In a practical quantum-key-distribution system, photon source and small operational errors cause intensity fluctuations inevitably, which cannot be ignored for a precise estimation on the single-photon fraction. In this paper, we demonstrated an efficient three-intensity decoy method scheme on top of the one-way Faraday-Michelson Interferometric system, combining an active monitoring with existing commercial apparatus to inspect fluctuations instantly. With this faithful detection for the upper bound of the fluctuation, the secure quantum key distribution is unconditionally realized with whatever type of intensity errors, which declares the utility and potential of decoy theory and active monitoring for quantum key distribution in practical use.

Entanglement swapping between photon pairs is a fundamental building block in schemes using quantum relays or quantum repeaters to overcome the range limits of long-distance quantum key distribution. We develop a closed-form solution for the actual quantum states prepared by realistic entanglement swapping, which takes into account experimental deficiencies due to inefficient detectors, detector dark counts, and multiphoton-pair contributions of parametric down-conversion sources. We investigate how the entanglement present in the final state of the remaining modes is affected by the real-world imperfections. To test the predictions of our theory, comparison with previously published experimental entanglement swapping is provided.

Hyperfine-structure splittings in the $2^{3}S\left(L=0\right)$ state of the six-body carbon muonic ${{}^{11}\text{C}}^{6+}{\mu }^{-}{e}_4^{-}$, ${{}^{12}\text{C}}^{6+}{\mu }^{-}{e}_4^{-}$, ${{}^{13}\text{C}}^{6+}{\mu }^{-}{e}_4^{-}$, and ${{}^{14}\text{C}}^{6+}{\mu }^{-}{e}_4^{-}$ ions are determined numerically with the use of variational wave functions. The variational wave functions for each of these carbon muonic ions are constructed with the use of six-body Gaussoids which explicitly depend upon all 15 relative coordinates $r_{12},r_{13},\dots ,r_{56}$ defined in an arbitrary six-body system.

The electronic resonant states of HCO and HOC are calculated using the complex Kohn variational method combined with structure calculations using multireference configuration interaction. No resonant state of HCO crosses the ion potential close to its minimum. Several resonances at higher energies are observed. There are clear indications of avoided crossings between the resonant states. There are resonant states that are repulsive with respect to both radial coordinates, but they remain relatively unchanged in energy as a function of the bending angle. For HOC, there are a manifold of resonant states with similar shapes as the potentials of HCO. However, the resonant states of HOC are lower in energy relative to the ion and could play an important role for dissociative recombination of ${\text{HOC}}^{+}$ at low collision energies.

We have carried out a detailed and systematic study of the correlation energies of inert gas atoms Ne, Ar, Kr, and Xe using relativistic many-body perturbation theory and relativistic coupled-cluster theory. In the relativistic coupled-cluster calculations, we implement perturbative triples and include these in the correlation energy calculations. We then calculate the dipole polarizability of the ground states using perturbed coupled-cluster theory.

The electronic stopping power, $S$, of ${\text{HfO}}_2$ films for proton and alpha particle beams has been measured and calculated. The experimental data have been obtained by the Rutherford backscattering technique and cover the range of 120–900 and 120–3000 keV for proton and alpha particle beams, respectively. Theoretical calculations of the energy loss for the same projectiles have been done by means of the dielectric formalism using the Mermin energy loss function—generalized oscillator strength (MELF-GOS) model for a proper description of the ${\text{HfO}}_2$ target on the whole momentum-energy excitation spectrum. At low projectile energies, a nonlinear theory based on the extended Friedel sum rule has been employed. The calculations and experimental measurements show good agreement for protons and a quite good one for alpha particles. In particular, the experimental maximums of both stopping curves (around 120 and 800 keV, respectively) are well reproduced. On the basis of this good agreement, we have also calculated the inelastic mean-free path (IMFP) and the stopping power for electrons in ${\text{HfO}}_2$ films. Our results predict a minimum value of the IMFP and a maximum value of the $S$ for electrons with energies around 120 and 190 eV, respectively.

Using a frequency-domain theory, we demonstrate that an angle-resolved high-order above-threshold ionization (HATI) spectrum carries three pieces of important information: the fingerprint of the molecular wave function in the direct above-threshold-ionization amplitude, the geometrical structure of the molecule in the potential scattering between two plane waves, and the interaction between the ionized electron and the laser field, manifested in a phase factor associated with laser-assisted collisions. As a result all main interference features in the HATI spectrum can be physically explained. As an application it is pointed out that the skeleton structure of a molecule can be better imaged using lasers of higher frequencies.

