This paper explores the application of Madelung hydrodynamic models to study two-dimensional electron gases, with a focus on nonlocal plasmonics and nonlinear optics. We begin by reviewing the derivation of the Madelung equations. Using the Madelung equations in conjunction with Poisson's equation, we calculate the spectrum of magnetoplasmons and the magneto-optical conductivity in the electrostatic regime, incorporating nonlocal corrections due to the Fermi pressure. In the absence of a magnetic field, we analyze nonlinear and nonlocal second-harmonic generation, demonstrating how plasmon excitation enhances this process. We further discuss the emergence of self-modulation phenomena driven by nonlinearity, leading to the renormalization of the plasmon dispersion. Notably, we show that nonlinearity amplifies nonlocal effects and, leveraging the hydrodynamic formalism, derive a simple analytic expression for the renormalized spectra.

Entanglement will be the key resource of future large-scale quantum networks, enabling quantum communication and advanced quantum applications like distributed quantum sensing and distributed quantum computing. To this end, entanglement will have to be distributed over large distances and efficiently coupled to quantum devices at the network nodes. This requires the entangled photons to have wavelengths and bandwidths compatible with the quantum memories in quantum repeater nodes or quantum devices at client nodes. Here, we present a cavity-enhanced source design using two nonlinear crystals inside a single cavity. We provide detailed considerations balancing the complexity of the cavity design with the photon bandwidth and the entanglement quality.

We propose a four-level tripod atom-light coupling scheme to manipulate the photonic spin Hall effect (PSHE) in the reflected probe light. Our study demonstrates that the tripod configuration enables enhanced PSHE near the Brewster angle upon reflection for the probe field, with significantly stronger transverse shifts at resonance compared to off-resonance conditions. The PSHE shift is maximized when all fields are resonant, aligning with the transparency point of the system, and is notably weaker under off-resonant conditions, where gain effects dominate. Additionally, the tripod scheme enables an enhanced transverse shift for the signal field at resonance, resulting in dual-field PSHE enhancement. This combined enhancement of transverse shift for both probe and signal fields underscores the superiority of the tripod configuration in achieving maximized PSHE shifts, surpassing prior schemes and opening new possibilities for advanced applications in spin-photonic devices.

The interface between quantum theory and gravity represents still uncharted territory. Recently, a tabletop experiment has been proposed for witnessing quantum features of gravity: Two masses in an initial superposition of spatially localized states interact only through gravity and it is measured whether the final state is entangled. Here we show that one can generate the same amount of entanglement in this setup by using the time evolution given by Newton's laws of motion. We argue that theories of quantum gravity that can be approximated by the Newtonian potential and time evolution given by Newton's laws of motion will generate gravity-mediated entanglement.

A fundamental problem in quantum information processing is the discrimination among a set of orthogonal quantum states of a composite system under local operations and classical communication (LOCC). Corresponding to the LOCC indistinguishable sets of four ququad-ququad orthogonal maximally entangled states constructed by Yu et al. [Phys. Rev. Lett. 109, 020506 (2012)], the maximum commutative sets were introduced as detectors for the local distinguishability of the set of generalized Bell states (GBSs) for which the detectors are sufficient to determine the LOCC distinguishability. In this work, we show how to determine all the detectors for a given GBS set. We also construct several four-GBS sets without detectors, most of which are one-way LOCC indistinguishable and only one of which is one-way LOCC distinguishable, indicating that the detectors are not necessary for LOCC distinguishability. Furthermore, we show that for four-GBS sets in the quantum system ${\mathbb{C}}^6\otimes {\mathbb{C}}^6$, the detectors are almost necessary for one-way LOCC distinguishability, except for one set in the sense of local unitary equivalence. The problem of one-way LOCC discrimination of four-GBS sets in ${\mathbb{C}}^6\otimes {\mathbb{C}}^6$ is completely resolved.

Quantum process tomography is a useful tool for characterizing quantum processes. This task is essential for the development of different areas, such as quantum information processing. In this work, we present a protocol for selective continuous-variable quantum process tomography. Our proposal allows one to selectively estimate any element of an unknown continuous-variable quantum process in the position representation, without requiring the complete reconstruction of the process. By resorting to controlled squeezing and translation operations, and adaptatively discretizing the process, a direct measure of an estimate of any process element can be obtained. Furthermore, we show, supported by numerical simulations, how the protocol can be used to partially reconstruct on a region a continuous-variable quantum process.

This work presents a detailed mathematical derivation of the hierarchically correlated orbital functional theory (HCOFT), a framework based on hypercomplex orbitals. Recent study [Phys. Rev. Lett. 133, 206402 (2024)] has demonstrated that hypercomplex orbitals in a determinant are equivalent to a set of real-valued orbitals that allow fractional occupations, making them desirable fundamental descriptors for many-electron systems. The algebraic properties of Clifford algebra are rigorously applied to derive key quantities within HCOFT, addressing the complexities introduced by the hypercomplex representation. It is shown that, despite this added complexity, the resulting density and kinetic energy remain physically meaningful and satisfy essential properties, including the Pauli exclusion principle. To establish the uniqueness of HCOFT, alternative definitions of hypercomplex orbitals within Clifford algebra are explored. These alternatives can lead to the loss of physical meaning in fundamental quantities for many-electron systems. Overall, this work demonstrates that HCOFT not only preserves the desired physical properties but also provides a single-determinant framework capable of describing multireference systems.

