We investigate the spontaneous emission of a two-level atom placed in the vicinity of a plasmonic cloak composed of a coated sphere. In the dipole approximation, we show that the spontaneous emission rate can be reduced to its vacuum value provided the atomic emission frequency lies within the plasmonic cloak frequency operation range. Considering the current status of plasmonic cloaking devices, this condition may be fulfilled for many atomic species so that we argue that atoms with a sufficiently strong transition can be used as quantum, local probes for the efficiency of plasmonic cloaks.

It has been shown by numerical simulation that the dissipative soliton resonance suppresses the appearance of new ultrashort pulses in the laser cavity that usually arise with increasing pumping. As a consequence, the energy of the pulses can reach an arbitrarily large value, which is determined by the corresponding pump power. The mechanism of the suppression is due to interaction between pulses through a saturable gain medium. The generation remains multistable: the number of pulses in steady-state operation depends on the initial conditions. In the case of multipulse generation, all pulses have identical durations, shapes, peak intensities, and chirps. The effect of maintaining single pulse operation with increasing pump power can be used to generate high-energy pulses.

We show that an antiferrosmectic-$C$ phase has lower energy at high densities than the nonmagnetized Fermi gas and ferronematic phases in an ultracold gas of fermionic atoms, or molecules, with large magnetic dipole moments. This phase, which is analogous to meson condensation in dense nuclear matter, is a one-dimensional periodic structure in which the fermions localize in layers with their pseudospin direction aligned parallel to the layers and staggered layer by layer.

We experimentally demonstrate the achievement of the entanglement-assisted capacity for classical information transmission over a depolarizing channel. The implementation is based on the generation and local manipulation of two-qubit Bell states, which are finally measured at the receiver by realizing projective measurements in the Bell basis. The depolarizing channel is realized by introducing quantum noise in a controlled way on one of the two qubits. This work represents an investigation into the amount of information that can be shared in the presence of noise.

We show that the standard quantum-walk quantum-to-classical transition, characterized by ballistic-to-diffusive spreading of the walker's position, can be controlled by externally modulating the coin state. We illustrate this by showing an oscillation between classical diffusive and quantum ballistic spreading using numerical and asymptotically exact closed-form solutions, and we prove that the walker is in a controllable incoherent mixture of classical and quantum walks with a reversible quantum-to-classical transition.

Local unitary invariance and the notion of negativity fonts are used as the principal tools to construct four-qubit invariants of degree 8, 12, and 24. A degree-8 polynomial invariant that is nonzero on pure four-qubit states with four-body quantum correlations and zero on all other states is identified. Classification of four-qubit states into seven major classes, using criteria based on the nature of correlations, is discussed.

The ionic fragmentation of a multiply charged CO molecule is studied under impact of 10-keV electrons using recoil-ion momentum spectroscopy. The kinetic-energy-release distributions for the various fragmentation channels arising from the dissociation of CO${}^{q+}$ ($q$ = 2–4) are measured and discussed in light of theoretical calculations available in the literature. It is observed that the present kinetic-energy-release values are much smaller than those predicted by the Coulomb explosion model. The kinetic-energy-release distribution for the C${}^{+}$+O${}^{+}$ channel is suggested to arise from the tunneling process. It is seen that the peak of kinetic-energy-release distribution is larger for that dissociation channel that arises from the same molecular ion which has higher charge on the oxygen atom. Further, the relative ionic fractions for seven ion species originating from ionization and subsequent dissociation of the CO molecule are obtained and compared with the existing data reported at low energy of the electron impact. The precursor-specific relative partial ionization cross sections are also obtained and shown to be about 66.4$\%$ from single ionization, 29.9$\%$ from double ionization, 3.3$\%$ from triple ionization, and about 0.4$\%$ from quadruple ionization of the precursor CO molecule contributing to the total fragment ion yield.

We theoretically investigate the electron transfer dynamics during the reflection of hydrogen atoms on an Al(100) surface for a wide range of collision energies below 6 keV. We find a nonmonotonic variation of the hydrogen-negative-ion fractions as functions of the projectile impact velocity due to nonadiabatic electron transfer. Our calculated anion fractions for projectiles scattered along high Miller-index crystal-surface directions are in good quantitative agreement with measured H${}^{-}$ fractions for a wide range of exit velocities.

