We propose a simple way to generate nonlinear excitations in a controllable way by managing interactions in Bose-Einstein condensates. Under the action of a quantum analog of a classical piston, the condensed atoms are pushed through the trap, generating vortex rings infully three-dimensional condensates or soliton trains in quasi-one-dimensional scenarios. The vortex rings form due to transverse instability of the shock-wave train, enhanced and supported by the energy transfer between waves. We elucidate in what sense the self-interactions within the atom cloud define the properties of the generated vortex rings and soliton trains. Based on the quantum-piston scheme we study the behavior of two-component Bose-Einstein condensates and analyze how the presence of an additional superfluid influences the generation of vortex rings or solitons in the other component, and vice versa. Finally, we show the dynamical emergence of skyrmions within two-component systems in the immiscible regime.

We theoretically investigated the ground states of coupled arrays of cavity-quantum-electrodynamical (cavity-QED) systems in the presence of two photon modes. Within the Gutzwiller-type variational approach, we found the first-order quantum phase transition between Mott-insulating and superfluid phases as well as the conventional second-order one. The first-order phase transition was found only for specific types of emitter models, and its physical origin is clarified based on the analytic arguments which are allowed in the perturbative and semiclassical limits. The first-order transition of the correlated photons is accompanied with discontinuous change in the emitter states, not only with the appearance of intercavity coherence in the superfluid phase. We also discuss the condition for the first-order transition to occur, which can lead to a strategy for future design of quantum-optical switching devices with cavity-QED arrays.

The optical ring-down time in a silica microsphere resonator is measured using phase-shift cavity ring-down spectroscopy. When detecting Rayleigh backscattered light that is coupled into the input waveguide it is found that the shift of the modulation phase angle, $\Delta \varphi$, is approximately linearly dependent on the amplitude modulation frequency. The microresonator therefore behaves as if it was a delay line. A model involving two different time scales for the buildup of the Rayleigh backscattered light and the decay of the whispering gallery modes is used to explain the observations.

We suggest a scheme to manipulate the generation dynamics of high-$Q$ microresonator-based frequency combs (microcombs). Our simulation demonstrates that a certain subfamily of comb teeth can be suppressed via selectively filtering out limited modes, from which high-quality mode-locked pulses can be generated. Mode-pulling in microcomb generation is also analyzed quantitatively to explain the results. These results can help us better understand microcomb generation dynamics and will be beneficial in controlling the generated comb line number and spacing.

The full Maxwell equations for a system of concentric shells of a noble metal and a semiconductor, with a very sharp excitonic resonance with a bare frequency in the gap between the bare plasmonic resonant frequency branches and having a strong oscillator strength, are solved. The plasmon-exciton coupling model is verified to produce for this dressed plexcition a spectrum that consists of a splitting of the bare excitonic resonance into two daughters and a shift in the plasmonic resonances from their bare values. The spectral region separating the excitonic daughters exhibits the features characteristic of induced transparency and slow light propagation. Furthermore, I show that the eigenvalues of this coupled system as herein are computed to significantly differ from those obtained in the electrostatic approximation where retardation and radiation reaction terms are neglected.

We analyze the nonequilibrium behavior of driven nonlinear photonic resonator arrays under the selective excitation of specific photonic many-body modes. Targeting the unit-filled ground state, we find a counterintuitive “superbunching” in the emitted photon statistics in spite of relatively strong on-site repulsive interaction. We consider resonator arrays with Kerr nonlinearities described by the Bose-Hubbard model, but also show that an analogous effect is observable in near-future experiments coupling resonators to two-level systems as described by the Jaynes-Cummings-Hubbard Hamiltonian. For the experimentally accessible case of a pair of coupled resonators forming a photonic molecule, we provide an analytical explanation for the nature of the effect.

We theoretically show how two impurity defects in a crystalline structure can be entangled through coupling with the crystal. We demonstrate this with a harmonic chain of trapped ions in which two ions of a different species are embedded. Entanglement is found for sufficiently cold chains and for a certain class of initial, separable states of the defects. It results from the interplay between localized modes which involve the defects and the interposed ions, it is independent of the chain size, and it decays slowly with the distance between the impurities. These dynamics can be observed in systems exhibiting spatial order; viable realizations are optical lattices, optomechanical systems, or cavity arrays in circuit QED.

