The dynamics of a cloud of ultracold two-level atoms is studied at off-resonant laser driving to a Rydberg state. We find that resonant excitation channels lead to strongly peaked spatial correlations associated with the buildup of asymmetric excitation structures. These aggregates can extend over the entire ensemble volume, but are in general not localized relative to the system boundaries. The characteristic distances between neighboring excitations depend on the laser detuning and on the interaction potential. These properties lead to characteristic features in the spatial excitation density, the Mandel $Q$ parameter, and the total number of excitations. As an application an implementation of the three-atom cswap or Fredkin gate with Rydberg atoms is discussed. The gate not only exploits the Rydberg blockade, but also utilizes the special features of an asymmetric geometric arrangement of the three atoms. We show that continuous-wave off-resonant laser driving is sufficient to create the required spatial arrangement of atoms out of a homogeneous cloud.

The influence of intermediate resonant dissociative channels in the few-photon ionization dynamics of H${}_2$${}^{+}$ is demonstrated in a pump-probe scenario. Two-photon ionization of H${}_2$${}^{+}$ by two sequentially applied pump and probe vuv femtosecond ${10}^{11}$ W/cm${}^2$ laser pulses is reported. The kinetic energy distribution of the ejected protons is calculated by solving the time-dependent Schrödinger equation within the Born-Oppenheimer approximation, including the electronic three-dimensional and vibrational one-dimensional motion. Population is effectively transferred from the $1s{\sigma }_g$ to the $3p{\sigma }_u$ potential surface, via resonant one-photon absorption, by applying a chirped pump pulse. The molecule is ultimately ionized by the probe pulse. It is found that a double-peaked structure appears in the resulting kinetic energy release spectra of the nuclear fragments. A corresponding modulated structure also appears in the dissociative channels, merely demonstrating that the double-peaked structure in the spectra originates from the molecular $1s{\sigma }_g$-$3p{\sigma }_u$ electronic dynamics, and an inherent two-center interference in the underlying electric-dipole coupling. It turns out that the dipole coupling between the $1s{\sigma }_g$ and $3p{\sigma }_u$ electronic states vanishes at an internuclear distance of $2.25$ a.u., i.e., close to the equilibrium internuclear separation. The resulting node in the $R$-dependent dipole coupling imposes a minimum in the corresponding kinetic energy release spectra of the dissociating nuclei. By creating a negative or positive chirp in the applied pump pulse, it is found that the modulations can be made more or less salient.

Coincidences between neutral particles and photoions have been measured at the C $1s$ edge of the CH${}_4$ molecule. These neutral particles are either vacuum ultraviolet (VUV) photons or metastable fragments and their contributions can be separated temporally. Observed VUV-photon–photoion coincidences were mostly triggered by Lyman emission in excited H atoms. Two metastable fragment-photoion channels were observed and assigned to H${}^{*}$-H${}^{+}$ and H${}^{*}$-CH${}_n{}^{+}$ ($n=0–3$) coincidences, where H${}^{*}$ signifies a metastable H atom. The coincidence yields as a function of photon energy revealed all the core excitations, but were particularly enhanced just above the C $1s$ ionization threshold and also displayed the vibrational structure related to the core-ionized C $1s^{-1}$ state. The metastable H atoms observed in the present study were produced in highly excited Rydberg states and could be partly field ionized. The approximate kinetic energy distributions of the metastable H${}^{*}$ atoms have been derived from the H${}^{*}$-H${}^{+}$ coincidence measurements performed at the C $1s\to 3p$ and $3d$ resonances as well as just above the C $1s$ threshold.

While classical or quantum interacting liquids become turbulent under sufficiently strong driving, it is not obvious what flow pattern an ideal quantum gas develops under similar conditions. Unlike classical noninteracting particles which exhibit rather trivial flow, ideal fermions have to satisfy the exclusion principle, which acts as a form of collective repulsion. We thus study the flow of an ideal Fermi gas as it is driven out of a narrow orifice of width comparable to the Fermi wavelength, employing both a microcanonical approach to transport, and solving a Lindblad equation for Markovian driving leads. Both methods are in good agreement and predict an outflowing current density with a complex microscopic pattern of vorticity in the steady state. Applying a bias of the order of the chemical potential results in a short-range correlated antiferromagnetic vorticity pattern, corresponding to local moments of the order of a tenth of a magneton, $e\hslash /2m$, if the fermions are charged. The latter may be detectable by magnetosensitive spectroscopy in strongly driven cold gases (atoms) or clean electronic nanostructures (electrons).

Nambu-Goldstone modes in immiscible two-component Bose-Einstein condensates are studied theoretically. In a uniform system, a flat domain wall is stabilized and then the translational invariance normal to the wall is spontaneously broken in addition to the breaking of two U(1) symmetries in the presence of two complex order parameters. We clarify the properties of the low-energy excitations and identify that there exist two Nambu-Goldstone modes: an in-phase phonon with a linear dispersion and a ripplon with a fractional dispersion. The signature of the characteristic dispersion can be verified in segregated condensates in a harmonic potential.

