We show that for pure and mixed states the problem of maximizing the correlation measure in the Clauser-Horne-Shimony-Holt inequality reduces to maximizing the perimeter of a parallelogram enclosed by an ellipse characterized by the entanglement contained in the bipartite system. Our geometrical description is valid for a nonmaximally entangled state. We also determine the corresponding optimal measurements.

We use a covariant formalism that is capable of describing the electric and magnetic versions of the Aharonov-Bohm effect, as well as the Aharonov-Casher effect, through local interactions of charges and currents with the quantum electromagnetic field. By considering that only local interactions of a quantum particle with the quantum field can affect its behavior, we show that the magnetic Aharonov-Bohm effect must be present even if the solenoid generating the magnetic field is shielded by a perfect conductor, as experimentally demonstrated.

We investigate lasing of a nitrogen gas induced by intense femtosecond laser pulses at around 400 nm. By examining both the self-induced and externally seeded forward emission spectra, we unambiguously identify the ${{\mathrm{N}}_2}^{+}$ lasing actions at 427.8 and 423.6 nm assigned respectively to the $B{}^2\mathrm{\Sigma }{{}_u}^{+}-X{}^2\mathrm{\Sigma }_g^{+}$ (0, 1) and (1, 2) emissions and show that the lasing mechanism is totally different from the lasing induced by near-infrared (800 nm) laser pulses, in which the population transfer from $X{}^2\mathrm{\Sigma }_g^{+}$ to $A{}^2\mathrm{\Pi }_u$ proceeds through the resonant $X{}^2\mathrm{\Sigma }_g^{+}-A{}^2\mathrm{\Pi }_u$ transition, facilitating the population inversion between the $B{}^2\mathrm{\Sigma }{{}_u}^{+}-X{}^2\mathrm{\Sigma }_g^{+}$ states [Phys. Rev. Lett. 123, 203201 (2019)]. We simulate the population distributions among the vibrational levels in the three lowest electronic states of ${{\mathrm{N}}_2}^{+}$ by the 400-nm laser pumping and find that the population is efficiently transferred between the $X{}^2\mathrm{\Sigma }_g^{+}$ state and the $B{}^2\mathrm{\Sigma }{{}_u}^{+}$ state by a Rabi oscillation combined with a Raman-type transition, leading to the population inversion so that the lasing occurs at the $B{}^2\mathrm{\Sigma }{{}_u}^{+}-X{}^2\mathrm{\Sigma }_g^{+}$ (0, 1) and (1, 2) emissions.

We demonstrate the use of multiple atomic-level Rydberg-atom schemes for continuous frequency detection of radio-frequency (RF) fields. Resonant detection of RF fields by electromagnetically induced transparency and Autler-Townes (AT) splitting in Rydberg atoms is typically limited to frequencies within the narrow bandwidth of a Rydberg transition. By applying a second field resonant with an adjacent Rydberg transition, far-detuned fields can be detected through a two-photon resonance AT splitting. This two-photon AT splitting method is several orders of magnitude more sensitive than off-resonant detection using the Stark shift. We present the results of various experimental configurations and a theoretical analysis to illustrate the effectiveness of this multiple level scheme. These results show that this approach allows for the detection of frequencies in a continuous band between resonances with adjacent Rydberg states.

The influence of hypothetical new interactions beyond the standard model on atomic spectra has attracted recent interest. In the present work, interelectronic photon-exchange corrections and radiative quantum electrodynamic corrections to the hypothetical contribution to the energy levels of few-electron ions from a new interaction are calculated. The ground states of H-like, Li-like, and B-like ions are considered, as motivated by proposals to use isotope shift spectroscopy of few-electron ions in order to set stringent constraints on hypothetical new interactions. It is shown that, for light Li-like and B-like ions, photon-exchange corrections are comparable to or even larger than, by up to several orders of magnitude, the leading one-electron contribution from the new interaction, when the latter is mediated by heavy bosons.

This paper reports experimental investigations of the spectral response of synchronous coherent population trapping (CPT) for magnetometry applications. Synchronous CPT is demonstrated using resonant two-photon excitation with a modulated bichromatic laser field in the ${}^{87}\mathrm{Rb} D_1$ manifold. Spectra containing multiple magnetic resonances are produced due to two-photon excitations created by various frequency components of the modulated beam. Theoretical modeling based on the atomic density-matrix equations is used to simulate the spectral response of synchronous CPT. Our results show potential advantages of synchronous CPT over synchronous optical pumping for magnetic field measurement. Magnetic resonance signals produced by synchronous CPT can be used in eliminating the dead-zone problem and also in determining the magnetic field direction.

