We propose and analyze an efficient scheme for the coherent transfer of optical vortices in a cold atomic ensemble with four-level double-$\mathrm{\Lambda }$ configuration. The orbital angular-momentum (OAM) information can be transferred from the incident vortex fields to the generated backward signal field via resonant backward four-wave mixing (FWM). Considering the single vortex probe field initially acting on one transition of the atomic ensemble and using experimentally achievable parameters, we identify the conditions under which the resonant backward FWM allows us to greatly improve the conversion efficiency of optical vortices beyond what is achievable in the resonant forward OAM transfers. It is found that the complete energy conversion of optical vortices originates from the perfect competition between linear absorption and parametric gain. We also demonstrate that the phase mismatching would be detrimental to the high-efficiency transfer of optical vortices. Furthermore, we show that the generated backward signal field develops a new pure or mixing OAM state when the control field is also a vortex beam. Finally, we investigate the composite vortex beam generated by collinear superposition of the incident vortex probe and signal fields, which can controlled via adjusting the intensity of the control field. Our scheme may have potential applications in OAM-based optical communication and optical information processing.

Production of electron-positron pairs in two oscillating strong electric field pulses with largely different frequencies and durations is considered. In a first scenario, the influence of a low-frequency background field on pair production by a short main pulse of high frequency is analyzed. The background field is shown to cause characteristic modifications of the momentum spectra of created particles, which, in turn, may be used for imaging of the background pulse. In a second scenario, an ultrashort, relatively weak assisting pulse is superimposed onto a strong main pulse. By studying the dependence of the pair production on the field parameters it is shown that duration and relative position of the ultrashort pulse modify the momentum spectra of produced particles in a distinctive way. Both scenarios enable one, moreover, to extract partial information about the time periods when pairs with certain momenta are produced predominantly.

Of indisputable relevance for nonequilibrium thermodynamics, fluctuations theorems have been generalized to the framework of quantum thermodynamics, with the notion of work playing a key role in such contexts. The typical approach consists of treating work as a stochastic variable and the acting system as an eminently classical device with a deterministic dynamics. Inspired by technological advances in the field of quantum machines, here we look for corrections to work fluctuations theorems when the acting system is allowed to enter the quantum domain. This entails including the acting system in the dynamics and letting it share a nonclassical state with the system acted upon. Moreover, favoring a mechanical perspective to this program, we employ a concept of work observable. For simplicity, we choose as theoretical platform the autonomous dynamics of a two-particle system with an elastic coupling. For some specific processes, we derive several fluctuation theorems within both the quantum and classical statistical arenas. In the quantum results, we find that, along with entanglement and quantum coherence, aspects of inertia also play a significant role since they regulate the route to mechanical equilibrium.

Using the single and double approximated relativistic coupled-cluster method, we calculate the first-order hyperfine structure constants for the $6p_{3/2}, 8p_{3/2}, 6d_{3/2}, 7d_{3/2}$, and $7d_{5/2}$ states in ${}^{133}\mathrm{Cs}$, and also evaluate the second-order magnetic dipole–magnetic dipole, magnetic dipole–electric quadrupole effects caused by the off-diagonal hyperfine interaction. With these calculations, we reanalyze some recent high-precision experimental measurements of the hyperfine splitting intervals of ${}^{133}\mathrm{Cs}$. We find that the second-order magnetic dipole–magnetic dipole effects are especially sizable at the order of kHz in the $B$ constant for the $d_{3/2,5/2}$ states. Our calculations can provide a reference for future higher precision experimental measurements.

We study the correlation functions in a one-dimensional attractive Fermi polaron system of spin-1/2 Fermi gas with one flipped spin as an impurity immersed in the fully polarized spin-up media. By means of the exact Bethe ansatz wave function with string solutions for the momentum, we manage to explicitly deduce the asymptotic form of the correlation functions in the infinite attractive limit and show that the system undergoes a polaron-molecule crossover when the attraction grows. The correlation functions in the infinite attractive limit can be decomposed into the free fermion term with the number of particles reduced by one and the molecular term indicated by the emergence of a sharp peak or dip in the central area of the correlation. These analytical results are checked by the numerical Monte Carlo methods for multiple integrals. The calculation is further extended to the excited states, i.e., the super-Tonks-Girardeau gas, and there is no peak or dip found in the vicinity of the correlation center. Tan contact is found to be proportional to the cubic of the interaction strength and the total energy is verified to satisfy the Tan adiabatic theorem in the strong attraction regime.

Two-dimensional spin-1/2 fermions with finite-range interactions are theoretically studied. Characterizing the attractive interaction in terms of the scattering length and the effective range, we discuss the finite-range effects on the ground-state properties in this system. The Brueckner $G$-matrix approach is employed to analyze the finite-range effects on an attractive Fermi-polaron energy and the equation of state throughout the BEC-BCS crossover in two dimensions, which can be realized in the population-imbalanced and -balanced cases between two components, respectively. The analytical formulas for these ground states obtained in this study would be useful for understanding many-body phenomena with finite-range interactions in low-dimensional systems.