We apply a functional renormalization group to systems of four bosonic atoms close to the unitary limit. We work with a local effective action that includes a dynamical trimer field, and we use this field to eliminate structures that do not correspond to the Faddeev-Yakubovsky equations. In the physical limit, we find three four-body bound states below the shallowest three-body state. The values of the scattering lengths at which two of these states become bound are in good agreement with exact solutions of the four-body equations and experimental observations. The third state is extremely shallow. During the evolution we find an infinite number of four-body states based on each three-body state which follow a double-exponential pattern in the running scale. None of the four-body states shows any evidence of dependence on a four-body parameter.

We study the phase diagram of a two-dimensional Fermi gas with the synthetic spin-orbit coupling that has recently been realized experimentally. In particular, we characterize in detail the properties and the stability region of the unconventional Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states in such a system, which are induced by spin-orbit coupling and Fermi surface asymmetry. We identify several distinct nodal FFLO states by studying the topology of their respective gapless contours in momentum space. We then examine the phase structure and the number density distributions in a typical harmonic trapping potential under the local density approximation. Our studies provide detailed information on the FFLO pairing states with spin-orbit coupling and Fermi surface asymmetry, and will facilitate experimental detection of these interesting pairing states in the future.

We investigate a hybrid quantum system that consists of a superatom coupled to a surface phonon-polariton. We apply this hybrid quantum system to subtract individual photons from a beam of light. A Rydberg atom blockade is used to attain absorption of a single photon by an atomic microtrap. Surface phonon-polariton coupling to the superatom then triggers the transfer of the excitation to a storage state, a single Rydberg atom. The approach utilizes the interaction between a superatom and a Markovian bath that acts as a controlled decoherence mechanism to irreversibly project the superatom state into a single Rydberg atom state that can be read out.

We study the generalized Jaynes-Cummings model of quantum optics at the inversion-symmetry breaking and in the ultrastrong coupling regime. With the help of a generalized multiphoton rotating-wave approximation, we study the stationary solutions of the Schrödinger equation. It is shown that the problem is reduced to resonant interaction of two position-displaced harmonic oscillators. Explicit expressions for the eigenstates and eigenvalues of generalized Jaynes-Cummings Hamiltonian are presented. We exemplify our physical model with analytical and numerical considerations regarding Rabi oscillations, collapse and revivals of the initial population of a two-level system, and photon distribution function at the direct multiphoton resonant coupling.

In a two-way deterministic quantum key distribution (DQKD) protocol, Bob randomly prepares qubits in one of four states and sends them to Alice. To encode a bit, Alice performs an operation on each received qubit and returns it to Bob. Bob then measures the backward qubits to learn about Alice's operations and hence the key bits. Recently, we proved the unconditional security of the final key of this protocol in the ideal device setting. In this paper, we prove that two-way DQKD protocols are immune to all detector-side-channel attacks at Bob's side, while we assume ideal detectors at Alice's side for error testing. Our result represents a step forward in making DQKD protocols secure against general detector-side-channel attacks.

The dispersion coefficients $C_6$, $C_8$, and $C_{10}$ for long-range interactions of He($n{}^{1,3}S$) and He($n{}^{1,3}P$), $2\le n\le 10$, with the ground-state noble-gas atoms Ne, Ar, Kr, and Xe are calculated by summing over the reduced matrix elements of multipole transition operators. The large-$n$ expansions for the sums over the He oscillator strength divided by the corresponding transition energy are presented for these series. Using the expansions, the $C_6$ coefficients for the systems involving He($13{}^{1,3}S$) and He($13{}^{1,3}P$) are calculated and found to be in good agreement with directly calculated values.

We demonstrate theoretically that by employing an intense few-cycle, circularly polarized laser pulse, two-dimensional localization of the electronic wave function in laser-driven H${}_3$${}^{2+}$ can be controlled by varying the carrier-envelope phase (CEP). In the reaction ${{\mathrm{H}}_3}^{2+}\to \mathrm{H}+{\mathrm{H}}^{+}+{\mathrm{H}}^{+}$, it is possible to select which one of the protons the electron finally binds to by tuning the value of the CEP. A complex asymmetry measure is introduced to quantify the degree of asymmetry induced by the laser field. In contrast to previous investigations of electron localization where only diatomic molecules were considered, our results extend CEP control of intramolecular electron motion to triatomic systems. This provides a step toward strong-field spatial and temporal control of electron dynamics in polyatomic molecules.