A quantum liquid in a heterogeneous mixture of ${}^{41}\mathrm{K}$ and ${}^{87}\mathrm{Rb}$ atoms is studied using the diffusion Monte Carlo method and density-functional theory. The perturbative Lee-Huang-Yang term for a heterogeneous mixture is verified and it is proved to be valid only near the gas-liquid transition. Based on the equations of state of the bulk mixture, calculated with diffusion Monte Carlo, extensions to Lee-Huang-Yang corrected mean-field energy functionals are presented. Using density-functional theory, a systematic comparison between different functionals is performed, focusing on the critical atom number, surface tension, surface width, Tolman length, and compressibility. These results are given as a function of the interspecies interaction strength, within the stability domain of the liquid mixture.

We investigate theoretically nonreciprocal chaos in a spinning optomechanical resonator. We find that under the Sagnac effect induced by spinning a resonator, the chaotic motion can exhibit unidirectional behavior. More specifically, the chaotic motion takes place when the spinning resonator is driven in a chosen direction, whereas it cannot emerge when the driving laser is applied in the opposite direction. Moreover, due to the Sagnac effect, we show that nonreciprocal chaos can resist the backscattering losses. The underlying reason for these phenomena is that the Sagnac effect induces an opposite frequency shift between clockwise and counterclockwise optical modes. Our work opens up a new route to achieve nonreciprocal chaos with the current experimental technology and may provide theoretical guidance for the production of a directional chaos signal generator.

We present a theoretical investigation on the quantum memory of photonic polarization qubits via electromagnetically induced transparency (EIT). The system we consider is a tripod-shaped four-level atomic system working under the condition of a double EIT, by which the storage and retrieval of a single-photon polarization qubit are implemented through the switching off and on of a control laser field, and the storage efficiency and the quantum-state fidelity for qubit memory are both calculated. We show that the optimal optical depth for acquiring the maximum efficiency and maximum fidelity of the qubit memory can be obtained simultaneously, which can be further improved by suppressing the optical absorption and dispersion via the choice of the time duration of the input qubit pulse and the amplitude of the control field. We also carry out a calculation on the quantum memory of a single-photon qudit by considering a multipod-shaped atomic-level configuration. The results reported here are useful for understanding the quantum transmission property of slow lights with multiple components and helpful for experimental realizations of high-quality memory of photonic qubits and qudits.

We present a scheme of quantum state truncation in the Fock basis (quantum scissors), based on the combined action of a nondegenerate optical parametric amplifier and a beam splitter. Differently from previously proposed linear-optics-based quantum scissors devices, which depend on reliable Fock state sources, our scheme requires only readily available Gaussian states, such as coherent state inputs (vacuum state included). A truncated state is generated after performing photodetections in the global state. We find that, depending on which output ports each of the two photodetectors is positioned, different types of truncated states may be produced: (i) states having a maximum Fock number of $N$, or (ii) states having a minimum Fock number $N$. In order to illustrate our method, we discuss an example having as input states a coherent state in the beam splitter and vacuum states in the amplifier, and show that the resulting truncated states display nonclassical properties, such as sub-Poissonian statistics and squeezing. We quantify the nonclassicality degree of the generated states using the Wigner-Yanase skew information measure. For complementarity, we discuss the efficiency of the protocol, e.g., generation probability as well as the effects of imperfections such as the detector's quantum efficiency and dark counts rate.

A battery is an energy storage device having utilities in a broad spectrum of fields. We consider batteries where quantum effects are inevitable, and compare the amount of energy extractable using local and global operations. We introduce the concept of local passivity for shared quantum batteries with local Hamiltonians, and characterize the set of all locally passive battery states. We find that the relation between energy extractions by local and global operations is closely connected with the entanglement content, for generic battery states.

The standard collisional model paradigm consists of a system that interacts sequentially with identically prepared ancillas. After infinitely many collisions, and under appropriate conditions, the system may converge to the same state as the ancillas. This process, known as homogenization, is independent of the ancilla initial state, being a property only of the underlying dynamics. In this paper we extend this idea to locally identical, but globally correlated, ancillas, and show that correlations break homogenization. This is done numerically using a minimal qubit model, and analytically using an exactly soluble Gaussian model. In both cases, we use Hamiltonian graph states with cyclic graphs as the prototypical method for building scalable many-body entangled ancillary states.

We propose a modified quantum teleportation scheme to increase the teleportation accuracy by applying a cubic phase gate to the displaced squeezed state. We describe the proposed scheme in the Heisenberg picture, evaluating it from the point of view of adding an error in teleportation, and show that it allows achieving less error than the original scheme. Repeating the description in the Schrödinger picture, we find the range of the displacement values at which our conclusions are valid. Using the example of teleportation of the vacuum state and the squeezed states, we show that the scheme allows one to achieve high-fidelity values.

