Entanglement-based quantum key distribution (EB-QKD) has several advantages over prepare-and-measure protocols, such as eliminating the need for decoy states and active modulators. At present, entanglement sources based on the spontaneous parametric down-conversion process have been widely used in EB-QKD experiments. Most of the existing models regard the photon-number distribution of pulse-pumped entanglement sources as thermal. However, doing so ignores any effects of spectral modes, which will lead to the overestimation of the secure key rate. In this paper, we analyze the correlation between the photon-number distribution and the joint spectral distribution and rewrite the quantum state of the photon pairs. Then we apply the results to EB-QKD and present a general model which can be used to calculate the secure key rate not only for any pulse-pumped systems but even for cw-pumped systems.

We present a framework of a multimode dense coding network with multiple senders and a single receiver using continuous variable systems. The protocol is scalable to arbitrary numbers of modes with the encoding being displacements while the decoding involves homodyne measurements of the modes after they are combined in a pairwise manner by a sequence of beam splitters, thereby exhibiting its potentiality to implement in laboratories with currently available resources. We compute the closed form expression of the dense coding capacity for the cases of two and three senders that involve sharing of three- and four-mode states, respectively. The dense coding capacity is computed with the constraint of fixed average energy transmission when the modes of the sender are transferred to the receiver after the encoding operation. In both the cases, we demonstrate the quantum advantage of the protocol using paradigmatic classes of three- and four-mode states. The quantum advantage increases with the increase in the amount of energy that is allowed to be transmitted from the senders to the receiver.

Time-dependent response theories are foundational to the development of algorithms that determine quantum properties of electronic excited states of molecules and periodic systems. They are employed in wave-function, density-functional, and semiempirical methods and are applied in an incremental order: linear, quadratic, cubic, etc. Linear response theory is known to produce electronic transitions from ground to excited state, and vice versa. In this work, a linear response approach, within the context of the coupled-cluster formalism, is developed to offer transition elements between different excited states (including permanent elements) and related properties. Our formalism, second linear response theory, is consistent with quadratic response theory and can serve as an alternative to develop and study excited-state theoretical methods, including pathways for algorithmic acceleration. This work also formulates an extension of our theory for general propagations under nonlinear external perturbations, where the observables are given by linked expressions which can predict their time evolution under arbitrary initial states and could serve as a means of constructing general state propagators. A connection with the physics of wave-function theory is developed as well, in which dynamical cluster operator amplitudes are related to wave-function linear superposition coefficients.

Vogt and Wannier [Phys. Rev. 95, 1190 (1954)] predicted that the capture rate of a polarizable neutral atom or molecule by an ion should increase by a factor of 2 compared to the classical Langevin rate as the collision energy approaches zero. This prediction has not been verified experimentally. The ${\mathrm{H}}_2^{+}$ $+$ ${\mathrm{H}}_2$ reaction is ideally suited to observe this effect because the small reduced mass makes quantum effects related to $s$-wave scattering observable at higher collision energies than in other systems. Moreover, the reaction rate for this barrierless, strongly exothermic reaction follows the classical Langevin capture model down to cold-collision conditions (about $k_{\mathrm{B}}\times 1$ K) and is not affected by short-range interactions. Below this temperature, a strong enhancement of the reaction rate resulting from charge–quadrupole interaction between ${\mathrm{H}}_2^{+}$ and ground-state ortho ${\mathrm{H}}_2$ ($J=1$) was observed. Here we present an experimental study of the reaction of ${\mathrm{H}}_2^{+}$ and para ${\mathrm{H}}_2$ ($J=0$), which has no dipole and no quadrupole moments, at collision energies below $k_{\mathrm{B}}\times 1$ K. We observe an enhancement at the lowest collision energies which is attributed to the quantum enhancement predicted by Vogt and Wannier. Measurements of the reaction of ${\mathrm{HD}}^{+}$ with HD support this conclusion.

