Photon pairs generated via spontaneous parametric down-conversion (SPDC) can exhibit entanglement in the Laguerre-Gaussian (LG) mode basis, which enables high-dimensional free-space quantum communication by exploiting the high-dimensional space spanned by the LG modes. For such free-space quantum communication, the brightness of the quantum light source plays an important role due to the atmospheric turbulence and photon loss. A variety of studies have analyzed the SPDC brightness by decomposing biphoton states into LG modes, but they have often relied on a degenerate state, a narrow-spectral-bandwidth approximation, or a thin-crystal approximation. However, these approaches are unsuitable for nondegenerate type-0 SPDC with a periodically poled nonlinear crystal, which offers higher brightness due to its superior nonlinear coefficients. In this study, we examine the spectrum of photon pairs in specific LG modes generated by a type-0 periodically poled potassium titanyl phosphate crystal while avoiding the constraints imposed by the aforementioned assumptions. In addition, we investigate the optimal focal parameters of the pump, signal, and idler to maximize the brightness for a given LG mode. Our findings show that it is not feasible to simultaneously optimize the brightness for different LG modes with a single pump focal parameter. The results of this study provide a comprehensive framework for developing high-brightness quantum light sources and contribute to the advancement of high-dimensional free-space quantum communication.

Quantum batteries have demonstrated remarkable charging properties, showing that a quantum advantage is possible in the realm of quantum thermodynamics. However, finding an effective strategy to store energy for long periods remains crucial in these systems. Here, we investigate different configurations of a waveguide QED system acting as a quantum battery and show that, in this context, collective effects can slow down the self-discharging time of the battery, thus improving the storage time. Specifically, when the artificial atoms of the array are arranged randomly, the energy and ergotropy of the optical system are shown to decay at a subexponential rate over long periods, in contrast to the energy decay of a single atom in a waveguide. In the case of atoms arranged in an ordered lattice, collective effects slow down dissipative discharging only for a specific lattice spacing. Thus, in both configurations, collective effects can be used to boost the energy-protection properties of optical systems.

We present a theoretical investigation of the Goos-Hänchen shift in an optical cavity containing a three-level atomic medium. Unlike conventional approaches that require external control or pump fields, transparency in our system is induced by the vacuum field itself through a phenomenon known as vacuum-induced transparency, distinguishing it from the traditional electromagnetically induced transparency. Inspired by this mechanism, we introduce the concept of the vacuum-induced Goos-Hänchen (VIGH) shift, where the lateral displacement of the reflected beam is governed by vacuum field interactions. Additionally, we explore the influence of a microwave field on the atomic medium, demonstrating its ability to modulate both the magnitude and direction of the VIGH shift. Notably, we establish a tunable control over positive and negative VIGH shifts through the microwave field, with the shifts in magnitude further influenced by the photon number inside the cavity. Moreover, we examine the impact of Doppler broadening, revealing a unique deviation in VIGH shift behavior. In contrast to the Doppler-free case, where the microwave field can transition the VIGH shift between positive and negative regimes, Doppler broadening suppresses this transition, enhancing only the magnitude of the negative VIGH shift as microwave strength increases. Our findings provide deeper insights into light-matter interactions at the quantum level and open pathways for precision control of optical shifts in cavity-based quantum systems.

We investigate the scattering of two distinguishable particles with unequal masses and a mutual short-range interaction with the aim of quantifying the impact of a tunneling “projectile” particle on the quantum mechanical state of the “barrier” particle. We find that the states of the barrier particle after the tunneling or reflection of the projectile are shifted by a distance which consists of a classical uniform motion and an aditional displacement that is given by the derivative of the phase of the transmission or reflection amplitudes multiplied by factors dependent on particles' masses, respectively. We demonstrate these results on a numerical example with a resonant interaction between a projectile and barrier. Our work demonstrates physical implications of the concept of phase time delay in the form of displacements of particles that are, in principle, experimentally measurable.

Higher-dimensional entanglement is a valuable resource for several quantum information processing tasks, and is often characterized by the Schmidt number and specific classes of entangled states beyond qubit-qubit and qubit-qutrit systems. We propose a criterion to detect higher-dimensional entanglement, focusing on determining the Schmidt number of quantum states and identifying significant classes of positive partial transposition and negative partial transposition entangled states. Our approach relies on evaluating moments of generalized positive maps which can be efficiently simulated in real experiments without the requirement of full-state tomography. We demonstrate the effectiveness of our detection scheme through various illustrative examples. As a direct application, we explore the implications of our moment-based detection schemes in identifying useful quantum-channels such as non-Schmidt-number breaking channels and nonentanglement breaking channels. Finally, we present an operational implication of our proposed moment criterion through its manifestation in channel discrimination tasks.

