Electron-impact excitation and ionization of H-like ions of nuclear charge $Z=2,\cdots ,8$ have been calculated from thresholds to high energies, with a particular focus on spin asymmetry of the cross sections. It is found that the importance of electron exchange is undiminished with increasing $Z$. Away from resonance regions, scaling considerations allow for accurate nonrelativistic estimates of the total-electron-spin-dependent cross sections for $Z>8$.

Molecular frame angular distributions of electrons released by interatomic Coulombic decay of ${\mathrm{He}}^{+*}\left(2\ell \right)\text{−}\mathrm{He}$ states are studied theoretically by means of electronic structure and nonadiabatic nuclear dynamics calculations. In previous experimental work [Phys. Rev. A 82, 063405 (2010)], distinct variations of the angular emission patterns have been observed for different ranges of kinetic-energy release of the fragment ions. Good agreement between the presently computed and these measured angular distributions can be achieved by assuming nonequal populations of ${}^2{\mathrm{\Sigma }}_{g/u}^{+}$ and ${}^2{\mathrm{\Pi }}_{g/u}$ decaying electronic states via initial shake-up ionization of the dimers.

Many-body systems undergoing quantum phase transitions reveal substantial growth of nonclassical correlations between different parties of the system. This behavior is manifested by characteristic divergences of the von Neumann entropy. Here we show that very similar features may be observed in one-dimensional systems of a few strongly interacting atoms when the structural transitions between different spatial orderings are driven by a varying shape of an external potential. When the appropriate adaptation of the finite-size scaling approach is performed in the vicinity of the transition point, few-fermion systems display a characteristic power-law invariance of divergent quantities.

The full statistical distribution of the superfluid fraction characterizing one-dimensional Bose gases in random potentials is discussed. Rare configurations with extreme fluctuations of the disorder potential can fragment the condensate and reduce the superfluid fraction to zero. The resulting bimodal probability distribution for the superfluid fraction is calculated numerically in the quasi-one-dimensional mean-field regime of ultracold atoms in laser speckle potentials. Using extreme value statistics, an analytical scaling of the zero-superfluid probability as a function of disorder strength, disorder correlation length, and system size is presented. It is argued that similar results can be expected for point-like impurities and that these findings are in reach for present-day experiments.

Measurement-free coherent feedback control is promising to manipulate various quantum optical systems for preparing nonclassical states of light. In this paper, three different coherent feedback control systems are constructed based on cascaded four-wave mixing processes. By utilizing a beam splitter as the feedback controller, we theoretically investigate the enhancement of tripartite quantum correlation and its pairwise correlation. The absorption effect of Rb vapor cells and the loss of optical propagation in the coherent feedback loop are also taken into account. The quantum correlation in the three structures can be characterized by the degree of relative intensity squeezing of the output fields. We find that the tripartite quantum correlation can be maximally enhanced by an optimal feedback ratio. In addition, two of the three pairwise correlations in the quantum regime can also be improved by tuning the strength of feedback, while the other pairwise correlation remains in the classical regime. Our results pave the way for the experimental implementation and may find potential applications in quantum communication and quantum metrology.

We demonstrate one-sided device-independent self-testing of any pure two-qubit entangled state based on a fine-grained steering inequality. The maximum violation of a fine-grained steering inequality can be used to witness certain steerable correlations, which certify all pure two-qubit entangled states. Our experimental results identify which particular pure two-qubit entangled state has been self-tested and which measurement operators are used on the untrusted side. Furthermore, we analytically derive the robustness bound of our protocol, enabling our subsequent experimental verification of robustness through state tomography. Finally, we ensure that the requisite no-signaling constraints are maintained in the experiment.

Any set of states which cannot be perfectly distinguished by local operations and classical communication (LOCC) alone can always be locally distinguished using quantum teleportation with a maximally entangled state as resource. However, in quantum information theory, entanglement is a very valuable resource, so it leaves the following problem: How to accomplish this task more efficiently than teleportation, that is, design the local discrimination protocol using less entanglement. In this paper, we first present two protocols to locally distinguish a particular unextendible product bases (UPB) in $5\otimes 5$ by using different entanglement resource. Then, we generalize the distinguishing methods for a class of UPB in $d\otimes d$, where $d$ is odd and $d⩾3$. Furthermore, for a class of UPB in $d\otimes d$, where $d$ is even and $d⩾4$, we prove that these states can also be distinguished by LOCC with multiple copies of low-dimensional entanglement resource. These results offer rather general insight into how to use entanglement resource more efficiently and also reveal the phenomenon of less nonlocality with more entanglement.