Characteristic fringe patterns of spectral interference in dynamic Rabi sidebands are used as a means to investigate decoherence phenomena in a highly nonequilibrium underdense microplasma formed in atmospheric-pressure oxygen gas in the wake of a strong-field (∼10${}^{13}$ W/cm${}^2$) femtosecond pump pulse. The same transient processes in the nonequilibrium plasma work both to create the effective two-level systems producing coherent sidebands and to destroy the coherence and smear the sideband spectral structure; the observed spectral fringe pattern is a result of the trade-off of these two effects. The sidebands generated on a moderately intense (∼10${}^{10}$ W/cm${}^2$) picosecond laser pulse reveal the decoherence rate as a function of the pump-pulse intensity and the pump-probe delay time. The rate increases with the pump laser intensity and decreases with the delay with corresponding decoherence times in the range of 750 fs to 3 ps, in good agreement with theory revealing that electron scattering dominates the dynamics for the subnanosecond relaxation processes.

We present a robust, ab initio method for addressing atom-light interactions and apply it to photoionization of argon. We use a close-coupling ansatz constructed on a multiconfigurational Hartree-Fock description of localized states and $B$-spline expansions of the electron radial wave functions. In this implementation, the general many-electron problem can be tackled thanks to the use of the atsp2k libraries [C. Froese Fischer et al., Comput. Phys. Commun. 176, 559 (2007)]. In the present contribution, we combine this method with exterior complex scaling, thereby allowing for the computation of the complex partial amplitudes that encode the whole dynamics of the photoionization process. The method is validated on the $3s3p^{6}np$ series of resonances converging to the $3s$ extraction. Then, it is used for computing the energy dependent differential atomic delay between $3p$ and $3s$ photoemission, and agreement is found with the measurements of Guénot et al. [Phys. Rev. A 85, 053424 (2012)]. The effect of the presence of resonances in the one-photon spectrum on photoionization delay measurements is studied.

Electric field noise is a hinderance to the assembly of large-scale quantum computers based on entangled trapped ions. Apart from ubiquitous technical noise, experimental studies of trapped ion heating have revealed additional limiting contributions to this noise, originating from atomic processes on the electrode surfaces. In a recent work [Safavi-Naini, Rabl, Weck, and Sadeghpour, Phys. Rev. A 84, 023412 (2011)] we described a microscopic model for this excess electric field noise, which points a way towards a more systematic understanding of surface adsorbates as progenitors of electric field jitter noise. Here, we address the impact of surface monolayer contamination on adsorbate-induced noise processes. Using exact numerical calculations for H and N atomic monolayers on a Au(111) surface, representing opposite extremes of physisorption and chemisorption, we show that an additional monolayer can significantly affect the noise power spectrum and, respectively, enhance and suppress the heating rates.

The purpose of this paper is to compare two unitary, deterministic approaches to the study of the interaction of quantized fields with atomic systems in optical cavities. In particular it is shown that the “modes of the universe” approach, in which each mode has both an intracavity and an outside part, is formally equivalent, in an appropriate limit, to the better known “input-output” formalism in its unitary (i.e., Hermitian-Hamiltonian based) form; the latter may be called a “quasimode” theory, since it treats the field inside the cavity as a separate degree of freedom. Differences that arise between the two approaches for numerical calculations involving a finite set of discrete modes are also pointed out. The formalism is illustrated with an in-depth discussion of the explicit solution for a single-photon pulse, of arbitrary shape and detuning, incident on a cavity containing a two-level atom.

We demonstrate a generic and robust mechanism that leads to an extreme output sensitivity to a deep subwavelength boundary perturbation in wavelength-scale microcavities. A deformation of the cavity boundary on the order of ten-thousandth of a wavelength may flip the output directions by ${180}^{\circ }$, corresponding to a variation of 0.1 nm for a 1-$\mu \text{m}$-radius cavity. Our analysis based on a perturbation theory reveals that such tiny structural change can cause a strong mixing of nearly degenerate cavity resonances with different angular momenta, and their interference is greatly enhanced to have a radical influence on the far-field pattern. Our finding opens the possibility of utilizing carefully designed wavelength-scale microcavities for fast beam steering and high-resolution detection.