Knowledge of the fundamental limitations on a magnetic trap for neutral particles is of paramount interest to designers as it allows for the rapid assessment of the feasibility of specific trap requirements or the quality of a given design. In this paper, performance limitations are defined for convexity of magnetic trapping potential and bias field using a local approximation in the trapping center. As an example, the fundamental bounds are computed for current supporting regions in the form of a spherical shell, a cylindrical region, and a box. A Pareto-optimal set considering both objectives is found and compared with known designs of the baseball trap and Ioffe-Pritchard trap. The comparison reveals a significant gap in the performance of classical trap designs from fundamental limitations. This indicates a possibility of improved trap designs and modern techniques of shape synthesis are applied in order to prove their existence. The topologically optimized traps perform almost two times better as compared to conventional designs. Last, but not least, the developed framework might serve as a prototype for the formulation of fundamental limitations on plasma confinement in a wider sense.

The local-density approximation (LDA) is the central technical tool in the modeling of quantum gases in trapping potentials. It consists in treating the gas as an assembly of independent mesoscopic fluid cells at equilibrium with a local chemical potential, and it is justified when the correlation length is larger than the size of the cells. The LDA is often regarded as a crude approximation, particularly in the ground state of the one-dimensional (1D) Bose gas, where the correlation length is “therefore said to be” infinite (in the sense that correlation functions decay as a power law). Here we take another look at the LDA. The local density $\rho \left(x\right)$ is viewed as a functional of the trapping potential $V\left(x\right)$, to which one applies a gradient expansion. The zeroth order in that expansion is the LDA. The first-order correction in the gradient expansion vanishes due to reflection symmetry. At second order, there are two corrections proportional to $d^{2}V/d{x}^2$ and ${(dV/dx)}^2$, and we propose a method to determine the corresponding coefficients by a perturbative calculation in the Lieb-Liniger model. This leads to an expression for the coefficients in terms of matrix elements of the density operator, which can in principle be evaluated numerically for an arbitrary coupling constant; here we show how to efficiently evaluate the coefficient associated to the curvature of the potential $d^{2}V/d{x}^2$, which dominates the deviation to LDA near local minima or maxima of the trapping potential. Both coefficients are evaluated analytically in the limits of infinite repulsion (hard-core bosons) and small repulsion (quasicondensate). The corrected LDA density profiles are compared to density-matrix renormalization group calculations, with significant improvement compared to zeroth-order LDA.

We consider a system of two identical fermions of general mass interacting with a third distinguishable particle via a contact interaction within an isotropic three-dimensional harmonic trap. We calculate time-dependent observables of the system after it is quenched in $s$-wave scattering length. To do this we use exact closed-form mass-imbalanced hyperspherical solutions to the static three-body problem. These exact solutions enable us to calculate two time-dependent observables, the Ramsey signal and particle separation, after the system undergoes a quench from noninteracting to the unitary regime or vice versa.

The rogue wave (RW) is a persistent topic in diverse fields of science including optics, hydromechanics, and electromagnetics. Despite the extensive experimental confirmations, the optical RW in the spectral domain is still elusive in aspects of its origination and evolutionary dynamics. Operating the fiber laser in a noiselike pulse (NLP) state, we observe the spectral RW according to the statistics of the shot-to-shot spectral evolution. Simulations consistent with experiments reveal that the spectrograms of NLPs are constituted by multiple localized speckles in the temporal-spectral domain, which originate from the interferences in the temporal and spectral domains. The collective motion of speckles is dominated by the Kerr effect and group-velocity dispersion. Several speckles can be in alignment at the same frequency, promoting the generation of the spectral RW. This paper sparks interest in the physics of incoherent wave packets widely existing in nonlinear optical systems.

A few-parameter expression for a light beam is found as a solution of the paraxial Helmholtz equation. It is achieved by exploiting appropriately chosen complex variables which entail the separability of the equation. Next, the expression for the beam is obtained independently by superimposing shifted Gaussian beams, whereby the shift can be made either by a real vector (in which case the foci of the Gaussian beams are located on a circle) or by a complex one. The solutions found depend on several parameters, the specific choice of which allows to obtain beams with quite different properties. For several selected parameter values figures are drawn, demonstrating the spatial distribution of the energy density and phase. In special cases, the effect of a shift of the intensity peak from one branch to another and phase singularities are observed.

