The present work is motivated by the question what aspect of correlation entailed by the two-qubit state serves as the appropriate quantitative resource for steering. To this end, considering Bell-diagonal states, suitable measures of simultaneous correlations in two and three complementary (mutually unbiased) bases are identified as the relevant resources for quantum steering. Quantitative relations between appropriate measures of quantum steering and corresponding measures of simultaneous correlations in complementary bases are demonstrated which ensure that for two-qubit steerable Bell-diagonal states, a higher value of simultaneous correlations in mutually unbiased bases necessarily implies a higher degree of quantum steering, both for two- and three-setting steering scenarios.

The Auger decay of the spin-orbit and molecular-field split Br $3d^{-1}$ core holes in HBr is investigated, both by a photoelectron–Auger-electron coincidence experiment and by ab initio calculations based on the one-center approximation. The branching ratios for the Auger decay of the five different core-hole states to the $4p{(\sigma ,\pi )}^{-2}$ dicationic final states are determined. Experimental and theoretical data are in good agreement and conform to results for the $4p{\pi }^{-2}$ final states from a previous analysis of the high-resolution conventional Auger-electron spectrum. The branching ratios for the Br $3d^{-1}$ Auger decay to the $4p{(\sigma ,\pi )}^{-2}$ with Σ symmetry follow the propensity rule of $L_{2,3}VV$ Auger decay [S. Svensson, A. Ausmees, S. J. Osborne, G. Bray, F. Gel'mukhanov, H. Ågren, A. Naves de Brito, O.-P. Sairanen, A. Kivimäki, E. Nõmmiste, H. Aksela, and S. Aksela, Phys. Rev. Lett. 72, 3021 (1994)] stating that the oriented core holes decay preferentially by involving a valence electron from an orbital with the same spatial orientation. For the $M_{4,5}VV$ decay in HBr this propensity rule has to be supplemented by the requirement that the Auger-electron channel and the other valence orbital have the same preferential orientation. We also probe the influence of the Auger kinetic energy on the distortion of the photoline caused by the postcollision interaction effect. For small kinetic energies, differences between experimental results and theoretical predictions are identified.

We investigate the out-of-equilibrium dynamics following a sudden quench of the interaction strength in a one-dimensional quasicondensate trapped at the surface of an atom chip. Within a linearized approximation, the system is described by independent collective modes and the quench squeezes the phase-space distribution of each mode, leading to a subsequent breathing of each quadrature. We show that the collective modes are resolved by the power spectrum of density ripples which appear after a short time of flight. This allows us to experimentally probe the expected breathing phenomenon. Our results are in good agreement with theoretical predictions which take the longitudinal harmonic confinement into account.

We study the dynamics of dilute and ultracold bosonic gases in a quasi-two-dimensional (quasi-2D) configuration and in the collisionless regime. We adopt the 2D Landau-Vlasov equation to describe a three-dimensional gas under very strong harmonic confinement along one direction. We use this effective equation to investigate the speed of sound in quasi-2D bosonic gases, i.e., the sound propagation around a Bose-Einstein distribution in collisionless 2D gases. We derive coupled algebraic equations for the real and imaginary parts of the sound velocity, which are then solved also taking into account the equation of state of the 2D bosonic system. Above the Berezinskii-Kosterlitz-Thouless critical temperature we find that there is rapid growth of the imaginary component of the sound velocity, which implies a strong Landau damping. Quite remarkably, our theoretical results are in good agreement with very recent experimental data obtained with a uniform 2D Bose gas of ${}^{87}\mathrm{Rb}$ atoms.

Flexible Rydberg aggregates, assemblies of a few Rydberg atoms coherently sharing electronic excitations while undergoing directed atomic motion, show great promise as a quantum simulation platform for nuclear motion in molecules or quantum energy transport. Here we study additional features that are enabled by the presence of more than a single electronic excitation, thus considering multiexciton states. We describe cases where these can be decomposed into underlying single-exciton states and then present dynamical scenarios with atomic motion that illustrate exciton-exciton collisions, exciton routing, and strong nonadiabatic effects in simple one-dimensional settings.

We introduce a theory to analyze the behavior of light emitters in nanostructured environments rigorously. Based on spectral theory, the approach opens the possibility to quantify precisely how an emitter decays to resonant states of the structure and how it couples to a background, also in the presence of general dispersive media. Quantification on this level is essential for designing and analyzing topical nanophotonic setups, e.g., in quantum technology applications. We use a numerical implementation of the theory for computing modal and background decay rates of a single-photon emitter in a diamond nanoresonator.