We investigate the effect of periodic driving by an external field on systems with attractive pairing interactions. These include spin systems (such as the ferromagnetic $XXZ$ model) as well as ultracold fermionic atoms described by the attractive Hubbard model. We show that a well-known phenomenon seen in periodically driven systems—the renormalization of the exchange coupling strength—acts selectively on bound pairs of spins (atoms), relative to magnon (bare atom) states. Thus one can control the direction and speed of transport of bound pair relative to magnon (unpaired) atom states, and thus coherently achieve spatial separation of these components. Applications to recent experiments on transport with fermionic atoms in optical lattices which consist of mixtures of bound pairs and bare atoms are discussed.

In the present work, we investigate, through ab initio molecular dynamics simulations, the molecular switching action (a hydrogen tautomerization reaction) in porphyrazine and phthalocyanine induced by electromagnetic pulses. We find that femtosecond pulses attainable from laser sources can induce the switching mechanism in porphyrazine and phthalocyanine through several reaction pathways. We investigate the dependence of the switching mechanism on the parameters (duration, frequency, and polarization) of the electromagnetic pulse and on isotopic substitutions. We also find that the dynamics of inner hydrogen switching in phthalocyanine can be induced by weaker electromagnetic pulses than in porphyrazine. This indicates that molecules of this class with larger numbers of aromatic rings will be more efficient in displaying the pulse-induced switching.

Nonequilibrium stimulation of superfluidity in trapped Fermi gases is discussed by analogy to the work of Eliashberg [Nonequilibrium Superconductivity, edited by D. N. Langenberg and A. I. Larkin (North-Holland, New York, 1986)] on the microwave enhancement of superconductivity. Optical excitation of the fermions balanced by heat loss due to thermal contact with a boson bath and/or evaporative cooling enables stationary nonequilibrium states to exist. Such a state manifests as a shift of the quasiparticle spectrum to higher energies and this effectively raises the pairing transition temperature. As an illustration, we calculate the effective enhancement of Cooper pairing and superfluidity in both the normal and superfluid phases for a mixture of ${}^{87}\text{R}\text{b}$ and ${}^6\text{L}\text{i}$ in the limit of small departure from equilibrium. It is argued that in experiment the desirable effect is not limited to such small perturbations and the effective enhancement of the pairing temperature may be quite large.

We develop two methods to calculate the momentum distribution of the insulating (Mott and charge-density-wave) phases of the extended Bose-Hubbard model with on-site and nearest-neighbor boson-boson repulsions on $d$-dimensional hypercubic lattices. First, we construct the random-phase approximation result, which corresponds to the exact solution for the infinite-dimensional limit. Then, we perform a power-series expansion in the hopping $t$ via strong-coupling perturbation theory, to evaluate the momentum distribution in two and three dimensions; we also use the strong-coupling theory to verify the random-phase approximation solution in infinite dimensions. Finally, we briefly discuss possible implications of our results in the context of ultracold dipolar Bose gases with dipole-dipole interactions loaded into optical lattices.

The Rayleigh-Taylor instability at the interface in an immiscible two-component Bose-Einstein condensate is investigated using the mean field and Bogoliubov theories. Rayleigh-Taylor fingers are found to grow from the interface and mushroom patterns are formed. Quantized vortex rings and vortex lines are then generated around the mushrooms. The Rayleigh-Taylor instability and mushroom-pattern formation can be observed in a trapped system.

We present a calculation of the spectral functions and the associated rf response of ultracold fermionic atoms near a Feshbach resonance. The single-particle spectra are peaked at energies that can be modeled by a modified BCS dispersion. However, even at very low temperatures their width is comparable to their energy except for a small region around the dispersion minimum. The structure of the excitation spectrum of the unitary gas at infinite scattering length agrees with recent momentum-resolved rf spectra near the critical temperature. A detailed comparison is made with momentum integrated, locally resolved rf spectra of the unitary gas at arbitrary temperatures and shows very good agreement between theory and experiment. The pair size defined from the width of these spectra is found to coincide with that obtained from the leading gradient corrections to the effective-field theory of the superfluid.