High-dimensional quantum key distribution (HD-QKD) offers high photon information efficiency and robust noise tolerance. Furthermore, Islam et al. [Phys. Rev. A 97, 042347 (2018)] proposed a variant of the HD-QKD protocol that can generate secret keys using any subset of monitoring states. However, there is a gap in the analysis of this protocol under finite-key scenarios, which is crucial for the practicality of any QKD protocol. In this work, we conduct a finite-key analysis of this protocol within the universally composable framework. Additionally, we analyze the performance of this protocol in actual experimental scenarios. The simulation results demonstrate that the protocol remains practical even with finite resources.

A systematic investigation of resonance strengths of $intershell$ higher-order (HO) electronic recombination for Li-like to N-like highly charged ions with atomic numbers ranging from $Z=10$ to 60 is carried out based on the relativistic configuration-interaction method. The calculated results are compared with the previous work of Beilmann et al. [Phys. Rev. Lett. 107, 143201 (2011)], revealing significantly lower HO recombination resonance strengths for low-$Z$ ions. In the present work, the correction to the HO resonance strength results from the inclusion of configuration interaction in the initial state of electronic recombination. Furthermore, the angular correction and linear polarization of HO electronic recombination are investigated to facilitate the comparison of the calculated resonance strengths with the electron-beam ion-trap measurements. An unexpected polarization behavior of dielectronic and trielectronic recombination has been shown, suggesting a promising opportunity for further study of the HO electronic recombination process.

In a linear radio-frequency (rf) ion trap, the rf null is the point of zero electric field in the dynamic trapping potential where the ion motion is approximately harmonic. When displaced from the rf null, the ion is superimposed by fast oscillations known as micromotion, which can be probed through motion-sensitive light-atom interactions. In this work, we report on the emergence of the rf pseudonull, a locus of points where the ion responds to light as if it were at the true rf null, despite being displaced from it. The phenomenon is fully explained by accounting for the general two-dimensional structure of micromotion and is experimentally verified under various potential configurations, with observations in great agreement with numerical simulations. The rf pseudonull manifests as a line in a two-dimensional parameter space, determined by the geometry of the incident light and its overlap with the motional structure of the ion. The true rf null occurs uniquely at the concurrent point of the pseudonull lines induced by different light sources.

We consider bosons interacting through a narrow $s$-wave resonance. Such a resonance is characterized by an infinite scattering length and a large and negative effective range $r_0$. We argue that any number $N\ge 3$ of bosons can form a self-bound cluster with the binding energy per particle increasing as $N^2$ for $1\ll N\ll {(-r_0/a_{\text{bg}})}^{1/2}$, where $a_{\text{bg}}$ is the background scattering length (between atoms and molecules). In the opposite limit $N\gg {(-r_0/a_{\text{bg}})}^{1/2}$, bosons form droplets with binding energy per particle saturating to a constant value independent of the particle number. The stability of clusters and droplets when the interaction is detuned from the resonance is also studied.

Multiple-phase estimation exploiting quantum states has broad applications in novel sensing and imaging technologies. However, the unavoidable presence of lossy environments in practical settings often diminishes the precision of phase estimations. To address this challenge, we propose an optimal multiple-phase estimation scheme that is inherently robust against photon loss, ensuring a persistent quantum advantage across all levels of photon loss. The scheme employs a multimode definite photon-number (DPN) state with weights optimized for given levels of photon loss. We theoretically demonstrate that the DPN state can sustain quantum enhancement in estimation precision under all levels of photon loss, compared to the classical benchmark that employs a coherent state input. The proposed scheme using DPN states generalizes earlier studies employing NOON states, which are only optimal when photon loss is small. We believe that our study, demonstrating persistent robustness to photon loss, paves the way for significant advancements in quantum-enhanced sensing technologies, enabling practical applications and quantum advantages in real-world scenarios.

We propose a randomized variant of the Greenberger-Horne-Zeilinger (GHZ) game where promise conditions for input distributions are selected uniform randomly from different possibilities, and disclosed to a single party only. Despite this asymmetry, the game is deterministically winnable using the GHZ correlation that yields perfect success in the original GHZ pseudotelepathy game. The introduced randomization, however, reveals a higher operational advantage in a class of multipartite communication complexity tasks, where inputs are distributed among distant parties and a designated function must be computed by one of them. Classical strategies require two bits of communication to the computing node from each of the other parties, while preshared GHZ states enable the same task with only one bit per party. This separation demonstrates a superior utility of GHZ correlations in multipartite protocols and showcases an enhanced operational power of genuine entanglement in multipartite quantum communication complexity tasks.