The dissociation dynamics of dissociative electron attachment to carbon disulfide is studied using the velocity map imaging technique. Ion yields of the ${\mathrm{S}}^{-}$ and ${\mathrm{CS}}^{-}$ fragments are measured as a function of incident electron energy between 2 and 12 eV. Energy and angle-differential yields of the ${\mathrm{S}}^{-}$ fragments are analyzed to investigate the dynamics of the ${\mathrm{CS}}_2{}^{-}$ anion around the 3.8-eV peak. The resonances involved in this fragmentation are identified and characterized in calculations using the well-known $R$-matrix method. Based on our measurements and the geometry dependence of the calculated resonances, we propose that a geometrical distortion of the transient negative-ion state of ${\mathrm{CS}}_2$ is necessary for S-CS bond dissociation to occur.

We predict a coherent superposition mediating a significant periodic energy flow between the fundamental and third-harmonic modes. It is found for the linearly polarized electromagnetic field, where a lower bound $S$ state $|g\rangle$ is coupled to an upper bound $S$ state $|b\rangle$, where the energy difference between these states is about two photons and the coupling occurs due to the above threshold ionization from these states together. Exceptional point on complex quasienergy surfaces of the dressed resonances $|g\rangle$ and $|b\rangle$ indicates coherent superposition that manifests itself dynamically as the Rabi cycle. The cycle is irreversible, being gradually quenched by the above threshold ionization while emitting strong third-harmonic and fundamental pulses. We provide a numerical evidence confirming the existence of such a process in the helium atom.

We propose dynamical Bragg mirrors as a means to compress intense short optical pulses. We show that strong-field photoexcitation of carriers changes the refractive index of the layers and leads to motion of the resonance-defined boundary of the Bragg mirror. In a reflection geometry, this counterpropagating motion leads to significant compression of the incident pulse. We utilize a finite-difference time-domain numerical model to predict up to a six-fold pulse compression in the few-femtosecond regime. Modification of the refractive index and properties of the compressed pulse as a function of the incident pulse parameters are investigated.

The anisotropic two-photon quantum Rabi model is studied using the Bogoliubov operator approach. Doubly degenerate exceptional states are identified using analytical methods. By adjusting the position of the last exceptional point belonging to two adjacent energy levels, we derive a condition for the absence of the discrete spectrum at the critical coupling where “spectral collapse” occurs. In this special case, the spectrum becomes fully continuous above a threshold energy, with no bound states existing, whereas the ground state remains gapped in the general case. More interestingly, we rigorously find that a finite number of bound states exist between two anisotropy-dependent critical atomic frequencies, with infinitely many bound states beyond this frequency regime. In this manner, all issues in the two-photon quantum Rabi model are resolved.

Bulk-edge correspondence constitutes a fundamental concept within the domain of topological physics, elucidating the profound interplay between the topological invariants that characterize the bulk states and the emergent edge states. A recent highlight along this research line consists of establishing bulk-edge correspondence under the eigenvalue's nonlinearity in a linear Hamiltonian by introducing auxiliary eigenvalues [T. Isobe et al., Phys. Rev. Lett. 132, 126601 (2024)]. The purpose of this work aims to extend Isobe's analysis to uncover bulk-edge correspondence of the eigenvalue's nonlinearity in intrinsic nonlinear Hamiltonians. To achieve this, we numerically solve the nonlinear Rice-Mele (RM) model and identify two distinct types of nonlinear eigenvalues: the intrinsically nonlinear eigenvalues and the eigenvalue's nonlinearity introduced through the incorporation of auxiliary eigenvalues. Furthermore, we establish a form of bulk-edge correspondence based on these auxiliary nonlinear eigenvalues, which we term the anomalous bulk-edge correspondence of a nonlinear physical system. The concept of the anomalous bulk-edge correspondence defined herein provides an alternative perspective on the intricate interplay between topology and nonlinearity in the context of bulk-edge correspondence.

We present a phase-space formalism to evaluate Bell inequality violations in continuous variable systems. By doing so we can generalize previous analyses (which have dealt only with pure states) to arbitrary mixed states. We leverage these results to analyze the effect of temperature on violations of Bell inequalities in a two-mode squeezed thermal state, which can become useful in tests of local realism in the presence of thermal noise. We also explore the nonmonotonic relationship between the violations of Bell inequalities and the amount of entanglement present in this family of mixed states. Additionally, we discuss the optimal choices of pseudospin operators for states beyond the two-mode squeezed vacuum.