Random samples of quantum channels have many applications in quantum information processing tasks. Due to the Choi-Jamiołkowski isomorphism, there is a well-known correspondence between channels and states, and one can imagine adapting state sampling methods to sample quantum channels. Here, we discuss such an adaptation, using the Hamiltonian Monte Carlo method, a well-known classical method capable of producing high-quality samples from arbitrary, user-specified distributions. Its implementation requires an exact parametrization of the space of quantum channels, with no superfluous parameters and no constraints. We construct such a parametrization, and demonstrate its use in three common channel sampling applications.

The family of $n$-bit Toffoli gates, with the two-bit Toffoli gate as the figurehead, are of great interest in quantum information as they can be used as universal gates and in quantum error correction, among other things. We present a single-step implementation of arbitrary $n$-bit Toffoli gates (up to a local change of basis), based on resonantly driving a single qubit that has a strong Ising coupling to $n$ other qubits. The setup in the two-qubit case turns out to be identical to the universal Barenco gate. The gate time and error are, in theory, independent of the number of control qubits, scaling better than conventional circuit decompositions. We note that our assumptions, namely, strongly coupling $n+1$ qubits and a driving frequency that scales with $n$, may break down for large systems. Still, our protocol could enhance the capabilities of intermediate-scale quantum computers, and we discuss the prospects of implementing our protocol on trapped ions, Rydberg atoms, and superconducting circuits. Simulations of the latter platform show that the Toffoli gate with two control bits attains fidelities of above 0.98 even in the presence of decoherence. We also show how similar ideas can be used to make a series of controlled-not gates in a single step. We show how these can speed up the implementation of quantum error correcting codes and we simulate the encoding steps of the three-qubit bit-flip code and the seven-qubit Steane code.

The nonrelativistic QED (NRQED) approach is applied to the calculation of relativistic corrections to the fine and hyperfine structure of hydrogenlike atoms at orders $m{\alpha }^6$ and $m{\alpha }^6(m/M)$. Results are found to be in agreement with those of the relativistic theory. This confirms that the derived NRQED effective potentials are correct, providing a reliable basis for studies in more complex atoms or molecules. Furthermore, we verify the equivalence between different forms of the NRQED Lagrangian used in the literature. It is found that the gauge-invariant form derived according to the initial formulation of NRQED by Caswell and Lepage [Phys. Lett. B 167, 437 (1986)] yields a different set of effective potentials, which are simpler for numerical computations.

It has been observed that the asymmetric quantum Rabi model (QRM), which does not possess any obvious symmetry, exhibits energy level crossings, which are often associated with symmetries. This observation suggests that there is in fact a symmetry in the asymmetric QRM, even though simple inspection of the model and its Hamiltonian does not reveal the nature of this symmetry. Here we present the results of numerical calculations on the energy eigenstates of the asymmetric QRM in an attempt to elucidate the nature of the symmetry. In particular, we note that the distribution of states in the Hilbert space among different symmetry classes is normally independent of system parameters and test whether this property holds for the asymmetric QRM. We find that it does not, which both helps explain why the symmetry is hidden and adds more intrigue as to what its nature might be.

We present an approach for generating cluster states on-chip, with the state encoded in the spatial component of the photonic wave function. We show that for spatial encoding, a change of measurement basis can improve the practicality of cluster-state algorithm implementation and demonstrates this by simulating the Grover's search algorithm. Our state generation scheme involves shaping the wave function produced by spontaneous parametric down-conversion in on-chip waveguides using specially tailored nonlinear poling patterns. Furthermore, the form of the cluster state can be reconfigured quickly by driving different waveguides in the array.

In the two-atom or multiatom system, the atoms can interact with each other through exchange of virtual photon. This kind of energy exchange is often referred as the dipole-dipole interaction (DDI). Here we consider this DDI system consisting of a pair of two-level atoms strongly coupled with a bimodal whispering-gallery-mode (WGM) microresonator which is driven by an external laser field. Our aim is to explore the photon correlation characteristics of the proposed architecture using realistic experimental parameter values. We compare in detail the quality of photon antibunching (i.e., the smallness of the second-order correlation function) from three involved configurations in cavity quantum electrodynamics (QED): (i) only one two-level atom, (ii) two far apart two-level atoms without DDI, and (iii) two DDI (dipole-coupled) two-level atoms are respectively coupled to the driven WGM microresonator through the evanescent field. We clearly show that the DDI between both atoms can distinctly enhance the photon antibunching even in the weak-coupling regime in configuration (iii) with feature-rich line shapes. We also find that the photon antibunching can be modulated by properly adjusting the atom-cavity coupling strength. In addition, we display that this strong photon antibunching is robust against the cooperative atomic decay. Our DDI-based cavity QED scheme may provide an alternative way to the construction of integrated on-chip single-photon sources.

