We study a system of qubits that are coupled to each other via only one degree of freedom, represented, e.g., by ${\sigma }_z$ operators. We prove that if, by changing the Hamiltonian parameters, a nondegenerate ground state of the system is continuously transformed in such a way that the expectation values of ${\sigma }_z$ operators of at least two coupled qubits change, this ground state is entangled. Using this proof, we discuss connection between energy level anticrossings and ground-state entanglement. Following the same line of thought, we introduce entanglement witnesses, based on cross susceptibilities, that can detect ground-state entanglement for any bipartition of the multiqubit system. A witness for global ground-state entanglement is also introduced.

In this work we report on ab initio theoretical results for the magnetic-field-induced $2s2p{}^3{P}_0\to 2s^{21}{S}_0$ E1 transition for ions in the beryllium isoelectronic sequence between $Z=5$ and 92. It has been proposed that the rate of the E1M1 two-photon transition $2s2p{}^3{P}_0\to 2s^{21}{S}_0$ can be extracted from the lifetime of the ${}^3{P}_0$ state in Be-like ions with zero nuclear spin by employing resonant recombination in a storage ring. This experimental approach involves a perturbing external magnetic field. The effect of this field needs to be evaluated in order to properly extract the two-photon rate from the measured decay curves. The magnetic-field-induced transition rates are carefully evaluated, and it is shown that, with a typical storage-ring field strength, it is dominant or of the same order as the E1M1 rate for low- and mid-$Z$ ions. Results for several field strengths and ions are presented, and we also give a simple $Z$-dependent formula for the rate. We estimate the uncertainties of our model to be within $5\%$ for low- and mid-$Z$ ions and slightly larger for more highly charged ions. Furthermore we evaluate the importance of including both perturber states, ${}^3{P}_1$ and ${}^1{P}_1$, and it is shown that excluding the influence of the ${}^1{P}_1$ perturber overestimates the rate by up to $26\%$ for the mid-$Z$ ions.

We present a comparative study of nonsequential double ionization (NSDI) of N${}_2$ and Ar exposed to near-single-cycle laser pulses. The NSDI process is investigated using carrier-envelope-phase-tagged electron-ion coincidence spectroscopy. The measured NSDI spectra of N${}_2$ and Ar exhibit a striking resemblance. In particular, the correlated two-electron momentum distribution arising from NSDI of N${}_2$ also displays a cross-shape very similar to that reported for Ar [Bergues et al., Nat. Commun. 3, 813 (2012)]. We interpret our results in terms of recollision-excitation with subcycle depletion and discuss how this mechanism accounts for the observed similarities and differences in the ionization behavior of the two species.

We consider the guiding of a few-cycle optical soliton by total internal reflexion, in a planar geometry. By means of numerical solution of a cubic generalized Kadomtsev-Petviashvili equation, we show that, for intensities high enough to induce soliton formation, the nonlinear effects considerably widen the guided mode and can even prevent guiding for the shortest pulses and the narrowest waveguides. However, waveguiding can be achieved by means of a steep variation of the nonlinear coefficients, e.g., by using a higher nonlinear coefficient in the cladding than that in the waveguide core. We further propose an analytical approach for extremely narrow guides, which allows us to derive a modified Korteweg–de Vries-type model for the propagation of few-cycle optical solitons in the planar waveguide.

The problem of separating classical from quantum correlations is, in general, intractable and has been solved explicitly only in a few cases. In particular, known methods cannot provide general solutions for an arbitrary number of settings. We provide the complete characterization of the classical correlations and the corresponding maximal quantum violations for the case of $n\ge 4$ observables $X_0,...,X_{n-1}$, where each consecutive pair $\{X_i,X_{i+1}\}$, sum $\mathrm{mod}n$, is jointly measurable. This generalizes both the Clauser-Horne-Shimony-Holt and the Klyachko-Can-Binicioğlu-Shumovsky scenarios, which are the simplest ones for locality and noncontextuality, respectively. In addition, we provide explicit quantum states and settings with maximal quantum violation and minimal quantum dimension.

One of the most important properties of orbital angular momentum (OAM) of photons is that the Hilbert space required to describe a general quantum state is infinite dimensional. In principle, this could allow for encoding arbitrarily large amounts of quantum information per photon, but in practice, this potential is limited by decoherence and errors. To determine whether photons with OAM are suitable for quantum communication, we numerically simulated their passage through a turbulent atmosphere and the resulting errors. We also proposed an encoding scheme to protect the photons from these errors, and characterized its effectiveness by the channel fidelity.

Experimental procedures are presented for the rapid detection of entanglement of unknown arbitrary quantum states. The methods are based on the entanglement criterion using accessible correlations and the principle of correlation complementarity. Our first scheme essentially establishes the Schmidt decomposition for pure states, with few measurements only and without the need for shared reference frames. The second scheme employs a decision tree to speed up entanglement detection. We analyze the performance of the methods using numerical simulations and verify them experimentally for various states of two, three, and four qubits.