It has recently been shown that vertical-cavity surface-emitting lasers with an intracavity saturable absorber are capable of forming binary localized structures called twin laser cavity solitons (LCSs). Apart from their asymmetric intensity distribution, they can spontaneously rotate about their center of mass with a frequency that is controllable via a bifurcation parameter representing the ratio of carrier lifetimes in the amplifier and in the absorber. Moreover, circles of maximum phase are found to form around the center of the binary structure which start rotation once the twin LCSs begin to do so. In this paper, we propose to use the fact that these phase circles continually shrink on the center of mass to trap microparticles at the center and subsequently rotate them through the dipole force provided by the asymmetric intensity distribution. This finding can be regarded as a first step in the realization of a localized microdimensional optical motor potentially useful in the manipulation of microparticles and nanofluids.

Photoelectron angular distributions (PADs) resulting from 800 and 1300 nm strong-field ionization of impulsively aligned ${\mathrm{CF}}_3\mathrm{I}$ molecules were analyzed using time-dependent density functional theory (TDDFT). The normalized difference between the PADs for aligned and antialigned molecules displays large modulations in the high-energy recollision plateau that are assigned to the diffraction of backscattered photoelectrons. The TDDFT calculations reveal that, in spite of their 2.6 eV energy difference, ionization from the HOMO-1 orbital contributes to the diffraction pattern on the same footing as ionization from the doubly degenerate HOMO orbital.

We investigate heat engines whose working substance is made of two coupled qubits performing a generalized Otto cycle by varying their applied magnetic field or their interaction strength during the compression and expansion strokes. During the heating and cooling strokes, the two qubits are coupled to local and common environments that are not necessarily at equilibrium. We find instances of quantum engines coupled to nonequilibrium common environments exhibiting nontrivial connections to quantum correlations as witnessed by a monotonic dependence of the work produced on quantum discord and entanglement.

The temporal evolution of entanglement between a noisy system and an ancillary system is analyzed in the context of continuous-time open quantum system dynamics. Focusing on a couple of analytically solvable models for qubit systems, we study how Markovian and non-Markovian characteristics influence the problem, discussing in particular their associated entanglement-breaking regimes. These performances are compared with those one could achieve when the environment of the system is forced to return to its input configuration via periodic instantaneous resetting procedures.

We propose and demonstrate a quantum control scheme for hybrid quantum registers that can reduce the operation time, and therefore the effects of relaxation, compared to existing implementations. It combines resonant excitation pulses with periods of free precession under the internal Hamiltonian of the qubit system. We use this scheme to implement quantum gates like controlled-NOT operations on electronic and nuclear spins of the nitrogen-vacancy center in diamond. As a specific application, we transfer population between electronic and nuclear spin qubits and use it to measure the Rabi oscillations of a nuclear spin in a system with multiple coupled spins.

We make precise calculation of hyperfine structure of $S$ states in muonic ions of lithium, beryllium, and boron in quantum electrodynamics. Corrections of orders ${\alpha }^5$ and ${\alpha }^6$ due to the vacuum polarization, nuclear structure, and recoil in first and second orders of perturbation theory are taken into account. We obtain estimates of the total values of hyperfine splittings, which can be used for a comparison with future experimental data.

The desorption of excited rubidium atoms off the surface of helium nanodroplets is studied in detail using femtosecond time-resolved photoion and photoelectron imaging spectroscopy in combination with quantum wave-packet simulations. The good agreement of the measured time-dependent velocity distributions with the simulation when exciting the Rb dopant atoms into the $6p$ state supports the pseudodiatomic model for the Rb-He droplet interaction, even on the level of quantum wave-packet dynamics. Time-resolved photoelectron spectra reveal the partitioning of excitation energy into the dopant and the droplet degrees of freedom.

We report on a kinematically complete experiment on strong-field double ionization of helium using laser pulses with a wavelength of 394 nm and intensities of 3.5–$5.7\times {10}^{14}$ W/${\mathrm{cm}}^2$. Our experiment reaches the most complete level of detail, which previously has been reached only for single-photon double ionization. We give an overview of the observables on many levels of integration, from the ratio of double to single ionization, the individual electron, and ion momentum distributions over the joint momentum and energy distributions to fully differential cross sections showing the correlated angular momentum distributions. Within the studied intensity range the ratio of double to single ionization changes from $2\times {10}^{-4}$ to $1.5\times {10}^{-3}$. We find the momentum distributions of the ${\mathrm{He}}^{2+}$ ions and the correlated two electron momentum distributions to vary substantially. Only at the highest intensity are both electrons emitted in the same direction, while at the lowest intensity back-to-back emission dominates. The joint energy distribution of the electrons shows discrete structures from the energy quantization of the photon field, which allows us to count the number of absorbed photons and thus access the parity of the final state. We find the energy of the individual electron to show a peak structure indicating a quantized sharing of the overall energy absorbed from the field. The joint angular momentum distributions of the two electrons show the highly directed emission of both electrons along the polarization axis as well as clear imprints of electron repulsion. They strongly change with the energy sharing between electrons. The aspect of selection rules in double ionization which are also visible in the presented data set is the subject of a preceding publication [Henrichs et al., Phys. Rev. A 97, 031405(R) (2018)].

