We propose to use the steered quantum coherence (SQC) as a signature of quantum phase transitions (QPTs). By considering various spin chain models, including the transverse-field Ising model, $\mathit{XY}$ model, and $\mathit{XX}$ model with three-spin interaction, we showed that the SQC and its first-order derivative succeed in signaling different critical points of QPTs. In particular, the SQC method is effective for any spin pair chosen from the chain, and the strength of SQC, in contrast to entanglement and quantum discord, is insensitive to the distance (provided it is not very short) of the tested spins, which makes it convenient for practical use as there is no need for careful choice of two spins in the chain.

We experimentally demonstrate a quantum receiver based on the Kennedy scheme for discrimination between two phase-modulated weak coherent states. The receiver is assembled entirely from standard fiber-optic elements and operates at a conventional telecom wavelength of 1.55 $\mu \text{m}$. The local oscillator and the signal are transmitted through different optical fibers, and the displaced signal is measured with a high-efficiency superconducting nanowire single-photon detector. We show the discrimination error rate is two times below that of a shot-noise-limited receiver with the same system detection efficiency.

Knowledge of the repulsive behavior of potential energy curves $V\left(R\right)$ at $R\to 0$ is necessary for understanding and modeling irradiation processes of practical interest. $V\left(R\right)$ is in principle straightforward to obtain from electronic structure calculations; however, commonly used numerical approaches for electronic structure calculations break down in the strongly repulsive region due to the closeness of the nuclei. In this work, we show by comparison to fully numerical reference values that a recently developed procedure [S. Lehtola, J. Chem. Phys. 151, 241102 (2019)] can be employed to enable accurate linear combination of atomic orbitals calculations of $V\left(R\right)$ even at small $R$ by a study of the seven nuclear reactions ${\mathrm{He}}_2\rightleftharpoons \mathrm{Be}, \mathrm{HeNe}\rightleftharpoons \mathrm{Mg}, {\mathrm{Ne}}_2\rightleftharpoons \mathrm{Ca}, \mathrm{HeAr}\rightleftharpoons \mathrm{Ca}, \mathrm{MgAr}\rightleftharpoons \mathrm{Zn}, {\mathrm{Ar}}_2\rightleftharpoons \mathrm{Kr}$, and $\mathrm{NeCa}\rightleftharpoons \mathrm{Zn}$.

We demonstrate a significant enhancement of frustrated double ionization (FDI) in the two-electron triatomic molecule ${\mathrm{D}}_3{}^{+}$ when driven by counter-rotating two-color circular (CRTC) laser fields. We employ a three-dimensional semiclassical model that fully accounts for electron and nuclear motion in strong fields. For different pairs of wavelengths, we compute the probabilities of the FDI pathways as a function of the ratio of the two field strengths. We identify a pathway of frustrated double ionization that is not present in strongly driven molecules with linear fields. In this pathway the first ionization step is “frustrated” and electronic correlation is essentially absent. This pathway is responsible for enhancing frustrated double ionization with CRTC fields. We also employ a simple model that predicts many of the main features of the probabilities of the FDI pathways as a function of the ratio of the two field strengths.

We show that atomic motion leads not only to noticeable quantitative, but in some cases also to qualitative modification of collective effects in dense and cold atomic ensembles even in the case when the characteristic Doppler shifts are tens of times smaller than the natural line width. The observed influence are explained as a result of the suppression of the impact of long-lived collective states caused by the displacement of the atoms.

We study theoretically the Floquet edge states in a photonic analog of the driven Su-Schrieffer-Heeger model implemented by an array of identical single-mode dielectric waveguides, where the time-dependent driving is modeled by periodically bended waveguides. We combine the coupled-mode theory with the Floquet-Bloch analysis and within this framework determine a band structure of the periodic system. We develop a theoretical approach for calculation of the edge states in semi-infinite systems and investigate their topological properties. In particular, we explore the dynamics of the 0- and $\pi$-edge states which reveal profound differences depending on their topological phase. To verify our observations, we simulate the power transport along the end of such a waveguide array and show that its spectra can be assigned to the excitation of the edge modes. The results obtained indicate that driving-induced topological properties of the edge modes can be exploited in controlling flow of light in periodically driven photonic structures and may provide insight into Floquet engineering of the realistic photonic systems.