Double photoionization spectra of Kr have been recorded using monochromatized synchrotron radiation of 88 eV photon energy and a versatile multielectron coincidence time-of-flight spectroscopy technique. The formation of the Kr${}^{2+}$ states of the lowest-energy configuration $4s^{2}4p^4$ is partly direct, producing electron pairs with a continuous distribution, and partly indirect via superexcited singly ionized states. The superexcited Kr${}^{+}$ states show strong and hitherto unexplained selectivity in branching to final Kr${}^{2+}$ states. Kr${}^{2+}$ states based on excited configurations are formed mainly by direct double photoionization.

The lowest three tune-out wavelengths of the four alkaline-earth-metal atoms Be, Mg, Ca, and Sr are determined from tabulations of matrix elements produced from large first-principles calculations. The tune-out wavelengths are located near the wavelengths for the ${}^3{P}_1^o$ and ${}^1{P}_1^o$ excitations. The measurement of the tune-out wavelengths could be used to establish a quantitative relationship between the oscillator strength of the transition leading to existence of the tune-out wavelength and the dynamic polarizability of the atom at the tune-out frequency. The longest tune-out wavelengths for Be, Mg, Ca, Sr, Ba, and Yb are 454.9813, 457.2372, 657.446, 689.200, 788.875, and 553.00 nm, respectively.

We analytically construct the wave function that, for a given initial state, produces a prescribed density for a quantum ring with two noninteracting particles in a singlet state. In this case the initial state is completely determined by the initial density, the initial time derivative of the density and a single integer that characterizes the (angular) momentum of the system. We then give an exact analytic expression for the exchange-correlation potential that relates two noninteracting systems with different initial states. This is used to demonstrate how the Kohn-Sham procedure predicts the density of a reference system without the need of solving the reference system's Schrödinger equation. We further numerically construct the exchange-correlation potential for an analytically solvable system of two electrons on a quantum ring with a squared cosine two-body interaction. For the same case we derive an explicit analytic expression for the exchange-correlation kernel and analyze its frequency dependence (memory) in detail. We compare the result to simple adiabatic approximations and investigate the single-pole approximation. These approximations fail to describe the doubly excited states, but perform well in describing the singly excited states.

The possibilities and perspectives of squeezed-light emission are studied for coherently driven single-photon sources, such as atoms and quantum dots. Maximal squeezing is realized, if the electronic subsystem of the emitter is in a pure quantum state. The purification is achieved by using a cavity as a second decay channel, aside from the incoherent coupling to the electromagnetic vacuum. For realistic cavities, this yields a purity of the electronic state of more than 99%. Aside from numerical calculations, we also derive approximate analytical results. Based on the approximations, effects are studied which originate from the environment of the emitter, including radiationless dephasing and incoherent pumping of the emitter and the cavity mode. The fragility of squeezing against decoherence is substantially reduced, so that squeezing persists even under hostile conditions. The measurement of squeezing from such light sources is also considered.

We review a scheme for performing a backaction-evading measurement of one mechanical quadrature in an optomechanical setup. The experimental application of this scheme has been limited by parametric instabilities caused in general by a slight dependence of the mechanical frequency on the electromagnetic energy in the cavity. We find that a simple modification to the optical drive can effectively eliminate the parametric instability even at high intracavity power, allowing realistic devices to achieve sub-zero-point uncertainties in the measured quadrature.

We study photon escape rates from two-dimensional atomic gases while taking into account cooperative effects between the scatterers. Based on the spectrum of the random Euclidean matrix $U_{ij}=J_0\left(x_{ij}\right)$, where $x_{ij}$ is the dimensionless random distance between any two atoms, a scaling behavior for the escape rates is obtained. This behavior is similar to the one obtained in the three-dimensional case, but qualitatively different from the result obtained in one dimension.

The quantum Zeno effect, i.e., the inhibition of coherent quantum dynamics by measurement operations, is one of the most intriguing predictions of quantum mechanics. Here we experimentally demonstrate the quantum Zeno effect by inhibiting the microwave-driven coherent spin dynamics between two ground-state spin levels of a single nitrogen vacancy center in diamond. Our experiments are supported by a detailed analysis of the population dynamics via a semiclassical model.

We study coherent adiabatic quantum state transfer through a multipath data bus by means of controlling on-site energies, and analyze the effect of pathway number on transfer efficiency. We first introduce a simple spacial geometry and demonstrate that the multipath data bus which contributes to the quantum state transfer can reduce to a tight-binding chain with three quantum dots. Simulation results show that the electron can be transferred more efficiently by properly increasing the number of transfer paths under the same shape of controlling pulses. The optimal pathway number and the corresponding high-fidelity transfer time scale are determined by the numerical analysis. Finally, we extend our proposal to a more general system. The results show that such a scheme is relatively insensitive to imperfect fabrications. Our discussions illuminate the possibility of system engineering to enhance control and transfer times.