The concept of chiral interactions has been intensively studied in various fields involving diverse scales of objects, e.g., nucleons, amino acids, and liquid crystals. One of the key problems is how chiral interactions induce synchronization effects among initially uncorrelated objects. In this study, we propose a model of chirality-selective synchronized fluorescence (superfluorescence) that involves spirally configured quantum emitters at the center of a spirally stacked metal nanostructure. The emitters mutually develop the correlation in time by repeatedly exchanging the radiated photons, which experience the chirality of their environment. We numerically show that the peak fluorescent intensity strongly depends on whether or not the chiralities of the emitters and the metals match, even though the localized surface plasmon decays sufficiently at the positions of the emitters. Our model of chirality-selective superfluorescence will provide an alternative tool for analyzing chiral interactions in synchronization processes.

We present theoretical and experimental results of coherent population trapping spectra on a multilevel $\mathrm{\Lambda }$-type configuration, adding a third beam to the standard two-laser $\mathrm{\Lambda }$ system to avoid undesired optical pumping. We show that the extra laser can preserve the nature of the dark resonances or introduce decoherence depending on its power. Experiments are carried out using a single trapped ${}^{40}\mathrm{Ca}^{+}$ ion in the $S_{1/2}\text{−}{P}_{1/2}\text{−}{D}_{3/2}$ manifold. Theoretically, the problem is solved with a Floquet-type expansion of the Liouvillian that correctly predicts all of the measured spectra without the need of full time integration. As a first application of the multilaser technique, we show that the richer spectra obtained can be used as a vectorial polarimeter of one of the beams, allowing one to measure the electrical field at the ion's position in any spatial direction. We also explain how our setup could realize a thermometer with tunable sensitivity and no laser-linewidth dependence.

Diagonalization of the effective Hamiltonian describing an open quantum system is the usual method of tracking its exceptional points (EPs). Although such a method is successful for tracking EPs in Markovian systems, it may be problematic in non-Markovian systems where a closed expression of the effective Hamiltonian describing the open system may not exist. In this work we provide an alternative method of tracking EPs in open quantum systems, using an experimentally measurable quantity, namely, the effective decay rate of a qubit. The quantum system under consideration consists of two nonidentical interacting qubits, one of which is coupled to an external environment. We develop a theoretical framework in terms of the time-dependent Schrödinger equations of motion, which provides analytical closed-form solutions of the Laplace transforms of the qubit amplitudes, enabling the study of various cases of environmental spectral densities. The link between the peaked structure of the effective decay rate of the qubit that interacts indirectly with the environment and the onset of the quantum Zeno effect is investigated, revealing the connections between the latter and the presence of exceptional points. Our treatment and results in addition reveal an intricate interplay between non-Markovian dynamics, the quantum Zeno effect, and non-Hermitian physics.

An approach to ab initio cavity quantum electrodynamics (QED) based on the reduced density matrix (RDM) is developed. The expectation value of the Pauli-Fierz Hamiltonian is expressed in terms of one- and two-body electronic and photonic RDMs, and the elements of these RDMs are optimized directly in polynomial time by semidefinite programming techniques, without knowledge of the full wave function. QED generalizations of important ensemble $N$-representability conditions are derived and enforced in this procedure. The resulting approach is applied to the description of classic ground-state strong-electron-correlation problems, augmented by the presence of ultrastrong light-matter coupling. First, we assess cavity-induced changes to the singlet-triplet energy gap of the linear oligoacene series; for a heptacene molecule, this gap can change by as much as 1.9 $\text{kcal}/\text{mol}$ (or 15%) when the molecule is aligned along the cavity mode polarization axis. We also explore the metal-insulator transition in linear hydrogen chains and demonstrate that strong electron-photon interactions increase the insulating character of these systems under all coupling strengths considered.