Optical nonreciprocity plays an essential role in optical information communication and information processing. Based on electromagnetically induced transparency, a nonreciprocal cavity dark-state polariton (DSP) could be achieved using spin-biased cold atoms. The DSP induces a nonreciprocal window with high transmission and low insertion loss around the cavity resonant frequency. By decreasing the strength of the control field, the contribution of the atom excitation dominates the behavior of the DSP, which exhibits a narrow cavity linewidth and thus a long lifetime of cavity photons in the form of the DSP. Further, we investigate the nonreciprocity for the statistical properties of the system in the single-atom ($N=1$) and multiatom ($N>1$) cases. Due to the existence of DSP, the photon statistics leads to totally different profiles for the light propagating in both directions in the two cases. In the single-atom ($N=1$) case, the light in the form of DSP shows apparent sub-Poissonian distribution simultaneously with relatively high transmission and a narrow bandwidth corresponding to the one-photon excitation due to quantum interference. Such a quasiparticle may provide a platform for novel applications in nonreciprocal quantum devices and quantum simulation.

The rapidly growing field of quantum information can benefit greatly from quantum nonlinear optics, which has the potential to generate highly correlated optical states with a sufficiently large interaction strength per photon. Such states play a crucial role in various quantum applications. While the traditional approach to generating correlated optical states involves controlled interactions between individual single photons through nonlinear mediums, a more efficient approach is to use the evolution of multiphotons in the highly nonlinear optical medium. To facilitate this approach, we propose a scheme that traces the time evolution of the photons encoded in Fock space to generate complex correlated states. Our approach mimics the dynamics of many-body systems with adjustable interaction strength, allowing us to simulate state transfer on a spin chain by tuning the interaction. We also investigate the feasibility of our scheme based on current technologies and explore the potential of quantum nonlinear optics in quantum simulation.

It is known that there are infinitely many distinguishability metrics for mixed quantum states. This freedom, in turn, leads to metric-dependent interpretations of physically meaningful geometric quantities such as complexity and volume of quantum states. In this paper, we first present an explicit and unabridged mathematical discussion on the relation between the Sjöqvist metric and the Bures metric for arbitrary nondegenerate mixed quantum states, using the notion of decompositions of density operators by means of ensembles of pure quantum states. Then, to enhance our comprehension of the difference between these two metrics from a physics standpoint, we compare the formal expressions of these two metrics for arbitrary thermal quantum states specifying quantum systems in equilibrium with a reservoir at nonzero temperature. For illustrative purposes, we show the difference between these two metrics in the case of a simple physical system characterized by a spin-qubit in an arbitrarily oriented uniform and stationary external magnetic field in thermal equilibrium with a finite-temperature bath. Finally, we compare the Bures and Sjöqvist metrics in terms of their monotonicity property.

Entanglement is known as an important indicator for characterizing different types of quantum phase transitions (QPTs); however, it faces some challenges in the Kosterlitz-Thouless (KT) phase transitions due to an essential singularity which cannot be identified in finite derivatives of the ground-state energy. In this paper, we consider global entanglement (GE) in a KT phase transition and show that while it does not indicate any clear signature of the phase transition, the conditional version of GE is a good indicator with strong signatures of the KT transition. In particular, we study a deformed version of the $Z_d$ Kitaev model which has an intermediate KT phase which separates a $Z_d$ topological phase from a magnetized phase at two different KT transition points. Using a mapping to the classical $d$-state clock model, we consider GE and the generalized GE and show that they do not provide a reliable indicator of transition points. However, their difference, called conditional global entanglement ($Q$), shows a peak at the first KT transition point. Additionally, we show that it can characterize various phases of the model as it behaves substantially differently in each phase. We therefore conclude that $Q$ is a useful measure that can characterize various phases of KT QPTs as well as their related critical points.

This paper considers a quantum node tasked with the teleportation of multiple information-carrying qubit (ICQ) streams, each to a different receiver, by means of entanglements, local operations, and classical communication. Our vision is that the node establishes entangled qubit pairs (EQPs) with the receivers before the arrival of ICQs, rather than waiting for their arrival. In this vein, the paper focuses on the class of protocols referred to as class $\mathcal{I}$ that instantaneously teleport arriving ICQs using preestablished EQPs, preventing arriving ICQs from decohering. The excess ratio ${\varepsilon }_{\mathrm{r}}$ is introduced as a quantifier of the system resources per arriving ICQ, and ${\varepsilon }_{\mathrm{r}}=1$ is shown to be a critical threshold. With ${\varepsilon }_{\mathrm{r}}>1$: for arrival streams characterized by interarrivals stochastically larger than exponential random variables, any member of class $\mathcal{I}$ teleports all arriving ICQs after a finite transient. With ${\varepsilon }_{\mathrm{r}}<1$: for stationary ergodic arrival streams, there exists no protocol that teleports all arriving ICQs after a finite transient. This work thus establishes the ultimate limit for distributing quantum states with finite lifetime. Within class $\mathcal{I}$, a protocol referred to as fresh information delivery (FID) is introduced and its optimality is proven. The operational characteristic of FID is provided in terms of the tradeoff between the waiting time of the EQP before it is utilized for teleportation and the excess ratio. Numerical experiments, comparing the proposed FID protocol with alternatives, corroborate the theoretical results. The results in this paper can be used for designing quantum nodes, paving the way for the implementation of the future quantum internet.