Complex quantum states of light are not only central to advancing our understanding of quantum mechanics, but are also necessary for a variety of quantum protocols. High-dimensional, or multipartite, quantum states are of specific interest, as they can exhibit unique properties both fundamentally and in application. The synthesis of high-dimensional, entangled photonic states can take the form of various schemes, which result in varying forms of entanglement. Frequency entanglement is specifically attractive due to compatibility with integrated systems and resistance to decoherence in fiber transportation; however, increasing the dimension of frequency-entangled states requires a system that offers quantum interactions between a large set of distinct frequencies. Here, we show how the phonon-photon interactions of forward Brillouin scattering, which offer access to a ladder of optical resonances permitted by a single mechanical mode, can be used for fast synthesis of frequency-entangled, single-photon $W$ states. In our proposed system, simultaneous laser pulses of different frequencies dynamically evolve either an injected single photon or a heralded single phonon, generating $W$ states of selected dimension and output frequency. This method enables the synthesis of “perfect” $W$ states by adjusting the pulse amplitudes. In addition, we show how this system can be used for quantum frequency translation.

While limitations on quantum computation under Markovian environmental noise are well understood in generality, their behavior for different quantum circuits and noise realizations can be less universal. Here we consider a canonical quantum algorithm—Grover's algorithm for unordered search on $L$ qubits—in the presence of systematic noise. This allows us to write the evolution as a random Floquet unitary, which we show is well characterized by random matrix theory (RMT). The RMT analysis enables analytical predictions for different regimes and critical disorders of the many-body dynamics. We find two critical disorders. At small disorder, finite-dimensional manifold remains nonergodic as long as the noise is smaller than an ergodicity-breaking transition, which scales as $O\left(L^{-1}\right)$. Computational power is lost at a much smaller disorder that scales as $O\left(2^{-L/2}\right)$. We comment on relevance to nonsystematic noise in realistic quantum computers, including cold atom, trapped ion, and superconducting platforms.

Dynamical correlations reveal important out-of-equilibrium properties of the underlying quantum many-body system, yet they are notoriously difficult to measure in experiments. While measurement protocols for dynamical correlations based on Hadamard tests for qubit quantum devices exist, they do not straightforwardly extend to qudits. Here, we propose a modified protocol to overcome this limitation by decomposing qudit observables into unitary operations that can be implemented and probed in a quantum circuit. We benchmark our algorithm numerically at the example of quench dynamics in a spin-1 $XXZ$ chain with finite shot noise and demonstrate advantages in terms of the signal-to-noise ratio over established protocols based on linear response. Our scheme can readily be implemented on various platforms and offers a wide range of applications like variational quantum optimization and probing thermalization in many-body systems.

We extend the interferometric method to multiphoton ionization, aiming to disentangle different ionization channels simultaneously for resonance-enhanced multiphoton ionization by numerically solving the time-dependent Schrödinger equation. The approach is based on the interference of the bichromatic ionization using an $\omega \text{−}4\omega$ pulse pair. By tuning the relative phase of the two-color laser pulse, we can extract the partial-wave complex amplitudes from the photoelectron angular distributions. Our analysis shows that controlling the fundamental frequency of the bichromatic pulses enables us to resolve resonant and nonresonant channels simultaneously. Taking the hydrogen atom as the prototype, at the resonant frequency, while the $g$ partial wave is resonance-free, the quantum phases of the $s$ and $d$ partial waves and the corresponding Wigner time delays are significantly influenced by the intermediate resonance. Given that the above pulses are now accessible through high-harmonic sources or free-electron lasers, such investigations could disentangle the ionization channels of more complex targets.

We investigate the scattering halos resulting from collisions between discrete momentum components in the time-of-flight expansion of interaction-tunable ${}^6\mathrm{Li}_2$ molecular Bose-Einstein condensates. A key highlight of this study is the observation of the influence of interactions on the collisional scattering process. We measure the production of scattering halos at different interaction levels by varying the number of particles and the scattering length, and quantitatively assess the applicability of perturbation theory. To delve into a general theory of scattering halos, we introduce a scattering factor and obtain a universal relation between it and the halo ratio. Furthermore, we simulate the formation of scattering halos under nonperturbative conditions and discuss the contribution of in-trap dynamics to the scattering halos through a return pulse experiment. This study enhances our understanding of the physical mechanisms underlying scattering processes in many-body systems and provides new perspectives for further theoretical research.

Experimental results are presented that provide insight into the physics of statistical imaging in heavily scattering random media based on measured speckle correlations as a function of the change in position of a moving object. In this way, definitive interpretation of a rather complex and earlier theory is achieved, making this work an experimental complement to that theory paper [Phys. Rev. A 101, 063827 (2020)]. Motion could be natural, where the set of positions is estimated or separately obtained, or directed, where a mechanical stage can be used to adjust the object's position. In the experiment, a coherent laser illuminates two scattering diffusers, while an object is translated between them in the resulting speckled field and images are collected in a transmission configuration. Results are shown for various objects of differing size and geometry, allowing the theory to be validated and interpreted with new understanding. This work demonstrates imaging opportunities, and applications include material characterization, environmental imaging and sensing, and deep-tissue in vivo imaging.