We present an integrated source of counterpropagating entangled states based on a coupled resonator optical waveguide that is pumped by a classical pulsed source incident from above the waveguide. We investigate theoretically the generation and propagation of continuous-variable entangled states in this coupled-cavity system in the presence of intrinsic loss. Using a tight-binding approximation, we derive analytic time-dependent expressions for the number of photons in each cavity, as well as for the correlation variance between the photons in different pairs of cavities, to evaluate the degree of quantum entanglement. We also derive simple approximate expressions for these quantities that can be used to guide the design of such systems, and discuss how pumping configurations and physical properties of the system affect the photon statistics and the degree of quantum correlation.

Anisotropic nanophotonic structures can couple the levels of a quantum emitter through the quantum interference effect. In this paper we study the coupling of quantum emitter excited states through the modes of a fully anisotropic structure, a structure for which all directions are physically nonequivalent. We consider an anisotropic metasurface as an illustrative example of such a structure. We point out another degree of freedom in controlling the temporal dynamics and spectral profiles of quantum emitters; namely, we show that a combination of the metasurface anisotropy and tilt of the emitter quantization axis with respect to the metasurface normal results in nonsymmetric dynamics between the transitions of electrons from the left-circular state to the right-circular state and in the inverse process. Our findings give an additional mechanism for control over light emission by quantum systems and can be utilized for probing active transitions of quantum emitters.

One of the fundamental features of the quantum world that distinguishes it from the classical counterpart is the existence of quantum correlations. In this paper, we study the quantum correlation from the viewpoint of partial coherence. Partial coherence in a bipartite system often contains local coherence of a subsystem. By removing the local coherence, we show that the remaining part of partial coherence can be regarded as quantum correlations. In this way, we introduce two quantum correlations: partial correlated coherence (PCC) and minimal partial correlated coherence (MPCC). Furthermore, we prove that PCC and MPCC induced by contractive pseudodistances are good candidates of quantum correlations. For contractive pseudodistances, we also show that the induced geometric partial correlated coherence (GPCC) and geometric minimal partial correlated coherence are actually the upper and lower bound of corresponding geometric quantum discord. Particularly, GPCCs based on a family of distances are proved to be bona fide measures of quantum correlations. By connecting to local quantum uncertainty, we provide an operational meaning for geometric quantum discord and GPCC induced by affinity of distance.

Quantum entanglement has been one of the hottest topics in current day physics, since it is the driving force behind quantum cryptography, quantum teleportation, and quantum computing. Several measures of quantification of entanglement have been proposed, each of which can often only be applied to a few specific systems. In this paper we derive a kinematic measure of entanglement that is capable of giving a full description of Einstein-Podolsky-Rosen entanglement in molecular systems. The associated “coupled entropy” energy contribution generates the correct amount of strong correlation energy for the ${\mathrm{H}}_2$ and ${\mathrm{N}}_2$ prototype systems, and is shown to be able to perform the same feat if it is explicitly used as the correlation component in the density-matrix functional context. And, finally, we propose a nonkinematic way to measure the entanglement of spins by using the conditional density of the system.

Anion and cation emission following water dissociation was studied for 6.6-keV ${}^{16}{\mathrm{O}}^{+}+{\mathrm{H}}_2\mathrm{O}$ collisions. Absolute cross sections for the emission of all positively and negatively charged fragments, differential in both energy and observation angle, were measured. The fragments formed in hard, binary collisions appearing in peaks were distinguishable from those created in soft collisions with many-body dynamics that result in a broad energy spectrum. A striking feature is that anions and cations are emitted with similar energy and angular distributions, with a nearly constant ratio of about 1:100 for ${\mathrm{H}}^{-}$ to ${\mathrm{H}}^{+}$. Model calculations were performed at different levels of complexity. Four-body scattering simulations reproduce the measured fragment distributions if adequate kinetic-energy release of the target is taken into account. Providing even further insight into the underlying processes, predictions of a thermodynamic model indicate that transfer ionization at small impact parameters is the dominant mechanism for ${\mathrm{H}}^{+}$ creation. The present findings confirm our earlier observation that in molecular fragmentation induced by slow, singly charged ions, the charge states of the emitted hydrogen fragments follow a simple statistical distribution independent of the way they are formed.

We investigate the Floquet dynamics of a cavity-spinor Bose-Einstein condensate coupling system via periodic modulation of the cavity pump laser. Parametric resonances are predicted, and we show that due to cavity feedback-induced nonlinearity, the spin oscillation can be amplified to all orders of resonance, thus facilitating its detection. Real-time observation of Floquet dynamics via cavity output is also discussed.

