One central study that constitutes a major branch of quantum resource theory is the hierarchy of states. This provides a broad understanding of resourcefulness in certain tasks in terms of efficiency. Here we investigate the maximal superposition states, i.e., golden states, of the resource theory of superposition. Golden states in the resource theory of coherence are very well established; however, it is a very challenging task for superposition due to the nonorthogonality of the basis states. We show that there are sets of inner product settings that admit a golden state in high-dimensional systems. We bridge the gap between the resource theory of superposition and coherence in the context of golden states by establishing a continuous relation by means of a Gram matrix. In addition, immediate corollaries of our framework provide a representation of maximal states which reduces to the maximal state of the coherence in the orthonormal limit of pure basis states.

Removing quantum hacking attacks is a highly challenging task for quantum key distribution (QKD) since one needs to correctly detect the types of attacks. There is a big gap between the idealized theoretical model and the practical physical system, which results in various hacking strategies exploited from security loopholes. To bridge this gap, previous continuous-variable (CV) QKD systems adopted a universal statistical model to represent different dimensional characters of the physical layer. The challenge is to discriminate different attacks from the exposed key physical details. Here we present a semisupervised deep-learning method to detect the known attacks or potential threats that are hard to be captured in previous CVQKD systems. The proof-of-principle experiment result shows that it could break the present limitations that different attacks are relatively independent, and key features computed from previous threshold-based methods are not correlated with each other. We anticipate that our proposal will give insight into how quantum hacking attacks can be detected under the real system modeling constraints, and will enable the present quantum communication systems to reach a higher level of practical security without adding new assumptions.

The formation and various disintegration modes of $2s$ states for muonic hydrogen and deuterium atoms at kinetic energies both above and below the $2s\text{−}2p$ threshold are studied. The cross sections of the collision-induced radiative quenching, elastic scattering, and Coulomb deexcitation of ${\left({\mu }^{-}p\right)}_{2s}$ and ${\left({\mu }^{-}d\right)}_{2s}$ atoms in collisions with ordinary hydrogen and deuterium atoms at collision energies below the $2s\text{−}2p$ threshold are calculated in the framework of the close-coupling approach. The basis set includes all open and closed channels corresponding to exotic-atom states with principal quantum number $n$ from 1 up to 30. Results of these numerical quantum-mechanical calculations of cross sections are used as input data for detailed cascade calculations. The kinetics of atomic cascade for ${\mu }^{-}p$ and ${\mu }^{-}d$ atoms is studied in the wide range of the relative target densities, $\phi ={10}^{-8}-1$ applying the improved version of the cascade model. It is shown that the collision-induced radiative quenching gives a significant contribution to the total disintegration of the $2s$ state of muonic atoms at energies below the $2s\text{−}2p$ threshold. The initial population of the $2s$ state for both muonic hydrogen and deuterium atoms, absolute intensities, and probabilities of the different disintegration modes as well as their lifetimes are calculated for the fractions with kinetic energies above and below the $2s\text{−}2p$ threshold. The obtained results are compared with the known experimental data.

An accurate description of electron momentum spectroscopy (EMS) for molecular targets remains unresolved since its advent several decades ago. Here, we report a multicenter three-distorted-wave calculation of electron momentum profiles for molecules. The accuracy of the calculation method is demonstrated by an EMS study on oxygen molecules at various electron impact energies ranging from 608 to 1808 eV. The calculations are in excellent agreement with the experiment for all the observed valence orbitals, especially the “turn-up” at zero momentum for the $1{\pi }_g$ orbital as well as the intensities at large momentum up to 6.0 a.u. where the cross sections are extremely low.

