated using quantum-chemical methods. The results show that, if such particles exist, the resulting signals would lead to a unique and unmistakable signature, for which we present event simulations as they would be seen by the JUNO or DUNE photomultipliers. Our analysis brings together two very different research areas, namely, fundamental particles physics and the search for a fundamental theory on the one hand, and methods of advanced quantum chemistry on the other.

Continuous-variable (CV) quantum information processing is a promising candidate for large-scale fault-tolerant quantum computation. However, analysis of CV quantum process relies mostly on direct computation of the evolution of operators in the Heisenberg picture, and the features of CV space has yet to be thoroughly investigated in an intuitive manner. One key ingredient for further exploration of CV quantum computing is the construction of a computational model that brings visual intuition and new tools for the analysis of quantum protocols, especially non-Gaussian processes. In this paper, we delve into a graphical computational model, inspired by a similar model for qubit-based systems called the ZX calculus, which enables the representation of arbitrary CV quantum process as a simple directed graph. We demonstrate the utility of our model as a graphical tool to comprehend CV processes intuitively by showing how equivalences between two distinct quantum processes can be proven as a sequence of diagrammatic transformations for various quantum protocols. We also examine possible applications of our model, such as measurement-based quantum computing, characterization of Gaussian and non-Gaussian processes, and circuit optimization.

Superconducting circuit quantization conventionally starts from classical Euler-Lagrange circuit equations of motion. Invoking the correspondence principle yields a canonically quantized description of circuit dynamics over a bosonic Hilbert space. This approach has been very successful for describing experiments, but implicitly starts from the classical Ginsberg-Landau mean-field theory for the circuit. Here, we employ a different approach that starts from a microscopic fermionic Hamiltonian for interacting electrons, whose ground space is described by the Bardeen-Cooper-Schrieffer (BCS) many-body wave function that underpins conventional superconductivity. We introduce the BCS ground space as a subspace of the full fermionic Hilbert space, and show that projecting the electronic Hamiltonian onto this subspace yields the standard Hamiltonian terms for Josephson junctions, capacitors, and inductors, from which standard quantized circuit models follow. This approach does not impose a spontaneously broken symmetry so that it consistently describes quantized circuits that support superpositions of phases, and the canonical commutation relations between phase and charge are derived from the underlying fermionic commutation properties, rather than imposed. By expanding the projective subspace, this approach can be extended to describe phenomena outside the BCS ground space, including quasiparticle excitations.

An energy equilibrium arises when an electron-positron pair plasma is generated by the nonlinear Breit-Wheeler interaction of a $\gamma$-ray bunch with laser photons and becomes radiatively confined. Ordinarily, ponderomotive scattering prevents leptons from accessing the high-field regions where quantum electrodynamic (QED) effects dominate, but if quantum (stochastic) radiation reaction is sufficiently strong, a balance of acceleration and recoil forces emerges. This unique scenario enables leptons to continuously mediate the transformation of laser photons into directed $\gamma$-rays via inverse Compton scattering as they copropagate with the field. Importantly, it is observed that the lepton energy stabilizes while the $\gamma$ energy increases in direct proportion to laser energy depletion. The proposed scheme enables fundamental tests of QED, and is investigated here using the particle-in-cell method in full dimensionality, in addition to test-particle calculations. The per-species energy conversion efficiency is analyzed as a function of the initial laser and $\gamma$-beam properties.

The one-dimensional rotor model is a classical momentum-conserving system that exhibits normal heat conduction, making it a valuable platform for studying thermal transport. Despite progress in understanding its transport properties, two key issues remain open: (1) whether a phase transition from normal to anomalous heat conduction occurs as temperature decreases and (2) how thermal conductivity $\kappa$ depends on temperature. Using molecular dynamics simulations and Green-Kubo formalism, we demonstrate that although normal conduction may persist theoretically at any nonzero temperature, the exponentially increasing mean free path at low temperatures renders it practically unobservable, supporting the existence of an effective phase transition. Furthermore, we find that $\kappa$ exhibits a double-exponential temperature dependence—$\kappa \sim e^{2.54T}$ at high temperatures and $\kappa \sim e^{0.49T}$ at low temperatures—with an intermediate power-law regime $\kappa \sim T^{-3.2}$ dominated by nonlinearity. To isolate the role of nonlinearity, we study a modified rotor model with suppressed phase jumps, which exhibits $\kappa \sim T^{-3.6}$, confirming the influence of interaction nonlinearity. These findings deepen the understanding of temperature-dependent transport in low-dimensional systems.

A Dirac electron passing through a heterojunction with spatially variable tilt experiences an effective curved spacetime. In this work, we show how this experience is manifested in its conductance. We investigate the propagation of electron waves in a two-dimensional tilted Dirac cone heterostructure where tilt depends on the coordinate $z$ along the junction. The resulting Dirac equation in an emergent curved spacetime for the spinor $\psi \left(z\right)$ can be efficiently solved using a fourth-order Runge-Kutta numerical method by a transformation to a suitable spinor $\phi$ where the resulting Dirac cone looks locally upright. The spatial texture of the tilt induces oscillatory behaviors in key physical quantities such as the norm ${\left|\phi \left(z\right)\right|}^2$ of the wave function, polar and azimuthal angles $\mathrm{\Theta }\left(z\right)$ and $\mathrm{\Phi }\left(z\right)$ of the pseudospin, and the integrated transmission $\tau$ where oscillation wavelengths get shorter (longer) in stronger (weaker) tilt regions. Such an oscillatory behavior that is reminiscent of gigantic gravitational redshift is an indicator of an underlying spacetime metric that can be probed in tunneling experiments. We derive analytical approximations for the position-depdendent wave numbers $\mathrm{\Delta }{k}_z\left(z\right)$ that explain the redshift patterns and corroborate it with numerical simulations. For a tilt bump spread over length scale $\ell$, upon increasing $\ell$, the amplitude of redshifted oscillations reduces whereas the number of peaks increases. The scale invariance of the Dirac equation allows us to probe these aspects of $\ell$ dependence by a voltage sweep in transmission experiments. Smooth variations of the tilt reduces impedance mismatch of the electron waves, thereby giving rise to very high transmission rates. This concept can be used in combination with a sigmoid-shaped tilt texture for a gigantic redshift or blueshift engineering of the transmitted waves, depending on whether the sigmoid is downswing or upswing.

Recently, it has become possible to compute real-frequency four-point correlation functions of quantum impurity models using a multipoint extension of the numerical renormalization group (mpNRG). In this work, we perform several numerical consistency checks of the output of mpNRG by investigating exact relations between two- and four-point functions. This includes the Bethe-Salpeter equations and the Schwinger-Dyson equation from the parquet formalism, which we evaluate in two formally identical but numerically nonequivalent ways. We also study the first-order U(1) Ward identity between the vertex and the self-energy for the first time in full generality in the real-frequency Keldysh formalism. We generally find good agreement of all relations, often up to a few percent, both at weak and at strong interaction.