The standard quantum limit (SQL) of a classical mechanical force detection results from quantum back action perturbing evolution of a mechanical system. In this paper we show that usage of a multidimensional optical transducer may enable a broadband quantum back action evading measurement. We study theoretically a corresponding technique of measurement of a resonant signal force acting on a linear mechanical oscillator coupled to a generic optical system with three optical modes with separation nearly equal to the mechanical frequency. The measurement is performed by optical pumping of the central optical mode and analyzing the light escaping the two other modes. By detecting optimal quadrature components of the optical modes and postprocessing the measurement results we are able to exclude the back action in a broad frequency band and characterize the force with sensitivity better than the SQL. We show that the proposed scheme is similar to the multidimensional system containing quantum-mechanics-free subsystems which can evade the SQL using the idea of the so-called negative mass [M. Tsang and C. M. Caves, Phys. Rev. X 2, 031016 (2012).].

Recent works on quantum resource theories of non-Gaussianity, which are based upon the type of tools available in contemporary experimental settings, put Gaussian states and their convex combinations on equal footing. Motivated by this, in this paper, we derive a model of dissipative time evolution based on unitary Lindblad operators which, while it does not preserve the set of Gaussian states, preserves the set of their convex combinations, i.e., so-called quantum Gaussian states. As we demonstrate, the considered evolution proves useful both as a description for random scattering and as a tool in dissipator engineering.

The combined use of quantum channels can grant communication advantages in the form of enhancements to communication capacities. One such channel composition is the quantum switch, which implements a system with indefinite causal order by coherent control of the orderings of two or more quantum channels, resulting in enhanced communication capacities. Here, using the monogamy relation, we studied the flow of entanglement monotone in these quantum compositions of channels in the environmental representation. We implemented the two-party quantum switch in this framework, and demonstrated that non-Markovianity is the source of revival of the entanglement monotone in this setup. The possibility and amount of revival was shown to depend on the entangling capability of the channels, and the perfect activation of coherent information with entanglement-breaking channels was also replicated with the perfect revival of entanglement monotone. Additionally, we showed that a more general non-Markovian circuit can still grant enhancements to coherent information and Holevo quantity without the presence of indefinite causal order.

Quantum state transfer is one of the basic tasks in quantum information processing. We here propose a theoretical approach to realize arbitrary entangled state transfer through a qubit chain, which is a class of extended Su-Schrieffer-Heeger models and accommodates multiple topological edge states separated from the bulk states. We show that an arbitrary entangled state, from two qubits to $\mathcal{N}$ qubits, can be encoded in the corresponding edge states, and then adiabatically transferred from one end to the other of the chain. The dynamical phase differences resulting from the time evolutions of different edge states can be eliminated by properly choosing evolution time. Our approach is robust against both the qubit-qubit coupling disorder and the evolution time disorder. For concreteness of discussions, we assume that such a chain is constructed by an experimentally feasible superconducting qubit system, but our proposal can also be applied to other systems.

Network nonlocality exhibits completely novel quantum correlations compared to standard quantum nonlocality. It has been shown that network nonlocality can be shared in a generalized bilocal scenario via weak measurements [Hou, Liu, and Ren, Phys. Rev. A 105, 042436 (2022)]. In this paper, we investigate network nonlocality sharing via weak measurements in a generalized star-shaped network configuration with arbitrary numbers of unbiased dichotomic input $k$, which includes $n$ branches and adds $(m-1)$ more parties in each branch to the original star network $(n,m=1,k=2)$ scenario. It is shown that network nonlocality sharing among all observers can be revealed from simultaneous violation of $2^n$ inequalities in the $(n,m=2,k=2)$ and $(n,m=2,k=3)$ scenarios for any $n$ branches. The noise resistance of network nonlocality sharing with a precise noise model is also analyzed.

High-order harmonic generation (HHG) in atomic systems by the highly nonlinear response to an intense laser field is a prominent pathway to the synthesis of ever shorter laser pulses at increasingly higher photon energies. Extensions of this process to molecules add to the complexity but also offer novel opportunities as multicenter effects and the coupling to nuclear degrees of freedom can influence HHG. In this paper, we theoretically explore the impact of coherent vibrational excitations of diatomic molecules on the HHG spectrum within the framework of time-dependent density-functional theory. We observe the appearance of interference structures in the HHG spectra controlled by resonances between the driving field and the vibrational wave packet.