Generation of ultrabroadband isolated attosecond pulses (IAPs) is essential for time-resolved applications in chemical and material sciences, as they have the potential to access the spectral water window region of chemical elements, which has yet to be established. Here we propose a numerical scheme for highly efficient high-order harmonic generation and hence the generation of ultrabroadband IAPs in the XUV and soft-x-ray regions. The scheme combines the use of chirped pulses with trapped electrons in copper transition-metal surfaces and takes advantage of the characteristic features of an infrared (IR) single-cycle pulse to achieve high conversion efficiencies and large spectral bandwidths. In particular, we show that ultrabroad IAPs with a duration of 370 as and with a bandwidth covering the photon energy range of 50–250 and 350–450 eV can be produced. We further show that introducing an additional IR single-cycle pulse permits us to enhance the harmonic yield in the soft-x-ray photon energy region by almost seven orders of magnitude. Our findings thus elucidate the relevance of trapped electrons in metal surfaces for developing stable and highly efficient attosecond light sources in compact solid-state devices.

The numerical simulation of multiple scattering in dense ensembles is the mostly adopted solution to predict their complex optical response. While the scalar- and vectorial-light-mediated interactions are accurately taken into account, the computational complexity still limits current simulations to the low saturation regime and ignores the internal structure of atoms. Here, we propose to go beyond these restrictions, at constant computational cost, by describing a multilevel system by an effective two-level system that best reproduces the coherent and total scattering properties in any saturation regime. The correspondence of our model is evaluated for different experimentally realistic conditions such as the modification of the driving field polarization, the presence of stray magnetic fields, or an incoherent resonant electromagnetic field background. The trust interval of the model is quantified for the ${\mathrm{D}}_2$ line of ${}^{87}\mathrm{Rb}$ atoms, but it could be generalized to any closed transition of a multilevel quantum system.

The creation of a Hamiltonian in the quantum regime which has nontrivial topological features is a central goal of the cold-atom community, enabling widespread exploration of novel phases of quantum matter. A general scheme to synthesize such Hamiltonians is based on dynamical modulation of optical lattices which thereby generate vector potentials. At the same time, the modulation can lead to heating and serious difficulties with equilibration. Here we show that these challenges can be overcome by demonstrating how a Hofstadter Bose-Einstein condensate (BEC) can be dynamically realized, using experimental protocols. From Gross-Pitaevskii simulations our study reveals a complex, multistage evolution; this includes a chaotic intermediate “heating” stage followed by a spontaneous reentrance to the BEC. The observed behavior is reminiscent of evolution in cosmological models.

We propose the concept of a spontaneous many-body Floquet state. This is a state that, in the absence of external periodic driving, self-oscillates like in the presence of a periodic Hamiltonian, this behavior being spontaneously induced by many-body interactions. In addition, its quantum fluctuations are described by regular Floquet theory. Furthermore, it is also a time crystal, presenting long-range time-periodic order. However, this crystalline behavior is very different to that of conventional Floquet discrete time crystals: here, there is no external periodic driving, energy is conserved, and the nature of the spontaneous symmetry breaking is continuous instead of discrete. We demonstrate that spontaneous many-body Floquet states can emerge in a variety of canonical many-body problems, ranging from interacting fermions to Bose-Hubbard models. We specifically show that a spontaneous many-body Floquet state is a universal intrinsic state of a one-dimensional flowing atom condensate, both subsonic and supersonic, resulting from a dynamical phase transition and robust against external perturbations and quantum fluctuations, proposing also realistic experimental scenarios for its observation. A spontaneous many-body Floquet state not only represents a realization of a continuous time crystal, but also another paradigm in Floquet physics.