We perform a simulation of the experiment [J. C. Jaskula et al., Phys. Rev. Lett. 109, 220401 (2012)] where the temporal modification of the effective one-dimensional interaction constant was used to create pairs of atoms with opposite velocities. The simulations clearly demonstrate the huge impact of the interaction between quasiparticles due to finite temperature on the pair production process, explaining the relatively small atomic pair production and the absence of number squeezing in the experiment.

Higher-order transitions can occur in the ultrastrong-coupling regime of circuit QED through virtual processes governed by the counter-rotating interactions. We propose a feasible way to probe higher-order transitions through the scattering of propagating microwave photons incident on the hybrid qubit-cavity system. The line shapes in the scattering spectra can indicate the coherent interaction between the qubits and the cavity, and the higher-order transitions can be identified in the population spectra. We further find that if the coupling strengths between the two qubits and the cavity are tuned to be asymmetric, the dark antisymmetric state with the Fano line shape can also be detected from the variations in the scattering spectra.

In the field of cavity optomechanics, proposals for quantum nondemolition (QND) measurements of phonon number provide a promising avenue by which one can study the quantum nature of nanoscale mechanical resonators. Here we investigate these QND measurements for an optomechanical system whereby quadratic coupling arises due to shared symmetries between a single optical resonance and a mechanical mode. We establish a relaxed limit on the amount of linear coupling that can exist in this type of system while still allowing for QND measurements of mechanical Fock states. The ability to perform optomechanical QND measurements of this nature would allow one to probe the decoherence of these mesoscopic states, providing an experimental test bed for quantum collapse theories.

Spectral characterization is a fundamental step in the development of useful quantum technology platforms. Here, we study an ensemble of interacting qubits coupled to a single quantized field mode, an extended Dicke model that might be at the heart of Bose-Einstein condensate in a cavity or circuit-QED experiments for large and small ensemble sizes, respectively. We present a semiclassical and quantum analysis of the model. In the semiclassical regime, we show analytic results that reveal the existence of a third regime in addition of the two characteristics of the standard Dicke model characterized by one logarithmic and two jump discontinuities in the derivative of the density of states. We show that the finite quantum system shows two different types of clustering at the jump discontinuities, signaling a precursor of two excited quantum phase transitions. These are confirmed using Peres lattices where unexpected order arises around the new precursor. Interestingly, Peres conjecture regarding the relation between the spectral characteristics of the quantum model and the onset of chaos in its semiclassical equivalent is valid in this model as a revival of order in the semiclassical dynamics occurs around the new phase transition.

For complex $\mathit{PT}$-symmetric scattering potentials (CPTSSPs) $V\left(x\right)=V_1{f}_{\mathrm{even}}\left(x\right)+i{V}_2{f}_{\mathrm{odd}}\left(x\right),f_{\mathrm{even}}(\pm \infty )=0=f_{\mathrm{odd}}(\pm \infty ),V_1,V_2\in \mathrm{Re}$, we show that complex $k$ poles of transmission amplitude $t\left(k\right)$ or zeros of $1/t\left(k\right)$ of the type $\pm k_1+i{k}_2,k_2\ge 0$ are physical which yield three types of discrete energy eigenvalues of the potential. These discrete energies are real negative, complex-conjugate pair(s) of eigenvalues (CCPEs: ${\mathcal{E}}_n\pm i{\gamma }_n$) and real positive energy called spectral singularity (SS) at $E=E_{*}$ where the transmission and reflection coefficient of $V\left(x\right)$ become infinite for a special critical value of $V_2=V_{*}$. Based on four analytically solvable and other numerically solved models, we conjecture that a parametrically fixed CPTSSP has at most one SS. When $V_1$ is fixed and $V_2$ is varied there may exist Kato's exceptional point(s) $\left(V_{\mathrm{EP}}\right)$ and critical values $V_{*m},m=0,1,2,\cdots$, so when $V_2$ crosses one of these special values a new CCPE is created. When $V_2$ equals a critical value $V_{*m}$ there exists one SS at $E=E_{*}$ along with $m$ or more number of CCPEs. Hence, this single positive energy $E_{*}$ is the upper (or rough upper) bound to the CCPEs: ${\mathcal{E}}_l⪅E_{*}$, here ${\mathcal{E}}_l$ corresponds to the last of CCPEs. If $V\left(x\right)$ has Kato's exceptional points (EPs: $V_{\mathrm{EP}1}<V_{\mathrm{EP}2}<V_{\mathrm{EP}3}<\cdots <V_{\mathrm{EPl}}$), the smallest of critical values $V_{*m}$ is always larger than $V_{\mathrm{EPl}}$. Hence, in a CPTSSP, real discrete eigenvalue(s) and the SS are mutually exclusive whereas CCPEs and the SS can coexist.