A long-lived quantum memory is a firm requirement for implementing a quantum repeater scheme. Recent progress in solid-state rare-earth-ion-doped systems justifies their status as very strong candidates for such systems. Nonetheless an optical memory based on spin-wave storage at the single-photon level has not been shown in such a system to date, which is crucial for achieving the long storage times required for quantum repeaters. In this paper we show that it is possible to execute a complete atomic frequency comb (AFC) scheme, including spin-wave storage, with weak coherent pulses of $\overline{n}=2.5\pm 0.6$ photons per pulse. We discuss in detail the experimental steps required to obtain this result and demonstrate the coherence of a stored time-bin pulse. We show a noise level of $(7.1\pm 2.3)\times {10}^{-3}$ photons per mode during storage, and this relatively low noise level paves the way for future quantum optics experiments using spin waves in rare-earth-doped crystals.

We propose an efficient method to generate entangled states of vibrational modes of membranes which are coupled to a single-mode cavity field via the radiation pressure. By using sideband excitations, we show that arbitrary entangled states of vibrational modes in different membranes can be produced in principle by sequentially applying a series of classical pulses with desired frequencies, phases, and durations. As examples, we show how to synthesize several typical entangled states, such as Bell states, NOON states, Greenberger-Horne-Zeilinger states, and W states. The environmental effect, information leakage, and experimental feasibility are briefly discussed. Our proposal can be applied to different setups of optomechanical systems, in which vibrating modes of many mechanical resonators are coupled to a single-mode cavity.

The concept of dynamical adiabatic states, originally proposed to describe one-electron atom(ion)-ion collision systems is developed and the properties of the corresponding dynamical adiabatic potential energy curves are studied for a complete range of internuclear separations $R$. The advantages of a dynamical adiabatic basis are threefold. First, it is compatible with the boundary conditions, whereas in standard adiabatic two-Coulomb center basis we have nonvanishing inelastic transitions when internuclear distance $R\to \infty$. Second, rotational transitions are transformed into radial transitions via a type of hidden crossings in contrast with the standard adiabatic basis, where these transitions could only be included by numerical close-coupling calculations. And third, the ionization process can be described using a basis of the complete discrete orthogonal wave packets, which is much more satisfactory for the process compared with the standard adiabatic approach where the continuum states which have no direct physical meaning are employed. Results of the calculation for the (HeH)${}^{2+}$ quasimolecular system are presented and discussed. Comparison is made with previous results derived by perturbation theory in the united-atom and separated atoms limits.

The improved strong-field approximation (ISFA) is a version of the strong-field approximation which takes into account an additional interaction of the ionized electron with the parent ion within the first Born approximation. The ISFA describes well the middle- and high-energy parts of the electron spectra in the above-threshold ionization process. We show, using an example of a short-range potential, that the ISFA is able to describe the low-energy structure in the energy spectra if it is calculated without additional approximations. We introduce two different forms of the $T$-matrix element which are appropriate for application of two widely used approximations: the saddle-point approximation [i.e., its more advanced version, the uniform approximation (UA)] and the pole approximation (PA). We show that both the PA and UA are not able to describe the low-energy structure. Furthermore, the UA describes better the plateau of the spectrum than the PA. We also identify the origin of a very-low-energy structure; it is connected to the laser-free (i.e., without exchange of the laser photons) electron forward scattering.

We have developed two configurations of an echo interferometer that rely on standing-wave excitation of a laser-cooled sample of rubidium atoms. Both configurations can be used to measure acceleration $a$ along the axis of excitation. For a two-pulse configuration, the signal from the interferometer is modulated at the recoil frequency and exhibits a sinusoidal frequency chirp as a function of pulse spacing. In comparison, for a three-pulse stimulated-echo configuration, the signal is observed without recoil modulation and exhibits a modulation at a single frequency as a function of pulse spacing. The three-pulse configuration is less sensitive to effects of vibrations and magnetic field curvature, leading to a longer experimental time scale. For both configurations of the atom interferometer (AI), we show that a measurement of acceleration with a statistical precision of 0.5% can be realized by analyzing the shape of the echo envelope that has a temporal duration of a few microseconds. Using the two-pulse AI, we obtain measurements of acceleration that are statistically precise to 6 parts per million (ppm) on a 25 ms time scale. In comparison, using the three-pulse AI, we obtain measurements of acceleration that are statistically precise to 0.4 ppm on a time scale of 50 ms. A further statistical enhancement is achieved by analyzing the data across the echo envelope so that the statistical error is reduced to 75 parts per billion (ppb). The inhomogeneous field of a magnetized vacuum chamber limited the experimental time scale and resulted in prominent systematic effects. Extended time scales and improved signal-to-noise ratio observed in recent echo experiments using a nonmagnetic vacuum chamber suggest that echo techniques are suitable for a high-precision measurement of gravitational acceleration $g$. We discuss methods for reducing systematic effects and improving the signal-to-noise ratio. Simulations of both AI configurations with a time scale of 300 ms suggest that an optimized experiment with improved vibration isolation and atoms selected in the $m_F=0$ state can result in measurements of $g$ statistically precise to 0.3 ppb for the two-pulse AI and 0.6 ppb for the three-pulse AI.