A system of a resonant scatterer described as a two-level atom coupled to a circular dielectric waveguide is studied. The system is characterized by the scattering properties of a guided mode (guided transmission and reflection, bulk radiation) and optomechanical properties (propelling force and torque on the scatterer). The results are based on a self-consistent solution of the scattering problem using the Green's functions and cover the cases of scatterers inside and outside waveguides. It is shown that while a scatterer with a fixed dipole orientation gives rise to scattering spectra described by Lorentzian line shapes, an isotropic scatterer can produce non-Lorentzian line shapes. The parameters defining the line shape are analyzed. The complicated line shapes are likely to arise in the coherent characterization of an emerging class of photonic devices based on photon emitters coupled to well-confined modes of dielectric waveguides. It is obtained that the propelling force and axial torque on the scatterer at resonance can be directly expressed in terms of the spontaneous emission coupling factor $\left(\beta \right)$ of the system. The balances of the linear and angular momentum in the system are discussed. Numerical results are presented for strongly confined guided modes which allow efficient mode scattering and conversion of the linear and angular mode momentum to the force and torque, respectively.

With advanced micro- and nanophotonic structures, the vacuum photon-photon coupling rate is anticipated to approach the intrinsic loss rate and lead to unconventional quantum effects. Here, we investigate the classical-to-quantum transition of such photonic nonlinear systems using the quantum cluster-expansion method, which addresses the computational challenge in tracking large photon number states of the fundamental and harmonic optical fields involved in the second-harmonic generation process. Compared to the mean-field approximation used in the weak-coupling limit, the quantum cluster-expansion method solves multimode dynamics efficiently and reveals the quantum behaviors of optical parametric oscillations around the threshold. This paper presents a universal tool to study quantum dynamics of multimode systems and explore the nonlinear photonic devices for continuous-variable quantum information processing.

We investigate the performance of a microscopic quantum heat engine consisting of $V$- or $\mathrm{\Lambda }$-type emitters interacting collectively or independently when being in contact with environmental thermal reservoirs. Though the efficiency of a Carnot's cycle is always higher than those associated with these setups, we have found that the performance of the cooperative $\mathrm{\Lambda }$-type heat engine may be larger than that of the $V$ type under similar conditions. Cooperativity among the emitters plays an important role for the $\mathrm{\Lambda }$-type setup, significantly improving its performance, while it is less relevant for a $V$-type thermal engine. This is because the population inversion on the working atomic transition as well as its off-diagonal elements behave differently for these two atomic ensembles.

In recent years, the reassessment of quantum physical phenomena under the framework of resource theories has triggered the design of novel quantum technologies that take advantage from quantum resources, such as entanglement and quantum coherence. Bearing this in mind, in this work we study the dynamics of quantum resources for two solid-state fermionic quantum devices: (i) a system composed by a pair of Majorana fermions and (ii) another comprising a pair of regular fermions. In both systems, the fermionic species are coupled to a single-level quantum dot. From the interaction of these tripartite systems with a dissipative reservoir, we were able to characterize the dynamics of the devices for some initial states. By employing a time-nonlocal master-equation approach, we obtain the evolution for fermionic occupations, quantum correlations, and quantum coherences in both the Markovian and non-Markovian dissipating regimes. We investigate the interconversion of local coherence and bipartite correlations for the marginal states in each device. While the dynamics of the entanglement and quantum coherence depend quite strongly on the temperature of the reservoir for regular fermions, we found these evolutions are qualitatively similar for the case of Majorana bound states regardless of temperature. Our results illustrate the use of quantum information-theoretic measures to characterize the role of quantum resources in fermion systems.

Reducing errors is critical to the application of modern quantum computers. We investigate the quantum error mitigation considering parametric circuits accessible by classical computations in some range of their parameters. We present a method of global randomized error cancellations mitigating quantum errors by summing a linear combination of additionally randomized quantum circuits with weights determined by comparison with the referent classically computable data. We illustrate the performance of this method with real quantum simulations of the $n=4$ spins antiferromagnetic Ising model in the transverse field.

We present a convenient and efficient scheme to generate multipartite continuous-variable entanglement via mechanical oscillator displacement induced by two strong input pump fields in a conventional single-cavity optomechanical system. It is shown that multipartite entanglement among the outputs of the two pump fields and any number of relatively weak probe fields can be realized and optimized when the two pump fields with suitable amplitude ratio and relative initial phase are, respectively, tuned to the red and blue mechanical sidebands of a single-cavity mode and each probe field is red detuned by the mechanical frequency with respect to a different neighboring cavity mode. This method can, in principle, be extended to flexibly and conveniently generate an arbitrary number of nondegenerate bright entangled fields by using only coherent laser fields, and may find promising applications in realistic quantum communication and networks.

