Birefringence and dichroism in atomic systems are active areas of research. Circular dichroism is typically observed in chiral atoms, molecules (such as DNA), and optically asymmetric materials. These phenomena enable polarization-based selectivity and high-precision measurements with applications in optical systems, biomedical diagnostics, and advanced photonics. Meanwhile, negative-refractive-index materials offer exotic applications in optics and photonics, including superresolution imaging, invisibility cloaking, and subwavelength imaging. Here we identify a route to achieving birefringence and dichroism in a left-handed system. We investigate the optical activity of an atomic left-handed medium, analyzing the interaction of left- and right-circularly polarized light with different atomic states while incorporating local field corrections. Notably, our results demonstrate simultaneous left-handedness for both polarization states. Furthermore, we find that electric birefringence and dichroism are significantly enhanced with increasing control field strength, while magnetic birefringence and dichroism can be fully controlled via strong laser coupling. These findings have promising implications in fields at the interface of left-handedness and dichroism.

In this paper, we propose a driven-dissipative scheme for generating non-Gaussian entangled mechanical states and consider the remote preparation of motional Schrödinger cat states via mechanical entanglement. The system under our study consists of a cavity optomechanical setup with two frequency-mismatched mechanical oscillators simultaneously coupled to a bichromatically driven cavity field. We show that under appropriate conditions, an effective Hamiltonian for nondegenerate parametric down-conversion involving the two mechanical oscillators and the cavity field can be engineered. We demonstrate analytically and numerically that the cavity dissipation drives the mechanical oscillators into a steady-state non-Gaussian entangled state. The properties of the mechanical state, including non-Gaussianity, Wigner negativity, entanglement, and quantum steering, are investigated in detail. We further reveal that through the non-Gaussian mechanical entanglement, the remote preparation of the Schrödinger cat states of one mechanical oscillator can be achieved by coupling the other one to an auxiliary probe cavity and homodyning the output of the probe cavity. We also discuss the robustness of the mechanical states against thermal fluctuations. Our scheme provides a feasible approach for the dissipative and remote preparation of non-Gaussian mechanical nonclassical states.

We propose an effective model leveraging cavity superradiance to achieve entanglement between two spatially separated two-level atomic ensembles trapped in a standing-wave optical cavity. This model is based on permutational symmetry breaking, which leads to the emergence of entangled quantum dark states with nonzero excitations but vanishing decay rate. This steady-state entanglement is created by purely dissipative dynamics, unlike the superradiance of permutationally symmetric two-level atoms, whose steady state is the ground state. Furthermore, we show that the entanglement is significantly enhanced near critical points in the superradiance potential because of the extremely high two-mode squeezing factor between two atomic ensembles. We also discuss spin squeezing in each atomic ensemble in the vicinity of specific critical points in the potential. These findings may play an important role in the generation of correlations immune to dissipation in separated atomic modes.

We study the collective dissipative dynamics of dipoles modeled as harmonic oscillators coupled to one-dimensional electromagnetic reservoirs. The bosonic nature of the dipole oscillators as well as the reservoir modes allows an exact numerical simulation of the dynamics for arbitrary coupling strengths. At weak coupling, apart from essentially recovering the dynamics expected from a Markovian Lindblad master equation, we also obtain non-Markovian effects for spatially separated two-level emitters. In the so-called ultrastrong coupling regime, we find the dynamics and steady state depends on the choice of the reservoir which is chosen as either an ideal cavity with equispaced, unbounded dispersion or a cavity array with a bounded dispersion. Moreover, at even higher coupling strengths, we find a decoupling between the light and matter degrees of freedom attributable to the increased importance of the diamagnetic term in the Hamiltonian. In this regime we find that the dependence of the dynamics on the separation between the dipoles is not important and the dynamics is dominated by the occupation of the polariton mode of lowest energy.

Quantum-classical transitions have long attracted much attention. We study such transitions in quantum spin-($j$,1/2) systems at thermal equilibrium. Unlike previous work [Phys. Rev. A 73, 064302 (2006)], we find that the threshold temperature of quantum entanglement decreases with increasing spin $j$ and completely disappears in the limit $j\to \infty$. In the ground state of systems with highly symmetric interactions, the discord-type quantum correlations can exist even for arbitrarily large spin. Such correlations turn out to be unstable and are destroyed by small perturbations that violate the symmetry of the Hamiltonian. The stable quantum correlations gradually degrade as the spin $j$ grows and eventually vanish when the classical limit is reached.

