We present an efficient scheme for the generation of entangled states of two spatially separated nitrogen-vacancy (NV) centers with two whispering-gallery-mode (WGM) microresonators, which are coupled either by an optical fiber-taper waveguide or by the evanescent fields of the WGM. We show that the steady state of the two NV centers can be steered into a singletlike state through a dissipative quantum dynamical process, where the cavity decay plays a positive role and can help drive the system to the target state. The protocol may open up promising perspectives for quantum communication and computation with this solid-state cavity quantum electrodynamic system.

In this paper, we study the unconditional security of the so-called measurement-device-independent quantum key distribution (MDIQKD) with the basis-dependent flaw in the context of phase encoding schemes. We propose two schemes for the phase encoding: The first one employs a phase locking technique with the use of non-phase-randomized coherent pulses, and the second one uses conversion of standard Bennett-Brassard 1984 (BB84) phase encoding pulses into polarization modes. We prove the unconditional security of these schemes and we also simulate the key generation rate based on simple device models that accommodate imperfections. Our simulation results show the feasibility of these schemes with current technologies and highlight the importance of the state preparation with good fidelity between the density matrices in the two bases. Since the basis-dependent flaw is a problem not only for MDIQKD but also for standard quantum key distribution (QKD), our work highlights the importance of an accurate signal source in practical QKD systems.

In a quest for benchmarking transition rate data on electric-dipole ($E1$) forbidden transitions to be used in collisional-radiative models of plasma spectra, the radiative decay rates of the metastable levels $2s^{2}2p^2$ ${}^1{D}_2$, $2s^{2}2p^3$ ${}^2{P}_{1/2,3/2}^{\mathrm{o}}$, and $2s^{2}2p^4$ ${}^1{D}_2$ in C-, N-, and O-like ions, respectively, have been measured for the elements Si, P, and S. Results with uncertainties mostly well below 1$\%$ are obtained using ions circulating in a storage ring. Precision results for Si${}^{7+}$, P${}^{7+}$, P${}^{8+}$, P${}^{9+}$, S${}^{8+}$, and S${}^{10+}$ obtained here complete the isoelectronic sequence data sets for these four metastable levels in all three elements.

We present the calculation of the self-energy of the isolated hydrogen atom within the $GW$ approximation starting from the noninteracting Green's function constructed from the exact wave functions of the hydrogen atom. The error in the electron removal energy of the $1s$ state is found to be about 0.02 eV, which is much smaller than what one would expect. This small error is explained by the cancellation of the self-screening errors between different $l$ contributions of the self-energy. The unbound continuum states are found to be crucial to get the correct self-energy.

Unstable four-boson states having an approximate dimer-atom-atom structure are studied using momentum-space integral equations for the four-particle transition operators. For a given Efimov trimer the universal properties of the lowest associated tetramer are determined. The impact of this tetramer on the atom-trimer and dimer-dimer collisions is analyzed. The reliability of the three-body dimer-atom-atom model is studied.

The long-range interaction between ${}^{\{1,3\}}S$ He${}^{*}\left(3s\right)$ and ${}^{1}S$ He$\left(1s\right)$ was studied in bulk liquid helium by fluorescence measurements and by combined theoretical electronic structure and bosonic density functional theory calculations. The excited He${}^{*}$ atoms were produced in the liquid by corona discharge with subsequent impact excitation instigated by hot electrons from the discharge. The long-range contribution to the repulsive “hump” near 5 Å in the ${}^{\{1,3\}}S$ He–${}^{1}S$ He potentials was interrogated by monitoring He${}^{*}$ ${}^{\{1,3\}}S\left(3s\right)\to {}^{\{1,3\}}P(2p$) fluorescence profile characteristics as a function of external pressure between 0.1 and 3.5 MPa. Fluorescence line shifts and widths as a function of pressure were extracted from the experimental data and compared to the theoretical predictions, establishing that the nascent He${}^{*}$ atoms reside in a bubble state within the liquid. It was observed that the experimental data could only be consistently reproduced if the excited He${}^{*}$ atoms emit in a less-dense environment as compared with the rest of the bulk liquid.

The electron-impact multiple ionization and subsequent dissociation of CO${}_2$ is studied for 12-keV electron energy using a linear time-of-flight mass spectrometer coupled with a multihit, position-sensitive detector. The complete as well as incomplete Coulomb explosion pathways for CO${}_2$${}^{2+}$ and CO${}_2$${}^{3+}$ ions are examined and identified. The kinetic energy release distributions for these precursor ions are obtained. The experimental kinetic energy release values for the complete Coulomb fragmentation channels are found to be overestimated by those calculated from the Coulomb explosion model. From the angular correlation studies, it is inferred that bent geometrical states are involved for most of the fragmentation channels of CO${}_2$${}^{2+}$ and CO${}_2$${}^{3+}$ ions. The concerted and/or sequential nature of all the dissociation pathways is also assigned. This study provides the first results on energetics associated with the charge separation in dissociative ionization of CO${}_2$ under the impact of electrons at a subrelativistic energy.