Single charge-exchange in collisions of heavy bare nuclei with the ground state of two-electron atomic targets is described perturbatively as a four-body problem. The employed four-body boundary-corrected continuum intermediate state (BCIS-4B) method considers the correlated and uncorrelated target wave functions ${\phi }_i.$ A thorough examination is performed for the formation of any final hydrogen-like $nlm$ state of the captured electron. For arbitrary projectile and target nuclear charges, the nine-dimensional integral in the transition amplitude is reduced to a two-dimensional numerical quadrature. The general analysis is applied to one-electron capture by protons from helium targets beginning with the lower edge (10 keV) of intermediate energies and extending to the higher (12.5 MeV) domain. These include the main peaks (Massey, Thomas) due to single and double scattering, respectively. The results encompass over 70 state-selective and state-summed cross sections $(n\le 6,0\le l\le n-1,-l\le m\le l).$ In comparison to measurements, the electronic correlations in ${\phi }_i$ greatly improve the overall performance of the BCIS-4B method around the Massey peak, below about 100 keV. Moreover, while largely outperforming the three-body boundary-corrected continuum intermediate state method, the cross sections in the BCIS-4B with the correlated ${\phi }_i$ compare excellently overall with the available experimental data at 10 to 12 500 keV. Hence, the BCIS-4B method, with its built-in two main capture mechanisms (one-step Massey and two-step Thomas) is capable of spanning impact energies covering three or more orders of magnitude at which the state-summed cross sections vary over 11 orders of magnitude.

We investigate the directional amplification of non-Hermitian single-band systems in one dimension by using two models. The first model is the Hatano-Nelson model with added cavity decay and the second model is a linear lattice with fully asymmetric next-nearest-neighbor hopping. Based on the Dyson equation, we calculate the Green's functions of the two models analytically and investigate the directional amplification of a weak probe field. We find that, although the periodic-boundary spectrum of the two models determines the frequency interval in which the probe field can be directionally amplified, the open-boundary spectrum influences not only the bandwidth of the amplification curve but also the shape of the transmission curves. The bandwidth and the gain can be tuned independently by the parameters of the system. We also find that, for lattices with finite size, the frequency interval of the directional amplification shrinks due to the finite-size effects. Our thorough analysis is expected to provide a deeper insight into the directional amplification in non-Hermitian single-band systems and a tool for designing directional amplifiers and sensors.

The idea of exploiting maximally entangled states as a resource lies at the core of several modalities of quantum information processing, including secure quantum communication, quantum computation, and quantum sensing. However, due to imperfections during or after the entangling gates used to prepare such states, the amount of entanglement decreases and their quality as a resource gets degraded. We introduce a low-overhead protocol to reverse this degradation by partially filtering out a specific type of noise relevant to many quantum technologies. We present two trapped-ion schemes for the implementation of a nonunitary probabilistic filter against amplitude damping noise, which can protect any maximally entangled pair from spontaneous photon scattering during or after the two-qubit trapped-ion entangling gates. This filter can be understood as a protocol for single-copy quasidistillation, as it uses only local operations to realize a reversal operation that can be understood in terms of weak measurements.

We consider bipartite quantum state discrimination using positive-partial-transpose measurements and show that minimum-error discrimination by positive-partial-transpose measurements is closely related to entanglement witness. By using the concept of decomposable entanglement witness, we establish conditions on minimum-error discrimination by positive-partial-transpose measurements. We also provide conditions on the upper bound of the maximum success probability over all possible positive-partial-transpose measurements. Finally, we illustrate our results using examples of multidimensional bipartite quantum states.

Quantum key distribution with multimode subcarrier wave encoding is propitious for being robust against environmental disturbance. For application in long-distance quantum communications, this encoding has to be compatible with entanglement-assisted quantum repeaters that are commonly designed to work with dual-rail encodings. We propose and demonstrate an interface between subcarrier wave and dual-rail encodings with a fidelity of $0.95\pm 0.03$ using a linear optical circuit. The developed scheme opens the way for future heterogeneous quantum networks that combine subcarrier wave quantum key distribution with trusted and nontrusted nodes.