We propose a simplified measurement scheme for a Gaussian modulated coherent state (GMCS) protocol for continuous variable quantum key distribution (CV-QKD), utilizing homodyne detection without quadrature switching. The reference frame of measurement is taken to be at an arbitrary angle, however, reconciliation converges the proposed scheme to GMCS with switching quadrature protocol. The arbitrary frame of measurement could also include the unknown random thermal drift within Bob's optical measurement setup. We found this scheme is advantageous for practical free-space and fiber-based GMCS protocol-based CV-QKD systems as it does not require a phase modulator for random measurement selection quadrature at Bob.

Quantum teleportation, a fundamental protocol in quantum information science, enables the transfer of quantum states through entangled particle pairs and classical communication channels. While ideal quantum teleportation requires maximally entangled states as resources, real-world implementations inevitably face environmental noise and decoherence effects. In this work, we investigate quantum teleportation in nonequilibrium environments with different temperatures or chemical potentials. We employ the Bloch-Redfield equation to characterize the nonequilibrium dynamics in both bosonic and fermionic reservoirs. Our analysis reveals that nonequilibrium conditions, namely, the temperature difference and chemical potential difference, can significantly enhance the teleportation fidelity beyond what is achievable in equilibrium scenarios. Notably, under certain nonequilibrium conditions, the fidelities for all input states converge to the same value. We term this behavior the teleportation with a fixed-point fidelity. Importantly, at this fixed point, fidelity can be further enhanced by tuning the nonequilibrium parameters. These findings offer crucial insights for the implementation of quantum communication protocols in realistic environments, demonstrating that nonequilibrium conditions not only improve the fidelity but also present a promising avenue for simplifying practical quantum teleportation schemes.

Graph kernels are essential for learning the properties of graph data, a fundamental task in the analysis of such data. However, a major limitation of current graph kernel methods is their high computational cost, which becomes prohibitive when dealing with large-scale graphs. In order to address this bottleneck, we propose a quantum algorithm designed to analyze the intrinsic structural information within graph data through an efficient quantum kernel method. The core idea of our kernel method involves encoding pertinent information from graphs into quantum states, utilizing quantum kernel functions to represent the similarities among various graphs. This process is facilitated through the application of quantum walks and quantum signal processing techniques. Building upon this framework, we propose a quantum kernel ridge regression algorithm that consists of two key steps. First, we employ our quantum kernel method to prepare a quantum state, in which the amplitudes encode the kernel functions. Second, we implement quantum feature mapping in parallel to encode the corresponding kernel matrix into the quantum circuit. Crucially, we demonstrate that the computational complexity of our regression algorithm scales primarily as $O\left(M^{3/2}\right)$ with respect to the size of the graph set $M$. This suggests that our quantum kernel method could process graph data with resource requirements scaling approximately as $O(\text{poly}log(N\left)\right)$, where $N$ denotes the number of nodes in the graph. Accordingly, this underscores the advantage of our approach over classical methods in the context of large-scale graph analysis. To validate our approach, we conduct numerical experiments on chemical molecular datasets, demonstrating the method's effectiveness and advantages. In summary, this work represents a significant advancement in quantum graph learning, enhancing its potential for analyzing large-scale graph data.

$k$-uniform states are valuable resources in quantum information, enabling tasks such as teleportation, error correction, and accelerated quantum simulations. However, the practical realization of $k$-uniform states, at scale, faces major obstacles: Verifying $k$ uniformity is as difficult as measuring code distances, and devising fault-tolerant logical preparation protocols further adds to the complexity. To address these challenges, we present a scalable fault-tolerant method for preparing encoded $k$-uniform states and we illustrate our approach using surface and color codes. We first present a technique to determine $k$ uniformity of stabilizer states directly from their stabilizer tableau. We then identify a family of Clifford circuits that ensures both fault tolerance and scalability in preparing these states. Building on the encoded $k$-uniform states, we introduce a hybrid physical-logical strategy that retains some of the error-protection benefits of logical qubits while lowering the overhead for implementing arbitrary gates compared to fully logical algorithms. We show that this hybrid approach can outperform fully physical implementations for resource-state preparation, as demonstrated by explicit constructions of $k$-uniform states.

This paper systematically analyzes the key noise sources limiting the sensitivity of Rydberg atomic receivers. First, the quantum projection noise inherent to the system is modeled, as this noise sets the theoretical limit for receiver sensitivity. Second, the paper establishes the constraints relationship between photodetector noise and system sensitivity under both direct detection and heterodyne detection methods. Subsequently, this paper establishes the constraint relationships between the laser's intensity noise and receiver sensitivity, and between the phase noise and receiver sensitivity, respectively. The research reveals that the intensity noise of the probe light significantly exceeds that of the coupling light in its effect on receiver performance, while its phase noise exerts a comparable level of influence.