Inhomogeneous superfluidity lies at the heart of many intriguing phenomena in quantum physics. It is believed to play a central role in unconventional organic or heavy-fermion superconductors, chiral quark matter, and neutron star glitches. However, so far even the simplest form of inhomogeneous superfluidity, the Fulde-Ferrell (FF) pairing state with a single center-of-mass momentum, is not conclusively observed due to the intrinsic complexibility of any realistic Fermi systems in nature. Here we theoretically predict that the controlled setting of ultracold fermionic atoms with synthetic spin-orbit coupling induced by a two-photon Raman process, demonstrated recently in cold-atom laboratories, provides a promising route to realize the long-sought FF superfluidity. At experimentally accessible low temperatures, the FF superfluid state dominates the phase diagram, in sharp contrast to the conventional case without spin-orbit coupling. We show that the finite center-of-mass momentum carried by Cooper pairs is directly measurable via momentum-resolved radio-frequency spectroscopy. Our work opens the way to direct observation and characterization of inhomogeneous superfluidity.

We address the experimental determination of entropic quantum discord for systems made of a pair of polarization qubits. We compare results from full and partial tomography and found that the two determinations are statistically compatible, with partial tomography leading to a smaller value of discord for depolarized states. Despite the fact that our states are well described, in terms of fidelity, by families of depolarized or phase-damped states, their entropic discord may be largely different from that predicted for these classes of states, such that no reliable estimation procedure beyond tomography may be effectively implemented. Our results, together with the lack of an analytic formula for the entropic discord of a generic two-qubit state, demonstrate that the estimation of quantum discord is an intrinsically noisy procedure. Besides, we question the use of fidelity as a figure of merit to assess quantum correlations.

The reduced time evolution of a single qubit under the influence of a classical environment is studied by means of the stochastic Liouville equation. The qubit initially correlates with the environment, which is created by a preparation process of an initial state. The phase fluctuation caused by the environment is characterized by means of the Gauss-Markov process and the two-state-jump Markov process. It is shown that depending on values of the parameters, the initial correlation enhances or reduces the decoherence of the qubit. In the case of the slow modulation, the effect of the initial preparation of the qubit becomes significant and it cannot be ignored in the reduced time evolution of the qubit.

We study the behavior of bipartite entanglement at fixed von Neumann entropy. We look at the distribution of the entanglement spectrum, that is, the eigenvalues of the reduced density matrix of a quantum system in a pure state. We report the presence of two continuous phase transitions, characterized by different entanglement spectra, which are deformations of classical eigenvalue distributions.

We study distribution schemes for a polarization entangled photon pair based on a decoherence-free subspace over lossy quantum channels and propose an efficient scheme that is robust against not only collective phase noises but also general collective noises for two qubits. While the proposed scheme employs two photons to build the decoherence-free subspace, the success probability is proportional to the channel transmittance of a single photon with the aid of a counter-propagating weak coherent light pulse. The key ingredient in the scheme is found to be the reciprocity of the channel. The proposed scheme shares the rest of the properties with the previously proposed schemes; i.e., it can be realized by linear optical elements and it is robust against the fluctuations in the optical circuits used by the two parties.

We present a localized solution to the problem of entanglement degradation in noninertial frames. A two-mode squeezed state is considered from the viewpoint of two observers, Alice (inertial) and Rob (accelerated), each observing a single localized mode of the field. We study the state of these modes to determine how much entanglement the observers can extract from the initial state. The dominant source of degradation is an inevitable mode mismatch between the mode of the squeezed state Rob is given and the mode he is able to observe from his accelerated frame. Leakage of the initial mode through Rob's horizon places a limit on his ability to fully measure the state, leading to an inevitable degradation of entanglement that even in principle cannot be fully retrieved by any measurement device.

We report on a technique that allows for the entanglement manipulation of qudit states encoded in the linear transverse momentum of photon pairs generated in the process of spontaneous parametric downconversion. It relies on the use of spatial light modulators and interferometric techniques for implementing the desired local measurements. As an application of our method we perform the optimal entanglement concentration of entangled qutrit states, where maximally entangled states are obtained with the maximal success probability. The procedure of entanglement concentration is fundamental for many quantum information tasks, thus showing the potential of our technique for future experimental investigations considering high-dimensional Hilbert spaces.