We study the dynamics of a two-dimensional Bose gas after an instantaneous quench of an initially ultracold thermal atomic gas across the Berezinskii-Kosterlitz-Thouless phase transition, confirming via stochastic simulations that the system undergoes phase-ordering kinetics and fulfills the dynamical scaling hypothesis at late-time dynamics. Specifically, we find in that regime the vortex number decaying in time as $〈N_{\mathrm{v}}〉\propto t^{-1}$, consistent with a dynamical critical exponent $z\approx 2$ for both temperature and interaction quenches. Focusing on finite-size boxlike geometries, we demonstrate that such an observation is within current experimental reach.

We report a combined theoretical and experimental investigation on the unambiguous role of electronic coherence underlying the generation of laserlike coherent emission from ${{\mathrm{N}}_2}^{+}$ ions launched in a strong laser field. The coherence manifests itself by giving rise to a dramatic enhancement of ${{\mathrm{N}}_2}^{+}$ laserlike radiation with two temporally separated pump laser pulses centered at 800- and 1580-nm wavelengths, respectively. We further show that coherent control of ${{\mathrm{N}}_2}^{+}$ lasing in a subfemtosecond timescale can be achieved by replacing the single 800-nm laser pulse with a pair of 800-nm laser pulses of adjustable time delay. The experimental results are well reproduced in theory using a three-level quantum model of ${{\mathrm{N}}_2}^{+}$ ions under the density matrix representation.

Multiple-copy state discrimination is a fundamental task in quantum information processing. If there are two, pure, nonorthogonal states then both local and collective schemes are known to reach the Helstrom bound, the maximum probability of successful discrimination allowed by quantum theory. For mixed states, it is known that only collective schemes can perform optimally, so it might be expected that these schemes are more resilient to preparation noise. We calculate the probability of success for two schemes, a local scheme based on Bayesian updating and quantum data gathering, the simplest possible collective measurement, and consider imperfect preparation fidelity. We find two surprising results. First, both schemes converge upon the same many-copy limit, which is less than unity. Second, the local scheme performs better in all cases. This highlights the point that one should take into account noise when designing state discrimination schemes.

We consider a quantum device $D$ interacting with a quantum many-body environment $R$ which features a second-order phase transition at $T=0$. Exploiting the description of the critical slowing down undergone by $R$ according to the Kibble-Zurek mechanism, we explore the possibility to freeze the environment in a configuration such that its impact on the device is significantly reduced. Within this framework, we focus on the magnetic-domain formation typical of the critical behavior in spin models and propose a strategy that allows one to protect the entanglement between different components of $D$ from the detrimental effects of the environment.

Phenomenological quantum Hamiltonians $H^{\left(N\right)}\left(\lambda \right)=J^{\left(N\right)}+\lambda V^{\left(N\right)}\left(\lambda \right)$ representing a general real $N^2$-parametric perturbation of an exceptional-point-related unperturbed Jordan-block Hamiltonian $J^{\left(N\right)}$ are considered. Tractable as non-Hermitian (in a preselected, unphysical Hilbert space) as well as, simultaneously, Hermitian (in another, “physical” Hilbert space), these matrices may represent a unitary, closed quantum system if and only if the spectrum is real. At small $\lambda$ we show that the parameters are then confined to a “stability corridor” $\mathcal{S}$ of the access to the extreme dynamical exceptional-point $\lambda \to 0$ regime. The corridors are narrow and $N$-dependent: they are formed by multiscale perturbations which are small in physical Hilbert space, i.e., which are such that $\lambda V_{j+k,j}^{\left(N\right)}\left(\lambda \right)=O\left({\lambda }^{(k+1)/2}\right)$ at $k=1,2,...,N-1$ and all $j$.

Calculations that achieve spectroscopic accuracy, i.e., at the level of ${10}^{-4}$ or better, are rare and typically restricted to few-electron systems. Here we present high-resolution grating spectrometer measurements of 11 $2s_{1/2}\text{−}2p_{3/2}$ transitions in neonlike ${\mathrm{Ge}}^{22+}$ that distinguish between two different implementations of the many-body perturbation theory (MBPT) method, including our own. The measurements show that our implementation of the MBPT method consistently achieves spectroscopic accuracy at the ${10}^{-4}$ level for all of the measured lines. Moreover, our calculations improve the ${10}^{-3}$ accuracy of prior MBPT calculations by an order of magnitude and allow us to make greatly improved predictions of the location of potential photopumped lines in ${\mathrm{Ge}}^{22+}$. The measurements also improve on previous measurements of $2s_{1/2}\text{−}2p_{3/2}$ transitions in fluorinelike ${\mathrm{Ge}}^{23+}$ and oxygenlike ${\mathrm{Ge}}^{24+}$. Measurements of $n=2\text{to}n=3$ transitions in ${\mathrm{Al}}^{7+}$, ${\mathrm{Al}}^{8+}$, and ${\mathrm{Al}}^{9+}$ are used to support our measurement accuracy at the ${10}^{-5}$ level.