Optical vector modes (VMs), characterized by spatially varying polarization distributions, have become essential tools across microscopy, metrology, optical trapping, nanophotonics, and optical communications. Here, we investigate the Hong-Ou-Mandel (HOM) interference of VMs providing a full spatial characterization of the output state. We find that, by carefully selecting the input VMs, it is possible to simultaneously observe all four polarization Bell states and their superpositions in the spatially varying polarization pattern of the two photons. These results represent a significant step in advancing our understanding of HOM interference within structured photons, offering promising avenues for high-dimensional quantum information processing and, in particular, high-dimensional quantum communication, quantum sensing, and advanced photonic technologies reliant on tailored quantum states of light.

Driven-dissipative nonlinear systems exhibit rich critical behavior, related to bifurcation, bistability, and switching, which underlie key phenomena in areas ranging from physics, chemistry, and biology [T. Wilhelm, BMC Syst. Biol. 3, 90 (2009)] to social sciences and economics. The importance of rare fluctuations leading to a dramatic jump between two very distinct states, such as survival and extinction in population dynamics [A. Taitelbaum et al., Phys. Rev. Lett. 125, 048105 (2020); A. Kamenev and B. Meerson, Phys. Rev. E 77, 061107 (2008)], success and bankruptcy in economics [S. Ghashghaie et al., Nature (London) 381, 767 (1996)], and the occurrence of earthquakes [L. Knopoff and Y. Kagan, J. Geophys. Res. 82, 5647 (1977)] or of epileptic seizures [K. Lehnertz, Extreme Events in Nature and Society (Springer, Berlin, Heidelberg, 2006), pp. 123–143], have been already established. In the quantum domain, switching is of importance in both chemical reactions and the devices used in quantum state detection and amplification [R. Vijay et al., Rev. Sci. Instrum. 80, 111101 (2009)]. In particular, the simplest driven single oscillator model serves as an insightful starting point. Here we describe switching induced by quantum fluctuations and illustrate that an instanton approach within Keldysh field theory can provide a deep insight into such phenomena. We provide a practical recipe to compute the switching rates semi-analytically, which agrees remarkably well with exact solutions across a wide domain of drive amplitudes spanning many orders of magnitude. Being set up in the framework of Keldysh coherent states path integrals, our approach opens the possibility of studying quantum activation in many-body systems where other approaches are inapplicable.

Bell measurements, which allow entanglement between uncorrelated distant particles, play a central role in quantum communication. Indeed, sharing, measuring, and creating entanglement lie at the core of various protocols, such as entanglement swapping and quantum teleportation. While for optical qubit systems a Bell measurement can be implemented using only linear components, the same result is no longer true for high-dimensional states, where one has to consider either auxiliary photons or nonlinear processes. Here, inspired by the latter approach, we propose a protocol for high-dimensional quantum teleportation based on nonlinear techniques. Moreover, we discuss the practical implementation of our proposed setup in the case of path-encoded qutrits, where nonlinear effects arise from sum-frequency generation. Finally, we compute the fidelity between quantum states to benchmark the validity of our model under the presence of crosstalk noise. Our approach is deterministic, versatile, and does not rely on the use of auxiliary photons, thus paving the way towards the practical implementation of quantum networks based on nonlinear effects.

We report an algorithm to calculate electron diffraction patterns for molecules with anisotropic angular distribution, which is significantly faster than existing methods. The algorithm uses a transform to convert the molecular orientation distribution, which is a function of three Euler angles, to the atom-pair distribution functions which depend on the polar and azimuthal angles. The diffraction signal can then be calculated from the atom-pair distributions. We demonstrate the computation method numerically by calculating electron diffraction patterns for a symmetric top molecule (trifluoroiodomethane) and an asymmetric top molecule (formaldehyde) and show that it reduces the calculation time by approximately two orders of magnitude compared to the standard brute-force method. The method can also be applied to the calculation of x-ray diffraction patterns.

We propose theoretically a simple two-dimensional system consisting of two square lattices of Rydberg atoms for the exploration of topological phenomena. In particular, we exploit the strong dipole-dipole interactions between Rydberg atoms to simulate a two-dimensional Su-Schrieffer-Heeger model. This model reveals a plethora of topological phases, which can be reached by varying the lattice geometry. We predict not only the emergence of edge and corner states, which characterize topological insulators, but also the appearance of pairs of topologically charged Dirac points and cones in gapless semimetallic phases. Moreover, the position, tilt, and degree of anisotropy of the cones, which may determine the transport properties in the system, are tunable via the lattice parameters. This establishes Rydberg lattice gases, which are already at the forefront of quantum simulation platforms, as the ideal setup for the exploration of topological phenomena.