We demonstrate dynamical near-complete inversion switching of a two-level quantum dot driven by picosecond optical pulses in a bimodal photonic band gap (PBG) waveguide at microwatt power levels. This is enabled by a sharp steplike discontinuity in the local (electromagnetic) density of states (LDOS) provided by a cutoff in one of the two waveguide modes. The atomic Bloch vector equations in this colored vacuum are derived using dressed state description of the atomic density operator in the presence of phonon-mediated dephasing. Instead of assuming phenomenological decay rates, we derive explicit expressions for decay terms of bare state atomic dipole moments and population in the Born-Markov approximation from a master equation approach. Giant Mollow splitting of the atom level due to subwavelength light localization of strong driving pulse causes the time-dependent Mollow bands to straddle the LDOS jump, leading to different radiative decay rates in the upper and lower Mollow sidebands. This results in remarkable field-dependent spontaneous emission and dipolar dephasing rates, combined with a novel “vacuum structure” term in the Bloch equations. Our Bloch equations predict ultrafast high-contrast inversion switching that is activated and deactivated by picosecond pulse trains detuned below and above the atomic resonance, respectively. This dynamic inversion is due to the rapid rise in relaxation rates as the pulse amplitude rises, causing the Bloch vector to switch from antiparallel to parallel alignment with the pulse “torque vector.” Subsequent near-complete inversion occurs through an adiabatic following process retained long after the pulse amplitude subsides and the system reverts to slow relaxation. For a 1% inhomogeneously broadened distribution of quantum dot with average of 100 D dipole moment, driven by $1.5\mu \text{m}$ picosecond pulses and coupled to a cutoff mode LDOS jump with radiative emission rates of ${\gamma }_{high}=2.5\text{THz}$ and ${\gamma }_{low}=5\text{GHz}$, a large average population switching contrast of 0.5 is demonstrated with a phonon dephasing rate ${\gamma }_p=0.5\text{THz}$. A 1.6 fJ control pulse is required per switching operation and a $30\mu \text{W}$ pulse train is sufficient to maintain the inversion. This switchable gain (loss) segment of the PBG waveguide can be used to controllably amplify (absorb) signal pulses conveying optical information. This provides a robust mechanism for ultrafast multiwavelength channel all-optical logic in PBG microchips.

We study the performance of linear and nonlinear optical schemes for the detection of weak signals for two classes of probe states. These are quadrature coherent squeezed states and the minimum uncertainty states of the generator of the transformation and the measured observable. Both for linear and nonlinear schemes we show that the generator-measurement minimum uncertainty states are far from being optimum, while the quadrature coherent squeezed states can reach maximum accuracy almost for the same amount of squeezing in both cases. The analysis is largely based on a suitable approximation treating the photon number as a continuous variable.

We analyze the existence and stability of nonlinear modes localized at phase-slip waveguide channels and their intersections in two-dimensional photonic lattices with rectangular symmetry. While in linear lattices only one localized mode of this type may exist under special conditions, nonlinearity supports a variety of localized modes including the modes which bifurcate from the symmetric states and describe nonlinear asymmetric localized states. We also study the surface modes localized at the waveguide channel edges in such lattices.

We present a system for the simulation of quantum phase transitions of collective internal qubit and phononic states with a linear crystal of trapped ions. The laser-ion interaction creates an energy gap in the excitation spectrum, which induces an effective phonon-phonon repulsion and a Jaynes-Cummings-Hubbard interaction. This system shows features equivalent to phase transitions of polaritons in coupled cavity arrays. Trapped ions allow for easy tuning of the hopping frequency by adjusting the axial trapping frequency and the phonon-phonon repulsion via laser detuning and intensity. We propose an experimental protocol to access all observables of the system, which allows one to obtain signatures of the quantum phase transitions even with a small number of ions.

We investigate the possibility of achieving the strong coupling regime of cavity quantum electrodynamics using rare-earth-metal-ions as impurities in monolithic optical resonators. We conclude that due to the weak oscillator strengths of the rare-earth-metals, it may be possible but difficult to reach the regime where the single photon Rabi frequency is large compared to both the cavity and atom decay rates. However, reaching the regime where the saturation photon and atom numbers are less than one should be much more achievable. We show that in this “bad cavity” regime, transfer of quantum states and an optical phase shift conditional on the state of the atom is still possible and suggest a method for coherent detection of single dopants.

We report on an experimental and theoretical systematic study of double ionization of helium by ion impact in terms of four-particle Dalitz plots. Several collision systems covering a broad range of perturbation parameters $\eta$ (projectile charge to speed ratio) were investigated. With increasing $\eta$ we observe a systematic trend from features, characteristic to correlated double-ionization mechanisms, to signatures of higher-order processes not requiring electron-electron correlations [the mechanism called “two-step–two projectile-electron interaction” (TS-2)]. The data for the largest $\eta$ can qualitatively be amazingly well described by a simple model only including the TS-2 mechanism.

A number of previous studies have suggested the possibility of two-center interference effects in the single ionization of diatomic molecules such as ${\text{H}}_2$ and ${\text{N}}_2$. While interference effects have been successfully observed in the ionization of ${\text{H}}_2$, to date evidence for interference in ${\text{N}}_2$ ionization has yet to be conclusively demonstrated. This study presents triply differential cross sections for electron impact ionization of ${\text{N}}_2$, measured using the $\left(e,2e\right)$ technique. The data are probed for signatures of two-center interference effects. Evidence for interference manifesting in the cross sections is observed.