We theoretically investigate the ground state and dynamics of a Rabi-coupled pseudospin-1/2 Bose-Einstein condensate, where only one spin component is subjected to an external potential. We show that in the quasiperiodic potential the Rabi coupling induces localization between the components as it is raised above the threshold value. Interestingly, the localization is mutually induced by both components for the quasiperiodic confinement, whereas for a harmonic trap the localization is induced in the potential-free component by interaction with that confined in the potential. Further, we explore the condensate dynamics by implementing a periodic driving of the Rabi frequency, where various frequency-dependent delocalization patterns, such as double- (triple)-minima, treelike (parquetlike), and frozen distributions with a correlated propagation of different spin populations are observed in the condensate density. These features pave the way to control the condensate mass and spin density patterns, both in the stationary and dynamical realizations.

Within the nonequilibrium Green's function framework, we systematically analyze the momentum relaxation dynamics of strongly coupled impurities in three-dimensional nondegenerate quantum gases. Using the Hartree-Fock approximation, we establish that the impurity velocity exhibits exponential decay asymptotically. When the impurity velocity is much lower than the thermal velocity of the gas atoms, the relaxation time $\tau$ is governed entirely by the self-energy evaluated at zero momentum and zero frequency, $\mathrm{\Sigma }(\mathbf{p}=0;\omega =0)$. Furthermore, via virial expansion theory, $\tau$ admits a systematic expansion in powers of the fugacity $z\equiv e^{\beta \mu }$. The leading-order term precisely recovers the Boltzmann semiclassical result, while higher-order corrections capture quantum many-body effects beyond the semiclassical approximation.

This article presents a comprehensive three-dimensional canonical quantization to treat quantum plasmonics for finite metallic or dielectric media of arbitrary shape. We use a microscopic model for the dissipative and dispersive medium coupled with the electromagnetic field, which is justified by the fact that if one integrates the degrees of freedom of the medium, one obtains the macroscopic Maxwell equations. Its quantization features a Hamiltonian formulation having the form of two infinite harmonic oscillators characterized by a double continuum. The diagonalized Hamiltonian is quantized by the correspondence principle, introducing creation-annihilation operators in a bosonic Fock space. The diagonal quantum Hamiltonian is the sum of two terms corresponding to the two continua. The physical observables, such as the electric field, are also the sum of two terms corresponding to the two continua, one of which had been omitted in the literature geared for an infinite bulk medium. In a second step, we show that the electric field operator can by written as linear combinations of the creation-annihilation operators with coefficients that satisfy integral equations of Fredholm type. We show that the solution of these equations can be expressed in terms of the classical Green tensor of the medium satisfying the Sommerfeld radiation condition. Finally, we consider the Purcell effect for the spontaneous emission of an atom close to the medium. We show that through an exact compensation of some terms, the Purcell factor for the system with the double continuum is proportional to the imaginary part of the Green tensor, which defines the local density of states. This result has the same form as the one obtained in the literature for bulk systems that involve a single continuum and a small dissipative background extending to infinity, and can be seen as a justification of this approach.

We study self-sustained cavity emission from driven atoms in collective strong coupling. The cavity emission occurs over a wide range of atom-cavity and drive-laser detunings without any external input to the cavity mode. Second-order correlation measurements $g^{\left(2\right)}\left(\tau \right)$ further reveal unanticipated phenomena in the observed cavity emission such as (1) damped oscillations at two frequencies and (2) significantly distinct $g^{\left(2\right)}\left(\tau \right)$ for different polarization components. The intricate relation between cavity-emission intensity, drive-laser detuning, and atom-cavity detunings is explained. A possible mechanism for the damped oscillations with two frequency components in $g^{\left(2\right)}\left(\tau \right)$ is suggested. Measurements show the existence of two separate polarization-decoupled mechanisms with distinct photon statistics, through which energy is transferred from the drive field to the cavity field. The statistical properties and mechanisms underlying cavity emission present valuable insights for extending nondestructive detection techniques to the regime of collective strong coupling.

A natural extension of the non-Hermitian qubit is to place it in a single-mode cavity. This setup corresponds to the quantum Rabi model (QRM) with a purely imaginary bias on the qubit, exhibiting parity-time ($\mathcal{P}\mathcal{T}$) symmetry. In this work, we first solve the $\mathcal{P}\mathcal{T}$-symmetric QRM using the Bogoliubov operator approach. We derive the transcendental function responsible for the exact solution, which can also be used to precisely identify exceptional points. The adiabatic approximation previously used can be easily formulated within this approach by considering transitions between the same manifolds in the space of Bogoliubov operators. By further considering transitions between the nearest-neighboring manifolds, we can analytically obtain more accurate eigensolutions. Moreover, these simple corrections can capture the main features of the dynamics, where the adiabatic approximation fails. Furthermore, the rich characteristics of the vacuum Rabi splitting in the emission spectrum are predicted. The width of the peaks increases with the coupling strength and the imaginary biases, reflecting the nature of open quantum systems. Additionally, we identify a quantum-criticality-enhanced effect by calculating the quantum Fisher information. Near the exceptional points, the quantum Fisher information in the $\mathcal{P}\mathcal{T}$-symmetric QRM is significantly higher than that of the non-Hermitian qubit component. This may open a new avenue for enhancing quantum sensitivity in non-Hermitian systems by incorporating coupling with an additional degree of freedom, enabling more precise parameter estimation.