Quantum vacuum noise is very detrimental for photon propagation and optical quantum memory, which particularly occurs in optically thick atomic media working under the condition of electromagnetically induced transparency (EIT) accompanied by a process of four-wave mixing (FWM). The suppression of quantum vacuum noise becomes now an imperative task for advancing the research and application of quantum memory. Here we present a scheme to suppress quantum vacuum noise and hence to improve the fidelity of photon propagation, by assuming that signal and idler laser fields are initially prepared in a two-mode squeezed vacuum state. With such a scheme, we show that the normal gain mode inherent in the EIT-based FWM system, which contributes optical gain to both the signal and idler fields, can be largely inhibited, giving rise to a significant suppression of the quantum vacuum noise along with their amplification and hence a large enhancement of the fidelity for photon propagation. The results reported here may have promising applications for high-fidelity quantum information processing and transformation, especially for improving the quality of multimode optical quantum memory based on EIT techniques.

In a nonlinear three-wave mixing process, the interacting waves can accumulate an adiabatic geometric phase (AGP) if the nonlinear coefficient of the crystal is modulated in a proper manner along the nonlinear crystal. This concept was studied so far only for the case in which the pump wave is much stronger than the two other waves, hence can be assumed to be constant. Here we extend this analysis for the fully nonlinear process, in which all three waves can be depleted and we show that the sign and magnitude of the AGP can be controlled by the period, phase, and duty cycle of the nonlinear modulation pattern. In this fully nonlinear interaction, all the states of the system can be mapped onto a closed surface. Specifically, we study a process in which the eigenstate of the system follows a circular rotation on the surface. Our analysis reveals that the AGP equals to the difference between the total phase accumulated along the circular trajectory and that along its vertical projection, which is universal for the undepleted (linear) and depleted (nonlinear) cases. Moreover, the analysis reveals that the AGPs in the processes of sum-frequency generation and difference-frequency generation have opposite chirality. Finally, we utilize the AGP in the fully nonlinear case for splitting the beam into different diffraction orders in the far field.

The uncertainty relation usually is one of the most important features in quantum mechanics and is the backbone of quantum theory, which distinguishes it from the rule in its classical counterpart. Specifically, entropy-based uncertainty relations are of fundamental importance in the region of quantum information theory, offering one nontrivial bound of key rate towards quantum key distribution. In this work, we experimentally demonstrate the entropic uncertainty relations and coherence-based uncertainty relations in an all-optics platform. By means of preparing two kinds of bipartite initial states with high fidelity, i.e., Bell-like states and Bell-like diagonal states, we carry out local projective measurements over a complete set of mutually unbiased bases on the measured subsystem. In terms of quantum tomography, the density matrices of the initial states and the postmeasurement states are reconstructed. It shows that our experimental results coincide with the theoretical predictions very well. Additionally, we also verify that the lower bounds of both the entropy-based and coherence-based uncertainty can be tightened by imposing the Holevo quantity and mutual information, and the entropic uncertainty is inversely correlated with the coherence. Our demonstrations might offer an insight into their uncertainty relations and their connection to quantum coherence in quantum information science, which might be applicable to the security analysis of quantum key distributions.

We present spectroscopic measurements and detailed theoretical analysis of inner-shell $LMn$ and $LNn$ ($n\ge$ 4) dielectronic resonances in highly charged $M$-shell ions of tungsten. The x-ray emission from ${\mathrm{W}}^{49+}$ through ${\mathrm{W}}^{64+}$ was recorded at the electron-beam ion trap (EBIT) facility at the National Institute of Standards and Technology with a high-purity Ge detector for electron-beam energies between 6.8 and 10.8 keV. The measured spectra clearly show the presence of strong resonance features as well as direct excitation spectral lines. The analysis of the recorded spectra with large-scale collisional-radiative modeling of the EBIT plasma allowed us to unambiguously identify numerous dielectronic resonances associated with excitations of the inner-shell $2s_{1/2}, 2p_{1/2}$, and $2p_{3/2}$ electrons.