We demonstrate the enhanced quantum noise reduction of the output field from an optical parametric amplifier (OPA) by employing the cascaded optical parametric amplifiers (COPA), in which the output field of one OPA is the input field of the subsequent one. The output field noise formulas of the cascaded degenerate and nondegenerate OPAs (CDOPA and CNOPA) are obtained. The analytical results show that a CDOPA and a CNOPA can generate squeezing beams and entangled beams with a wider bandwidth under a low pump parameter, respectively. The cavity output efficiency and the intracavity loss are the main factors for improving the degrees of squeezing and entanglement. Our work will provide improved schemes for designing COPAs to efficiently produce high-quality squeezed and entangled states, which have potential applications in quantum-information processing.

We study the quantum Rabi model within the framework of the analytical solution developed in Phys. Rev. Lett. 107, 100401 (2011). In particular, through time-dependent correlation functions, we give a quantitative criterion for classifying two regions of the quantum Rabi model, involving the Jaynes-Cummings, the ultrastrong-coupling, and the deep strong-coupling regimes. In addition, we find a stationary qubit-field entangled basis that governs the whole dynamics as the coupling strength overcomes the mode frequency.

We have simultaneously studied electromagnetically induced transparency (EIT) and Raman Ramsey fringes (RRF) in sodium vapor, in the same theoretical framework and the same experimental scheme. RRF spectra and EIT spectra can be simply distinguished by whether the gate timing of the detection is set at the beginning (RRF) or at the end (EIT) of the excitation pulses. The EIT spectral peaks exhibited pronounced ac Stark shifts linearly depending upon the pumping power (8.8 kHz/mW), while in the RRF spectra such shifts were almost negligible (0.19 kHz/mW), provided that the pulse separation is sufficiently large. This fact promotes RRF as a strong candidate for next-generation compact atomic clocks. Transient spectra in between RRF and EIT have also been investigated by changing the gate timing within the excitation pulse, and the transition time from RRF to EIT was found to be of the order of a few microseconds.

The eigenmodes of a nanosphere configuration where the core is an ensemble of two-level atoms with resonance frequency ${\omega }_C$, and the shell is an ensemble of two-level atoms but with shifted resonant frequency ${\omega }_B$ are obtained for different values of the ratio of the core radius to the shell outer radius as function of $\left({\omega }_C-{\omega }_B\right)$. I show that the eigenmodes belong to one of two branches. The dominant decay mode for superradiant emission may belong to either branch, depending on the system's parameters. In instances of configurations where the cooperative decay rate values for the two branches become equal, the cooperative exponential amplification (decay) is accompanied by temporal oscillations.

We extend the security analysis of continuous variable quantum-key-distribution protocols using a family of post selection schemes to account for arbitrary eavesdropping attacks. We show that the postselection protocol is equivalent to a virtual entanglement-based protocol including a distillation stage. We introduce a particular ‘Gaussian' post selection and demonstrate that the security can be calculated using only experimentally accessible quantities. Finally, we explicitly evaluate the performance for the case of a noisy Gaussian channel in the limit of unbounded key length and find improvements over all pre-existing continuous variable protocols in realistic regimes.

We study the quantum phases of one-dimensional Bose-Fermi mixtures in optical lattices. Assuming repulsive interparticle interactions, equal mass, and unit total filling, we calculate the ground-state phase diagram by means of both the Tomonaga-Luttinger liquid theory and time-evolving block decimation method. We demonstrate the existence of a counterflow superfluid phase of polaron pairs, which are composite particles consisting of two fermions and two bosonic holes, in a broad range of the parameter space. We find that this phase naturally emerges in ${}^{174}$Yb-${}^{173}$Yb mixtures, realized in recent experiments, at low temperatures.