Quantum battery (QB) is an application of quantum thermodynamics which uses quantum effects to store and transfer energy, overcoming the limitations of classical batteries and potentially improving the QB's performance. However, due to the interaction with the external environment, it leads to decoherence and thus reduces both the lifetime and the charging efficiency of QB. Here, we propose suppressing the environmental dissipation in the energy-storage process of the QB by exploiting both the electromagnetically-induced-transparency (EIT) effect and bound states. By constructing a hybrid system composed of a four-level atom and a coupled-cavity array, two bound states are formed in the system when the energy of the QB is in the energy band of the cavity array. Environmental dissipation is significantly inhibited due to the bound state and EIT effects, which extends the lifetime of the QB. We show that the charging efficiency of the QB is optimal when the energy of the QB is in resonance with the cavity. In addition, there is an optimal coupling strength between the two adjacent cavities, which helps to improve the performance of the QB. Our research has achieved the realization of long-term energy storage while improving the charging efficiency of QB, which has practical implications for the realization of QB.

We present a surface plasmon resonance scheme for the excitation of surface plasmon polaritons (SPPs) employing vacuum-induced transparency. The system under consideration consists of a metal-dielectric interface placed within an optical cavity, where the dielectric medium is composed of three-level atoms in a $\mathrm{\Lambda }$-type configuration. These atoms interact coherently with an incident probe and quantized cavity fields. Our theoretical analysis demonstrates that SPPs can be excited even in the absence of classical control fields, i.e., when the cavity is in its vacuum state. We also show that the electric-field-enhancement factor increases with the number of cavity photons. Furthermore, we analyze the propagation length and lifetime of the SPPs and observe that both are enhanced as the cavity photon number increases.

We evaluate the maximal Clauser-Horne-Shimony-Holt (CHSH) violation for a generic (typically mixed) qubit-qudit state, obtaining easily computable expressions in arbitrary qudit dimension. This represents the optimal (2-2-2) Bell nonlocality for this kind of systems. The work generalizes the well-known Horodeckis result for a qubit-qubit setup. We also give simple lower and upper bounds on that violation. We apply our general results to address a number of issues, namely, we obtain a bound on the degree of purity required in a system to exhibit nonlocality and study the statistics of nonlocality in random density matrices. In addition, we show the impossibility of improving the amount of CHSH violation by embedding the qudit in a Hilbert space of larger dimension. Finally, the general result is illustrated with a family of density matrices in the context of a qubit-qutrit system.

Negativity in certain quasiprobability representations is a necessary condition for a quantum computational advantage. Here we define a quasiprobability representation exhibiting this property with respect to quantum computations in the magic state model. It is based on generalized Jordan-Wigner transformations, and it has a close connection to the probability representation of universal quantum computation based on the $\mathrm{\Lambda }$ polytopes. For each number of qubits, it defines a polytope contained in the $\mathrm{\Lambda }$ polytope with some shared vertices. It leads to an efficient classical simulation algorithm for magic state quantum circuits for which the input state is positively represented, and it outperforms previous representations in terms of the states that can be positively represented.

Systematic QED calculations of ionization energies of the $2s$, $2p_{1/2}$, and $2p_{3/2}$ states as well as the $2p_{1/2}\text{–}2s$ and $2p_{3/2}\text{–}2s$ transition energies are performed for Li-like ions with the nuclear charge numbers $Z=10–100$. The convergence of QED perturbative expansion is improved by using the extended Furry picture, which starts from the Dirac equation with a local screening potential. An ab initio treatment is accomplished for one- and two-photon electron-structure QED effects and the one-photon screening of the self-energy and vacuum-polarization corrections. This is complemented with an approximate treatment of the two-photon QED screening and higher-order (three or more photon) electron-structure effects. As a result, the obtained theoretical predictions improve upon the accuracy achieved in previous calculations. Comparison with available experimental data shows a good agreement between theory and experiment. In most cases, the theoretical values surpass the experimental results in precision with only a few exceptions. In the case of uranium and bismuth, the comparison provides one of the most stringent tests of bound-state QED in the strong-field regime. Alternatively, the obtained results can be employed for high-precision determinations of nuclear charge radii.