Scaling of quantum capacitances and valence electron detachment energies is studied for icosahedral and nonicosahedral fullerenes. Scaling trends are considered from zero to infinite average radius, where a fullerene's local surface properties are similar to those of graphene. Detailed density-functional-theory calculations are performed to determine the geometries and detachment energies of icosahedral fullerenes, while values of these quantities are obtained for nonicosahedral species from previously published experimental results. Strongly linear, quasiclassical scaling versus average radii ${\overline{r}}_n$ is seen for the quantum capacitances, but on two different scaling lines for icosahedral and nonicosahedral species, respectively. By contrast, nonclassical, nonlinear scaling versus $1/{\overline{r}}_n$ is seen for the electron detachment energies, i.e., the valence ionization potentials and electron affinities. This nonlinearity is not accounted for by classical theories that are used to explain trends in electronic properties of fullerenes and usually give accurate quantitative estimates. Instead, simple quantum equations are derived to account for nonlinearities in the metal-particle-like electron detachment energy scaling and to show that these are responsible for nonclassical, nonzero intercepts in the capacitance scaling lines of the fullerenes. Last, it is found that points representing the carbon atom and the graphene limit lie on scaling lines for icosahedral fullerenes, so their quantum capacitances and their detachment energies scale smoothly from one C atom, to C${}_{60}$, to graphene.

A combined experimental and theoretical investigation of the dissociative-electron-attachment (DEA) dynamics in methanol are presented for the Feshbach resonance at 6.5-eV incident electron energy. Highly differential laboratory-frame momentum distributions have been measured for each fragmentation channel using a DEA reaction microscope. These measurements are combined with calculations of the molecular-frame electron attachment probability in order to investigate the dynamics of the dissociating methanol transient negative anion. In contrast to previous comparisons between water and methanol [Curtis and Walker, J. Chem. Soc., Faraday Trans. 88, 2805 (1992); Prabhudesai, Nandi, Kelkar, and Krishnakumar, J. Chem. Phys. 128, 154309 (2008)], we find subtle differences in the dissociation dynamics of the two fragment channels that are direct evidence of planar symmetry-breaking of warm methanol in its electronic ground state. We also find that the DEA fragmentation does not strictly follow the axial recoil approximation and we describe the dynamics that enable an accurate prediction of the fragment angular distributions.

Based on the analytic model of Feshbach resonances in harmonic traps described in Schneider et al. [Phys. Rev. A 83, 030701(R) (2011)] a Bose-Hubbard model is introduced that provides an accurate description of two atoms in an optical lattice at a Feshbach resonance with only a small number of Bloch bands. The approach circumvents the problem that the eigenenergies in the presence of a delta-like coupling do not converge to the correct energies, if an uncorrelated basis is used. The predictions of the Bose-Hubbard model are compared to nonperturbative calculations for both the stationary states and the time-dependent wave function during an acceleration of the lattice potential. For this purpose, a square-well interaction potential is introduced, which allows for a realistic description of Feshbach resonances within nonperturbative single-channel calculations.

This work proposes a theoretical study of the $\mathrm{Li}(2p\leftarrow 2s)$ photoabsorption spectra perturbed by ground hydrogen atoms. The temperature effect on the far-wing spectra is examined in the temperature range $4000–20000\text{K}.$ For this purpose, the ground and excited LiH potential curves and the transition dipole moments that connect them are constructed from ab initio data. The investigation shows that the profile spectra are dominated at all temperatures by the singlet $A\leftarrow X$ transitions and exhibit in the red wing a satellite structure near the wavelength $1000\text{nm}.$ The spectra revealed also the appearance beyond $14000\text{K}$ of a second satellite structure in the blue wing close to the wavelength $510\text{nm}$ originating from the triplet $c\leftarrow a$ transitions.

Superelastic electron-collision studies have been carried out with a transition-metal target. ${}^{107}\mathrm{Ag}$ atoms were excited from the $5{}^2{S}_{1/2}(F=1)$ ground state to the $5{}^2{P}_{3/2}(F^{\prime }=2)$ excited state using continuous-wave circularly polarized laser radiation at ∼328.1 nm. An electron beam of well-defined energy $E_{\mathrm{inc}}$ then deexcited the targets, and the superelastically scattered electrons that gained energy from the collision were detected as a function of scattering angle and laser polarization. Results for the angular momentum $L_{\perp }$ transferred to the target are presented over a range of scattering angles, for six outgoing energies from $E_{\mathrm{out}}=100\mathrm{eV}$ down to $20\mathrm{eV}$.

We observed elastic collisions between laser-cooled fermionic lithium atoms and calcium ions at the energy range from 100 mK to 3 K. Lithium atoms in an optical-dipole trap were transported to the center of the ion trap using an optical-tweezer technique, and a spatial overlap of the atoms and ions was realized in order to observe the atom-ion interactions. The elastic-scattering rate was determined from the decay of atoms due to elastic collisions with ions. The collision-energy dependence of the elastic-scattering cross section was consistent with semiclassical collision theory.