We present an experimental and theoretical study of the structure and autoionization dynamics of the ${\left(3p_{j_{\text{c}}}n{d}_j\right)}_{J=1,3}$ core-excited Rydberg states of Mg. Spectra were recorded experimentally for principal quantum numbers $n$ in the range from 30 to 100 using the isolated-core-excitation technique. Large-scale configuration-interaction calculations combined with the exterior-complex-scaling method were also carried out, which do not rely on the assumptions used in the usual $R$-matrix multichannel-quantum-defect-theory treatment of core-excited Rydberg states. Agreement between theory and experiment is excellent over the entire range of principal quantum numbers studied and allows us to elucidate in detail the structure of the core-excitation spectra. The dominant autoionization mechanisms are identified, and in particular the very fast spin-orbit autoionization of some ${\left(3p_{3/2}n{d}_j\right)}_J$ states above the $3p_{1/2}$ ionization threshold. We discuss the influence of the principal and total-angular-momentum quantum numbers $n$ and $j$ of the Rydberg electron and the total-angular-momentum quantum number $J$ of the atom on the autoionization dynamics. We also identify previously unobserved resonances attributed to members of the ${\left(3p_{j_{\text{c}}}n{g}_j\right)}_J$ series.

We consider a degenerate Bose gas confined in a double-well potential in interaction with a trapped ion in one dimension and investigate the impact of two relevant sources of imperfections in experiments on the system dynamics: ion motion and thermal excitations of the bosonic ensemble. Particularly, their influence on the entanglement generation between the spin state of the moving ion and the atomic ensemble is analyzed. We find that the detrimental effects of the ion motion on the entanglement protocol can be mitigated by properly choosing the double-well parameters as well as timings of the protocol. Furthermore, thermal excitations of the bosons affect significantly the system's tunneling and self-trapping dynamics at moderate temperatures; i.e., thermal occupation of a few double-well quanta reduces the protocol performance by about 10%. Hence, we conclude that finite temperature is the main source of decoherence in such junctions and we demonstrate the possibility to entangle the condensate motion with the ion vibrational state.

Subradiance is the cooperative inhibition of the radiation by several emitters coupled to the same electromagnetic modes. It was predicted by Dicke in 1954 and only recently observed in cold atomic vapors. Here we address the question to what extent this cooperative effect survives outside the limit of frozen two-level systems by studying the subradiant decay in an ensemble of cold atoms as a function of the temperature. Experimentally, we observe only a slight decrease of the subradiant decay time when increasing the temperature up to several millikelvins, and in particular we measure subradiant decay rates that are much smaller than the Doppler broadening. This demonstrates that subradiance is surprisingly robust against thermal decoherence. The numerical simulations are in good agreement and allow us to extrapolate the behavior of subradiance at higher temperatures.

The generation of ultrafast laser pulses and the reconstruction of their electric fields is essential for many applications in modern optics. Quantum optical fields can also be generated on ultrafast timescales; however, the tools and methods available for strong laser pulses are not appropriate for measuring the properties of weak, possibly entangled pulses. Here, we demonstrate a method to reconstruct the joint-spectral amplitude of a two-photon energy-time entangled state from joint measurements of the frequencies and arrival times of the photons, and the correlations between them. Our reconstruction method is based on a modified Gerchberg-Saxton algorithm. Such techniques are essential to measure and control the shape of ultrafast entangled photon pulses.

In this paper we concentrate on the remarkable role of a gradient waveguide in two-color light bullet formation. We study generation of the second harmonic in such an inhomogeneous nonlinear medium, taking into account diffraction and relatively weak temporal dispersion. Using the averaged Lagrangian method we consider all possible combinations of the range of group velocities (normal or anomalous dispersion) and waveguide geometry (focusing or defocusing waveguide). Stability conditions for a propagating two-color light bullet are derived analytically. We demonstrate the formation of a stable two-component light bullet in a parabolic planar quadratically nonlinear waveguide either at anomalous or at normal group velocity dispersion. We discuss also the results of numerical simulation confirming our analytical findings. Besides that, simulation allows us to expand the scope of the study and to show light bullet propagation at a certain phase mismatching and the formation of a stable coupled localized structure from a signal at the fundamental frequency as well.

By using an exact analytical non-Hermitian approach in terms of resonance states, we show that the exponential-nonexponential transition of decay at long times physically represents a process of interference in the time domain of the decaying particle with itself. We also show that actually the regeneration mechanism proposed by Fonda and Ghirardi [L. Fonda and G. C. Ghirardi, Nuovo Cimento A 7, 180 (1972)] is not the relevant mechanism for nonexponential decay.

We introduce a control method for off-resonant robust quantum information processing suited for quantum integrated photonics. We utilize detunings as control parameters to derive a family of composite pulses for high-fidelity complete population transfer. The presented detuning-modulated composite sequences can be implemented within the decay lifetime of the qubit and correct for control inaccuracies in various parameters including pulse strength, duration, detuning, phase jitter, Stark shift, and unwanted frequency chirps. We implement the proposed robust sequences in an integrated photonics platform to achieve complete light transfer insensitive to fabrication errors.