In this work we investigated the competition between the local Auger decay and core-level interatomic Coulombic decay (ICD) processes in core ionized rare-gas dimers. We computed the respective partial decay widths for the $4d$ vacancy in ${\mathrm{Xe}}_2$, XeKr, and XeAr, as well as for the $3d$ vacancy in ${\mathrm{Kr}}_2$. We found that the efficiency of ICD is strongly increased with decreasing interatomic distance and decreasing energy transfer in the decay step. The ICD-to-Auger ratio in the Franck-Condon region, where the decay occurs, is at most 0.26%. However, it reaches a few percentage points in larger clusters and becomes amenable for experimental observation. The small value of the branching ratio is due to large interatomic distances in the dimers (4–4.4 Å). Our results also indicate, in accordance with previous measurements, that in hydrogen-bonded and microsolvated clusters, where the distances between the monomers are 2–3 Å, core-level ICD should become an important pathway for charge redistribution following the absorption of hard x-rays.

We investigate the probe response of a cavity-optomechanical system with its mechanical oscillator coupling to a bosonic bath characterized by the Ohmic spectrum density. By employing the spectrum decomposition scheme, we simplify the original non-Markovian equation of motion for the system and then examine the dynamics of the intracavity field. Furthermore, we work out the probe response function and simplify it further with the linear fitting method. Based on this simplification, we obtain the details of the probe spectral profile and find that the probe response changes smoothly from the optomechanically induced transparency (OMIT) regime to the normal mode splitting regime as the driving strength increases due to the lifting of frequency degeneracy of the OMIT system by the non-Markovian effect. We also find notable probe amplification under the cavity driving resonant condition. Our method can be applied to solving a non-Markovian problem in quantum optics or other fields of physics where a bath with any frequency-dependent structure must be considered.

Entanglement and quantum interference are key ingredients in a variety of quantum information processing tasks. Harnessing the generation and characterization of entanglement in high-dimensional state spaces is a necessary prerequisite towards practical quantum protocols. Here, we use quantum interference on a beam splitter to engineer hyperentanglement in polarization and discrete frequency degrees of freedom (DOF). We show how independent measurements of polarization and frequency DOF allow for the verification of high-dimensional entanglement in the combined state space. These results may indicate new paths towards practical exploitation of entanglement stored in multiple degrees of freedom, in particular in the context of high-dimensional quantum information processing protocols.

We experimentally investigate the laser-induced dissociative frustrated multiple ionization of CO molecules; in this process one of the released electrons does not escape to the continuum but is eventually trapped into high-lying Rydberg states of the outgoing positively charged nuclear fragments at the ends of the laser pulse, leading to the formation of neutral Rydberg fragments, i.e., ${\mathrm{C}}^{*}$ or ${\mathrm{O}}^{*}$. By measuring the ejected neutral Rydberg and ionic nuclear fragments as well as the freed electrons in coincidence, we trace the probability of electron trapping by different nuclear fragments and find that formation of ${\mathrm{C}}^{*}$ is preferred to ${\mathrm{O}}^{*}$. Furthermore, the electron trapping to one of the outgoing nuclear fragments of the breaking molecule can be steered by using a phase-controlled two-color laser pulse. Our results show that Rydberg fragments are more likely to be produced for CO molecules when they are exposed to a laser field pointing from C to O. This orientation-dependent asymmetric emission of the neutral Rydberg fragments is further confirmed in the multiple ionization of CO molecules by using a single-color elliptically polarized femtosecond laser pulse.

In this work we theoretically study pairing in two-dimensional Fermi gases, a system which is experimentally accessible using cold atoms. We start by deriving the mean-field pairing gap equation for a coordinate-space potential with a finite interaction range and proceed to solve this numerically. We find that for sufficiently short effective ranges the answer is identical to the zero-range one. We then use diffusion Monte Carlo to evaluate the total energy for many distinct particle numbers; we employ several variational parameters to produce a good ground-state energy and then use these results to extract the pairing gap across a number of interaction strengths in the strongly interacting two-dimensional crossover. Extracting the gap via the odd-even energy staggering, our microscopic results can be used as benchmarks for other theoretical approaches.