It is known that the violation of Svetlichny's inequality (SI), rather than the usual Mermin's inequality (MI), is a robust criterion to confirm the existence of genuine multipartite quantum nonlocality. In this paper, we propose a feasible approach to test SI with three two-level atoms (TLAs) dispersively coupled to a driven cavity. The proposal is based on the joint measurements of the states of three TLAs by probing the steady-state transmission spectra of the driven cavity: each peak marks one of the computational basis states and its relative height corresponds to the probability superposed in the detected three-TLA state. With these kinds of joint measurements, the correlation functions in SI can be directly calculated, and thus the SI can be efficiently tested for typical tripartite entanglement, i.e., genuine tripartite entanglement [e.g., Greenberger-Horne-Zeilinger (GHZ) and W states] and biseparable three-qubit entangled states (e.g., ${|\chi \rangle }_{12}{|\xi \rangle }_3$). Our numerical experiments show that the SI is violated only by three-qubit GHZ and W states, not by biseparable three-qubit entangled state ${|\chi \rangle }_{12}{|\xi \rangle }_3$, while the MI can still be violated by biseparable three-qubit entangled states. Thus the violation of SI can be regarded as a robust criterion for the existence of genuine tripartite entanglement.

The absolute value of the hyperdeterminant of four qubits is a useful measure of genuine entanglement. We prove a recent conjecture of Gour and Wallach describing the pure maximally entangled four-qubit states with respect to this measure.

Many physical systems considered promising qubit candidates are not, in fact, two-level systems. Such systems can leak out of the preferred computational states, leading to errors on any qubits that interact with leaked qubits. Without specific methods of dealing with leakage, long-lived leakage can lead to time-correlated errors. We study the impact of such time-correlated errors on topological quantum error correction codes, which are considered highly practical codes, using the repetition code as a representative case study. We show that, under physically reasonable assumptions, a threshold error rate still exists; however, performance is significantly degraded. We then describe simple additional quantum circuitry that, when included in the error detection cycle, restores performance to acceptable levels.

We derive an algebraic framework which identifies the minimal information required to assess how well a quantum device implements a desired quantum operation. Our approach is based on characterizing only the unitary part of an open system's evolution. We show that a reduced set of input states is sufficient to estimate the average fidelity of a quantum gate, avoiding a sampling over the full Liouville space. Surprisingly, the minimal set consists of only two input states, independent of the Hilbert-space dimension. The minimal set is, however, impractical for device characterization, since one of the states is a totally mixed thermal state and extracting bounds for the average fidelity is impossible. We therefore present two further reduced sets of input states that allow for, respectively, numerical and analytical bounds on the average fidelity.

Two-loop QED corrections with closed fermion loops are calculated for the $1s$ bound-electron $g$ factor. Calculations are performed to all orders in the nuclear binding strength parameter $Z\alpha$ (where $Z$ is the nuclear charge and $\alpha$ is the fine-structure constant) except for the closed fermion loop, which is treated within the free-loop (Uehling) approximation in some cases. Comparison with previous $Z\alpha$-expansion calculations is made and the higher-order remainder of order ${\alpha }^2{\left(Z\alpha \right)}^5$ and higher is separated out from the numerical results.

The average charge states $\overline{q}$ of heavy and superheavy ions (atomic numbers $Z=80$–114) passing through He gas are studied experimentally and theoretically. Experimental data were measured at the gas-filled recoil separator, i.e., the TransActinide Separator and Chemistry Apparatus (TASCA) at GSI Darmstadt, for ion energies of a few hundred keV/u at gas pressures of 0.2 to 2.0 mbar. An attempt is made to describe experimental $\overline{q}$ values by means of atomic calculations of the binding energies and electron-loss and electron-capture cross sections. The influence of the gas-density effect is included in the calculations. The calculated $\overline{q}$ reproduce the experimental values for elements with $Z=80$–114 within 20%. A comparison with different semiempirical models is presented as well, including a local fit of high accuracy, which is often used in superheavy-element experiments to estimate the average charge states of heavy ions, e.g., at the gas-filled recoil separator TASCA. The $\overline{q}$ values for elements with $Z=115$, 117, 119, and 120 at He-gas pressure of 0.8 mbar are predicted.

For atomic hydrogen, an interference structure has been theoretically found to appear in the photoelectron spectra induced by strong extreme ultraviolet (XUV) laser pulses [Demekhin and Cederbaum, Phys. Rev. Lett. 108, 253001 (2012)]. For molecular hydrogen ion H${}_2{}^{+}$, our present work provides the results of the numerical simulation on its photoelectron spectra in an XUV photoionization process. The interference feature in the photoelectron spectra is investigated with respect to the dynamic Stark effect and the XUV laser parameters. The research outcome is that our numerical findings corroborate the prediction for the emergence of the modulation structures in the photoelectron spectra, similar to those already found for a hydrogen atom. For an XUV laser pulse with photon energy well above the ionization threshold of H${}_2{}^{+}$, the dynamic Stark effect is prominently demonstrated for certain values of laser parameters such as intensity and pulse duration. The deployed one-dimensional model and the plane-wave description for the continuum states for the molecular hydrogen ion are of crucial significance in modeling the molecular system. The numerical computation is made feasible by using a hybrid algorithm. We advocate an ensemble physical picture to decipher the physical mechanism for the formation of the interference photoelectron spectra by taking into account the nuclear degree of freedom. It is anticipated that more quantitative investigations on the Stark effect concealed in the photoelectron spectra created by single XUV photons are needed plus experimental verifications.