We consider photoionization in a system of two atoms of different species caused by a laser field. In this process an inner-valence vacancy, created by photoexcitation in one of the atoms, decays not only due to intra- but also interatomic electron-electron correlations with the energy release in the latter case being transferred to the other atom which is ionized. We show that when the intensity of the laser field increases this photoionization process—due to Rabi oscillations and Pauli blocking—may acquire qualitatively new features in its time evolution and the electron emission spectra.

We address a prototype integrated quantum circuit model system: a multifunctional quantum junction comprising three-level atomic nodes that couple two waveguides. We consider nodes with the $\mathrm{\Lambda }$ scheme of the allowed optical transitions, one of which is driven by an external classical electromagnetic field. We demonstrate that the latter field can be used to control the mode of the device operation: studying the dynamics of optical pulses, we show that the proposed integrated junction can operate as a controlled router or a switch, a 1/4 splitter, a delay line, or a storage node. The system offers also a possibility of intrinsic parallelism of quantum operations and can provide useful guidelines for making possible future quantum circuitry building blocks more scalable and integrated.

We investigate the problem of estimating the tunneling frequency of a two-level atomic system embedded in a dissipative environment by employing a numerically rigorous hierarchical equations-of-motion method. The effect of counter-rotating-wave terms on the attainable precision of the noisy quantum metrology is systematically studied beyond the usual framework of perturbative treatments. We find the counter-rotating-wave terms are able to boost the noisy quantum metrological performance in the intermediate- and strong-coupling regimes, whether the dissipative environment is composed of bosons or fermions. The results presented in this paper may pave the way to design a high-precision quantum estimation scenario under practical decoherence.

In their recent paper [Phys. Rev. A 103, 043106 (2021)], Zang et al. theoretically investigated high harmonic generation (HHG) in benchmark two-electron systems that are inversion symmetric with time-dependent density functional theory (TDDFT) in the Kohn-Sham formulation. They found that the theory wrongly predicted the emission of symmetry-forbidden even harmonics and concluded that this error originates from an inherent problem of TDDFT that unphysically populates one- and two-electron excited states. They further claimed that this effect results in an incorrect HHG cutoff energy. We reproduced their main results, but found that the unphysical even harmonics that they observed originated from numerical errors introduced by the boundary conditions. We show that contrary to their claims, the HHG cutoff energy calculated within TDDFT agrees perfectly with the standard and well-established models of HHG.

In reply to the Comment by O. Neufeld et al. [Phys. Rev. A 105, 047101 (2022)], we argue that the conclusions of Phys. Rev. A 103, 043106 (2021) remain valid. We disprove the claim that the unphysical even-order harmonics originate from convergence issues related to reflections at the boundary of the simulation box. By additional calculations, we show that such reflections perturb the high-order harmonic generation spectra by oscillations with periods much smaller than the distance between the harmonics. We also demonstrate that the convergence argument of the Comment, in contrast to our multielectron excitations argument, cannot explain why there are no unphysical even-order harmonics in one-electron systems. Moreover, we show that the argument put forward in the Comment to conclude that the time-dependent Kohn-Sham equations are superior to the time-dependent natural Kohn-Sham equations is not valid.

A quantum decaying system can reveal its nonclassical behavior by being noninvasively measured. Correlations of weak measurements in the noninvasive limit violate the classical bound for a universal class of systems. The violation is related to the incompatibility between exponential decay and unitary evolution. The phenomenon can be experimentally observed by continuous weak measurements and a large class of observables. The nonclassical nature of such a system allows us to treat it as a potential quantum resource.