Direct-frequency comb spectroscopy (DFCS) is currently the highest-resolution absolute frequency spectroscopic technique known. In general, one does DFCS by scanning the repetition rate, $f_{\mathrm{rep}}$, of a comb laser and measuring fluorescence from the excited states of the species under study. The technique has already been successfully characterized by a theoretical model that starts with the optical Bloch equations and, with a few simplifying assumptions converts them into linear coupled iterative equations. In the present work we build on that successful model to predict the characteristics of the ion yield from photoionization by the comb laser, as a function of $f_{\mathrm{rep}}$. We show that the ion spectrum yields the same atomic structure as the fluorescence spectra, but with greater efficiency.

We treat the nonequilibrium motion of a single-impurity atom in a low-temperature single-species Fermi sea, interacting via a contact interaction. In the nonequilibrium regime, the impurity does a superdiffusive geometric random walk where the typical distance traveled grows with time as $\sim t^{d/(d+1)}$ for the $d$-dimensional system with $d\ge 2$. For nonzero temperature $T$, this crosses over to diffusive motion at long times with diffusivity $D\sim T^{-(d-1)/2}$. These results apply also to a nonzero concentration of impurity atoms as long as they remain dilute and nondegenerate.

We demonstrate that a coherent superposition state of two temporally separated optical pulses, called a time-bin state, can be transferred to that of up-down electron spins in a semiconductor by synchronizing the time separation to the precession period of either electrons or holes. The time-bin transfer scheme does not require polarization-mode degeneracy and can map the time-bin state to the electron spin state that is not accessible directly using only polarization. The scheme offers an alternate approach for quantum interfaces between photons and electron spins.

Quantum spin networks can be used to transport information between separated registers in a quantum-information processor. To find a practical implementation, the strict requirements of ideal models for perfect state transfer need to be relaxed, allowing for complex coupling topologies and general initial states. Here we analyze transport in complex quantum spin networks in the maximally mixed state and derive explicit conditions that should be satisfied by propagators for perfect state transport. Using a description of the transport process as a quantum walk over the network, we show that it is necessary to phase-correlate the transport processes occurring along all the possible paths in the network. We provide a Hamiltonian that achieves this correlation and use it in a constructive method to derive engineered couplings for perfect transport in complicated network topologies.

For every electronic many-body Hamiltonian with a one-body potential that goes to zero at infinity, there is a maximum number of bound electrons ($J_{\max }$) that can be sustained in the system with an infinite lifetime. At this integer $J_{\max }$, the derivative discontinuity in the exact energy of ensemble density functional theory (DFT) is ill defined. However, we investigate the derivative discontinuity of the energy within the framework of density functional resonance theory [D. L. Whitenack and A. Wasserman, Phys. Rev. Lett. 107, 163002 (2011)], which reduces to ground-state DFT as a coordinate scaling parameter is taken to zero, and find that the exact exchange-correlation potential experiences discontinuous jumps at integer particle numbers, including $J_{\max }$. For integers below $J_{\max }$ the jump is purely real because of the real shift in the chemical potential. At $J_{\max }$, the jump has a nonzero imaginary component reflecting the metastability of the system upon addition of one more electron. In addition, the magnitude of the derivative discontinuity at $J_{\max }$ is larger than would have been expected by simply setting the affinity to zero.

A description of fine-structure patterns in nonpenetrating high-$L$ Rydberg atoms and ions is derived in a perturbative model in which the energy denominators occurring in the second-order perturbation theory are evaluated using the adiabatic expansion. The patterns of Rydberg energies that result are dominated by the expectation value of an effective potential containing a range of tensor orders and increasing negative powers of the Rydberg electron's radial coordinate. The coefficients of each term in the effective potential are expressed in terms of matrix elements and energies of the free ion at the core of the Rydberg system. Smaller corrections to these patterns due to application of the effective potential in second order and due to relativistic and spin contributions are also described. The effective potential provides a framework for extracting ion properties from measurements of high-$L$ fine-structure patterns.