Although many different entanglement measures have been proposed so far, much less is known in the multipartite case, which leads to the previous monogamy relations in the literature being not complete. We establish here a strict framework for defining the multipartite entanglement measure (MEM): apart from the postulates of the bipartite measure, i.e., vanishing on separable measures and nonincreasing under local operations and classical communication, a complete MEM should additionally satisfy the unification condition and the hierarchy condition. We then come up with a complete monogamy formula for the unified MEM (an MEM is called a unified MEM if it satisfies the unification condition) and a tightly complete monogamy relation for the complete MEM (an MEM is called a complete MEM if it satisfies both the unification condition and the hierarchy condition). Consequently, we propose MEMs which are multipartite extensions of entanglement of formation (EoF), concurrence, tangle, Tsallis $q$ entropy of entanglement, Rényi $\alpha$ entropy of entanglement, the convex-roof extension of negativity, and negativity. We show that (i) the extensions of EoF, concurrence, tangle, and Tsallis $q$ entropy of entanglement are complete MEMs; (ii) multipartite extensions of Rényi $\alpha$ entropy of entanglement, negativity, and the convex-roof extension of negativity are unified MEMs but not complete MEMs; and (iii) all these multipartite extensions are completely monogamous, and the ones which are defined by the convex-roof structure (except for the Rényi $\alpha$ entropy of entanglement and the convex-roof extension of negativity) are not only completely monogamous but also tightly completely monogamous. In addition, as a byproduct, we find a class of states that satisfy the additivity of EoF. We also find a class of tripartite states one part of which can be maximally entangled with the other two parts simultaneously according to the definition of mixed maximally entangled state (MMES) in Li et al. [Z. Li, M. Zhao, S. Fei, H. Fan, and W. Liu, Quantum Inf. Comput. 12, 0063 (2012)]. Consequently, we improve the definition of maximally entangled state (MES) and prove that the only MES is the pure MES.

We implement several plasma screening potentials to calculate the level energy shifts of highly charged ions in warm dense plasmas. The Stark widths of transitions are treated with an empirical interpolation scheme combining the impact approximation and the quasicontiguous approximation. The resulting shift-width relationship is compared with recent experimental measurements of the $K\beta$ transition of He-like Cl from the Orion laser facility.

The exact static and time-dependent Kohn-Sham (KS) exchange-correlation potential is extremely challenging to approximate as it is a local multiplicative potential that depends on the electron density everywhere in the system. The KS approach can be generalized by allowing part of the potential to be spatially nonlocal. We take this nonlocal part to be that of unrestricted Hartree-Fock theory. The additional local correlation potential in principle ensures that the single-particle density exactly equals the many-body density. In our case, the local correlation potential is predominantly nearsighted in its dependence on the density and hence an (adiabatic) local-density approximation to this potential yields accurate ground-state properties and real-time densities for one-dimensional test systems.

The interaction between superconducting qubits and one-dimensional microwave transmission lines has been studied experimentally and theoretically in the past two decades. In this work, we investigate the spontaneous emission of an initially excited artificial atom which is capacitively coupled to a semi-infinite transmission line, shorted at one end. This configuration can be viewed as an atom in front of a mirror. The distance between the atom and the mirror introduces a time delay in the system, which we take into account fully. When the delay time equals an integer number of atom oscillation periods, the atom converges into a dark state after an initial decay period. The dark state is an effect of destructive interference between the reflected part of the field and the part directly emitted by the atom. Based on circuit quantization, we derive linearized equations of motion for the system and use these for a semiclassical analysis of the transient dynamics. We also make a rigorous connection to the quantum optics system-reservoir approach and compare these two methods to describe the dynamics. We find that both approaches are equivalent for transmission lines with a low characteristic impedance, while they differ when this impedance is higher than the typical impedance of the superconducting artificial atom.

We investigate a coherent feedback squeezer that uses quantum coherent feedback (measurement-free) control. Our squeezer is simple, easy to implement, robust to the gain fluctuation, and broadband compared to the existing squeezers because of the negative coherent feedback configuration. We conduct a feasibility study that looks at the stability conditions for a feedback system to optimize the designs of real optical devices. The feasibility study gives fabrication tolerance necessary for designing and realizing the actual device. Our formalism for the stability analysis is not limited to optical systems but can be applied to the other bosonic systems.