Topologically protected waves in the linearly polarized Maxwell's equations with gyrotropic, magneto-optic media were studied a decade ago both computationally and experimentally. This paper develops a robust tight-binding model for this system that makes careful use of Wannier function representations. The model provides very good approximations to the underlying band structure. When solved on a semi-infinite strip, it produces exponentially localized edge modes whose corresponding eigenvalues span the frequency band gaps. A set of coupled differential equations are derived which allows one to find how the electromagnetic field propagates unidirectionally, without backscatter from defects. Furthermore, the discrete model predicts topologically protected edge modes with nontrivial Chern number which are consistent with direct simulation.

It is well known that although the group velocity of structured light pulses propagating in vacuum can be subluminal or superluminal, the upper limit of the energy flow velocity is $c$, the speed of light in vacuum. This inequality can be explained in terms of the reactive energy left behind by the fields. Energy and reactive energy densities have been calculated for vector-valued two-dimensional (light sheet) superluminal electromagnetic nondiffractive pulses, as well as scalar-valued and transverse magnetic three-dimensional superluminal and subluminal spatiotemporally localized electromagnetic waves. Emphasis is placed on the physical formation of the reactive energy due to interference of the plane-wave constituents of the structured light waves.

Digital quantum computing paradigm offers highly desirable features such as universality, scalability, and quantum error correction. However, physical resource requirements to implement useful error-corrected quantum algorithms are prohibitive in the current era of NISQ devices. As an alternative path to performing universal quantum computation, within the NISQ era limitations, we propose to merge digital single-qubit operations with analog multiqubit entangling blocks in an approach we call digital-analog quantum computing (DAQC). Along these lines, although the techniques may be extended to any resource, we propose to use unitaries generated by the ubiquitous Ising Hamiltonian for the analog entangling block and we prove its universal character. We construct explicit DAQC protocols for efficient simulations of arbitrary inhomogeneous Ising, two-body, and $M$-body spin Hamiltonian dynamics by means of single-qubit gates and a fixed homogeneous Ising Hamiltonian. Additionally, we compare a sequential approach where the interactions are switched on and off (stepwise DAQC) with an always-on multiqubit interaction interspersed by fast single-qubit pulses (banged DAQC). Finally, we perform numerical tests comparing purely digital schemes with DAQC protocols, showing a remarkably better performance of the latter. The proposed DAQC approach combines the robustness of analog quantum computing with the flexibility of digital methods.

We study the interplay between interactions and finite-temperature dephasing baths. We consider a double well with strongly interacting bosons coupled, via the density, to a bosonic bath. Such a system, when the bath has infinite temperature and instantaneous decay of correlations, relaxes with an emerging algebraic behavior with exponent $1/2$. Here we show that because of the finite-temperature baths and the choice of spectral densities, such an algebraic relaxation may occur for a shorter duration and the characteristic exponent can be lower than $1/2$. These results show that the interaction-induced impeding of relaxation is stronger and more complex when the bath has finite temperature and/or nonzero timescale for the decay of correlations.

We describe an efficient approach to modeling cavity quantum electrodynamics (QED) with a time-delayed coherent feedback using quantum trajectory simulations. An analytical set of equations is derived to exploit the advantages of trajectories in the presence of the non-Markovian dynamics, where adjustments to the standard stochastic dynamics are discussed. In the weak excitation regime, we first verify that our approach recovers known results obtained with other simulation methods and demonstrate how a coherent feedback loop can increase the photon lifetime in typical cavity-QED systems. We also highlight the underlying stochastic dynamics. We then explore the nonlinear few-photon regime of cavity QED, under the restriction of at most one photon at a time in the feedback loop. In particular, we show how feedback affects the cavity photoluminescence (populations versus laser detuning) and describe how one must account for conditioning in the presence of feedback; specifically, the system observables must be conditioned on no photon detections at the feedback output channel occurring.

Recent advances in the development of powerful transition-metal-doped chalcogenide femtosecond lasers made it possible to produce laser radiation directly in the vicinity of molecular vibrational resonances. Here we demonstrate the use of such light for low-order harmonic generation in a dense gas mixture, containing carbon dioxide molecules resonantly interacting with pump radiation. We report on the manifold increase of third (25) and fifth (10) harmonic generation efficiency and the appearance of subsequent harmonics (from 7th to 11th) in xenon (48 bars) in the presence of carbon dioxide (1 bar). Final conversion efficiency obtained experimentally and confirmed numerically reaches $1\%$ for third harmonic and $0.3\%$ for higher ones (from 5th to 11th in total) under a very moderate vacuum intensity of $1.8\times {10}^{13}$ W/${\mathrm{cm}}^2$. These experiments open new perspectives for optimization of harmonic generation efficiency without significant perturbation of the media. In particular, phase-matching conditions may be also found in extreme ultraviolet and soft x-ray regions, where conversion efficiency from a mid-IR pump stays quite low.