The ionization potentials, transition energies, and oscillator strengths of Li and Li-like C${}^{3+}$ and Al${}^{10+}$ are computed at different plasma environments with the Fock-space multireference coupled-cluster theory to examine the parametric dependence of these properties on plasma density and/or temperature. The results presented here show that the ionization and transition energies as well as the oscillator strengths are very sensitive to the plasma environment. It further shows that the spectral lines corresponding to $\Delta n=0$ transitions for Li-like C${}^{3+}$ and Al${}^{10+}$ are blueshifted, whereas the lines associated to $\Delta n\ne 0$ are redshifted ($n$ is the principal quantum number).

The random phase approximation (RPA) is thought to be a successful method; however, basic errors have been found that have massive implications in the simplest molecular systems. The observed successes and failures are rationalized by examining its performance against exact conditions on the energy for fractional charges and fractional spins. Extremely simple tests reveal that the RPA method satisfies the constancy condition for fractional spins that leads to correct dissociation of closed-shell molecules and no static correlation error (such as in H${}_2$ dissociation) but massively fails for dissociation of odd electron systems, with an enormous delocalization error (such as H${{}_2}^{+}$ dissociation). Other methods related to the RPA, including the Hartree-Fock response (RPAE) or range-separated RPA, can reduce this delocalization error but only at the cost of increasing the static correlation error. None of the RPA methods have the discontinuous nature required to satisfy both exact conditions and the full unified condition (e.g., dissociation of H${{}_2}^{+}$ and H${}_2$ at the same time), emphasizing the need to go beyond differentiable energy functionals of the orbitals and eigenvalues.

State-selective single electron capture in collisions of He${}^{+}$ with H${}_2$ molecules is investigated using the cold-target recoil-ion momentum spectroscopy technique. The energy range considered is 6.25 keV/amu (projectile velocity $v_p$ $=$ 0.5 a.u.) to 50 keV/amu ($v_p$ $=$ 1.41 a.u.). In addition, we explore the transverse recoil momentum distributions that provide direct access to the angular scattering of the receding projectile. The present experimental data are contrasted to the theoretical results obtained by means of the classical trajectory Monte Carlo model with which we find good overall agreement in the energy range explored.

We point out that the widely accepted condition $g_{11}{g}_{22}<g_{12}^2$ for phase separation of a two-component Bose-Einstein condensate is insufficient if kinetic energy is taken into account, which competes against the intercomponent interaction and favors phase mixing. Here $g_{11}$, $g_{22}$, and $g_{12}$ are the intra- and intercomponent interaction strengths, respectively. Taking a $d$-dimensional infinitely deep square well potential of width $L$ as an example, a simple scaling analysis shows that if $d=1$ ($d=3$), phase separation will be suppressed as $L\to 0$ ($L\to \infty$) whether the condition $g_{11}{g}_{22}<g_{12}^2$ is satisfied or not. In the intermediate case of $d=2$, the width $L$ is irrelevant but again phase separation can be partially or even completely suppressed even if $g_{11}{g}_{22}<g_{12}^2$. Moreover, the miscibility-immiscibility transition is turned from a first-order one into a second-order one by the kinetic energy. All these results carry over to $d$-dimensional harmonic potentials, where the harmonic oscillator length ${\xi }_{\mathrm{ho}}$ plays the role of $L$. Our finding provides a scenario of controlling the miscibility-immiscibility transition of a two-component condensate by changing the confinement, instead of the conventional approach of changing the values of the $g$'s.

We explore the connection between adiabatic rapid passage, a technique that allows for robust complete population transfer (CPT) in a resonant absorber, and self-induced transparency solitons. We employ the auxiliary differential equation finite-difference time-domain method to fully resolve the dynamics of femtosecond light pulses propagating through an absorbing two-level medium. Using the example of linearly chirped Gaussian pulses we find that the pulses achieving robust CPT for a single two-level system are also pulses that will transform into solitary waves when incident on a resonantly absorbing medium. At the entrance into the absorber strongly chirped, adiabatic pulses completely transfer the electronic population from the ground to the upper state. As the pulses suffer absorption they undergo a dynamic transition and quickly split into an off-resonant radiation part and one or several unchirped solitary pulses that may carry away some of the initially achieved population inversion. This spectral and temporal transformation process limits the spatial depths of the region of CPT in a dense absorber regardless of the initial pulse amplitude. We also show that the frequency-domain pulse area, defined as the spectral amplitude at the resonance frequency, allows for a better distinction between the regimes of $2\pi$-pulse and $0\pi$-pulse formation than time-domain measurements.

We show a systematic construction for implementing general measurements on a single qubit, including both strong (or projection) and weak measurements. We mainly focus on linear optical qubits. The present approach is composed of simple and feasible elements, i.e., beam splitters, wave plates, and polarizing beam splitters. We show how the parameters characterizing the measurement operators are controlled by the linear optical elements. We also propose a method for the implementation of general measurements in solid-state qubits.