Relativistic corrections of order ${\alpha }^2$ to the $g$ factor of ${\mathrm{H}}_2^{+}$ are calculated with a high accuracy of nine significant digits for a wide range of rovibrational states. The precision of previous calculations [R. A. Hegstrom, Phys. Rev. A 19, 17 (1979) is improved by about five orders of magnitude by performing nonadiabatic variational calculations and by including recoil corrections. These results allow for nondestructive identification of the internal state through the measurement of spin-flip transition frequencies, which is a crucial requirement for proposed spectroscopy experiments on ${\mathrm{H}}_2^{+}$ and its antimatter counterpart ${\overline{\mathrm{H}}}_2{}^{-}$ in Penning traps [E. G. Myers, Phys. Rev. A 98, 010101(R) (2018)]. Further, they pave the way towards precision calculations of the $g$ factor through the calculation of higher-order QED corrections and hence to an alternative precision route to obtaining the proton-electron mass ratio.

Stability and the dynamical behavior of binary Bose-Einstein condensed mixtures trapped on the surface of a rigid spherical shell are investigated at the mean-field level, exploring the miscibility with and without vortex charges and considering repulsive and attractive interactions. To compute the critical points for stability, we follow the Bogoliubov–de Gennes method for the analysis of perturbed solutions, with the constraint that initially the stationary states are in a complete miscible configuration. For the perturbed equal-density mixture of a homogeneous uniform gas and when hidden vorticity is verified, with the species having opposite azimuthal circulation, we consider a small perturbation analysis for each unstable mode, providing a complete diagram with the intra- and interspecies interaction role on the stability of the miscible system. Finally, beyond small-perturbation analysis, we explore the dynamics of some repulsive and attractive interspecies states by full numerical solutions of the time-dependent Gross–Pitaevskii equation.

Quantum optical cluster states have been increasingly explored in the light of their importance for measurement-based quantum computing. Here we set forth a method for generating quantum controlled cluster states: pumping an optical parametric oscillator with spatially structured light. We show that state-of-the-art techniques for producing clusters in the spectral and temporal domains are improved by a structured light pump, which manipulates the spatial mode couplings in the parametric interaction. We illustrate the method considering a second-order pump structure and show that a simple mode rotation yields different cluster states, including but not limited to the two-dimensional square and hexagonal lattices. We also introduce a novel technique for generating nonuniform cluster states and propose a simple setup for its implementation.

In spinor Bose-Einstein condensates, spin-changing collisions are a remarkable proxy to coherently realize macroscopic many-body quantum states. These processes have been, e.g., exploited to generate entanglement, to study dynamical quantum phase transitions, and proposed for realizing nematic phases in atomic condensates. In the same systems dressed by Raman beams, the coupling between spin and momentum induces a spin dependence in the scattering processes taking place in the gas. Here we show that, at weak couplings, such modulation of the collisions leads to an effective Hamiltonian which is equivalent to the one of an artificial spinor gas with spin-changing collisions that are tunable with the Raman intensity. By exploiting this dressed-basis description, we propose a robust protocol to coherently drive the spin-orbit-coupled condensate into the ferromagnetic stripe phase via crossing a quantum phase transition of the effective low-energy model in an excited state.

Using the approach offered by quantum speed limit, we show that geometric measure of multipartite entanglement for pure states [T.-C. Wei and P. M. Goldbart, Phys. Rev. A 68, 042307 (2003)] can be interpreted as the minimal time necessary to unitarily evolve a given quantum state to a separable one.

We propose and analyze a method for efficient dissipative preparation of matrix product states that exploits their symmetry properties. Specifically, we construct an explicit protocol that makes use of driven-dissipative dynamics to prepare a many-body quantum state that features symmetry-protected topological order and nontrivial edge excitations. The preparation protocol is protected from errors that respect the symmetry, allowing for robust experimental implementation without fine-tuned control. Numerical simulations show that the preparation time scales polynomially in system size $n$. Furthermore, we demonstrate that this scaling can be improved to $O\left({log}^{2}n\right)$ by using parallel preparation of individual segments and fusing them via quantum feedback. A concrete scheme using excitation of trapped neutral atoms into the Rydberg state via electromagnetically induced transparency is proposed and generalizations to a broader class of matrix product states are discussed.

The distribution of the quantum states to remote receivers is an indispensable technology in quantum communication, quantum networks, and quantum information processing. Perfect distribution does not allow any quantum information loss of the distributed quantum state. Quantum telecloning, which is made up of quantum cloning and quantum teleportation, is a remote quantum information distribution protocol. Here, we experimentally demonstrate unconditional $1\to 3$ quantum telecloning of coherent states, and the fidelities of the cloned states are $0.64\pm 0.01, 0.64\pm 0.01$, and $0.49\pm 0.01$, respectively. No information about the unknown state is lost during the telecloning process by exploiting the quadripartite entangled modes as an auxiliary resource. Deterministic $1\to 3$ quantum telecloning has great potential for applications in advanced quantum technology.