The violation of a classical Cauchy-Schwarz (CS) inequality is identified as an unequivocal signature of spontaneous Hawking radiation in sonic black holes. This violation can be particularly large near the peaks in the radiation spectrum emitted from a resonant boson structure forming a sonic horizon. As a function of the frequency-dependent Hawking radiation intensity, we analyze the degree of CS violation and the maximum violation temperature for a double barrier structure separating two regions of subsonic and supersonic condensate flow. We also consider the case where the resonant sonic horizon is produced by a space-dependent contact interaction. In some cases, CS violation can be observed by direct atom counting in a time-of-flight experiment. We show that near the conventional zero-frequency peak, the decisive CS violation cannot occur.

We study dynamical and stationary states of solitons in dual-core waveguides which are locally coupled (fused) at one or several short segments. The model applies to planar optical waveguides, and to Bose-Einstein condensate in dual traps. Collisions of an incident soliton with single and double locally fused couplers are investigated by means of systematic simulations and several analytical methods (quasilinear, fast-soliton, and adiabatic approximations). Excitation dynamics of a soliton trapped by a local coupler is studied by means of the variational approximation, and verified by simulations. Shuttle motion of a soliton trapped in a cavity between two local couplers, and in a finite array of couplers, is studied too.

The ultrashort-range force, the van der Waals force (VWF), rises rapidly when one nanoscale waveguide (NW) is close to another one, and is stronger than the external transverse gradient force. We theoretically investigate the great influence of the VWF on the device performance in a typical NW optomechanical system consisting of a suspended silicon waveguide and a silica substrate including waveguide deformation stiction and failure mechanism. The device shows unique optically activated plastic and elastic behaviors and stiction due to the VWF. When the input optical power is above the critical power, the waveguide is sticking to the substrate and the deformation is plastic and unrecoverable, even though the total force is less than the yield strength of the waveguide material. This is important and helpful for the design and applications of optomechanical devices.

We consider a square optical lattice in two dimensions and study the effects of both the strength and symmetry of spin-orbit coupling and Zeeman field on the ground-state, i.e., Mott-insulator (MI) and superfluid (SF), phases and phase diagram, i.e., MI-SF phase-transition boundary, of the two-component Bose-Hubbard model. In particular, based on a variational Gutzwiller ansatz, our numerical calculations show that the spin-orbit-coupled SF phase is a nonuniform (twisted) one, with its phase (but not the magnitude) of the order parameter modulating from site to site. Fully analytical insights into the numerical results are also given.

We derive the statistics of scattered photons from a $\Lambda$- or ladder-type three-level emitter (3LE) embedded in a one-dimensional open waveguide. The weak probe photons in the waveguide are coupled to one of the two allowed transitions of the 3LE, and the other transition is driven by a control beam. This system shows electromagnetically induced transparency, which is accompanied with the Autler-Townes splitting (ATS) at a strong driving by the control beam, and some of these effects have been observed recently. We show that the nature of second-order coherence of the transmitted probe photons near two-photon resonance changes from bunching to antibunching to constant as the strength of the control beam is ramped up from zero to a higher value where the ATS appears.

We report experimental generation of a radially polarized Bessel-Gauss (RPBG) beam of order 1 with the help of a spatial light modulator, a spiral phase plate, and a radial polarization converter. Furthermore, we carry out a comparative study of the self-healing properties of a RPBG beam and a linearly polarized Bessel-Gauss (LPBG) beam which are blocked by a sector-shaped opaque obstacle both experimentally and numerically. Our results clearly show that the self-healing ability of a RPBG beam indeed is superior to that of a LPBG beam, and some physical interpretations are given. Our results will be useful for particle trapping and microscopy.

Quantum-information processing requires a high degree of isolation from the detrimental effects of the environment as well as an extremely precise level of control of the way quantum dynamics unfolds in the information-processing system. In this paper we show how these two goals can be ideally achieved by hybridizing the concepts of noiseless subsystems and of holonomic quantum computation. An all-geometric universal computation scheme based on nonadiabatic and non-Abelian quantum holonomies embedded in a four-qubit noiseless subsystem for general collective decoherence is proposed. The implementation details of this synergistic scheme along with the analysis of its stability against symmetry-breaking imperfections are presented.

We report results of ion neutralization of low-energy positive He ions interacting with a surface of highly oriented pyrolitic graphite as a function of the impinging energy. We found a neutralization probability close to unity for a wide energy range. This behavior is reproduced by our theory only if we take into account the electronic correlation introduced by the He first excited states considered as possible neutralization channels. The finite occupation of these excited states opens the Auger deexcitation process as a source of emitted electrons. The calculation of this electron emitted spectrum is complex as it requires the knowledge of the energies and occupations of the atomic configurations as a function of the ion trajectory. Our calculation shows the presence of high energetic secondary electrons contributing to the Auger electron emission spectra of this system [N. Bajales, L. Cristina, S. Mendoza, R. A. Baragiola, E. C. Goldberg, and J. Ferrón, Phys. Rev. Lett. 100, 227604 (2008)].

Quantum-mechanical interference with distant neighboring resonances is found to cause shifts for precision saturated fluorescence spectroscopy of the atomic helium $2^{3}S$-to-$2^{3}P$ transitions. The shifts are significant (larger than the experimental uncertainties for measurements of the intervals) despite the fact that the neighboring resonances are separated from the measured resonances by 1400 and 20$$000 natural widths. The shifts depend strongly on experimental parameters such as the angular position of the fluorescence detector, the intensity and size of laser beams, and the properties of the atomic beam. These shifts must be considered for the ongoing program of determining the fine-structure constant from the helium $2^{3}P$ fine structure.

We report on the coexistence of atomic line emission (ALE) and high-order harmonic generation (HHG) from argon with experimental conditions bridging the multiphoton and tunnel ionization regimes. Driven by sub-$\mu$J femtosecond laser pulses in tight-focusing geometry, characteristic spectra of ALE from highly excited neutral argon as well as from singly ionized argon are detected in the presence of the harmonics. The results are discussed with respect to the electronic structure of argon and the phase-matching condition of the HHG process.

We study the spin distribution and macroscopic entanglement in spinor condensates confined in an anisotropic potential under external magnetic fields. Different types of magnetic phases can be reached by tuning the magnetic dipolar interaction strength by modifying the trapping geometry. We investigate the atom-number fluctuations of the ground state and show that the different internal hyperfine states exhibit super-Poissonian, Poissonian, and sub-Poissonian distributions with different trapping geometries and strengths of the external magnetic field. We also propose a scheme to create a macroscopic maximally entangled spin state by slowly sweeping the external magnetic field.

Materials with nontrivial lattice geometries allow for the creation of exotic states of matter like topologically insulating states. Therefore searching for such materials is an important aspect of current research in solid-state physics. In the field of ultracold gases there are ongoing studies aiming to create nontrivial lattices using optical means. In this paper we study two species of fermions trapped in a square optical lattice and show how nontrivial lattices can emerge due to strong interaction between atoms. We theoretically investigate regimes of tunable parameters in which such self-assembly may take place and describe the necessary experimental conditions. Moreover, we discuss the possibility of such emergent lattices hosting topologically insulating states.

Quantum information processing, from cryptography to computation, based upon linear quantum-optical circuit elements relies heavily on the ability offered by the Hong-Ou-Mandel (HOM) effect to “route” photons from separate input modes into one of two common output modes. Specifically, the HOM effect accomplishes the path entanglement of two photons at a time such that no coincidences are observed in the output modes of a system exhibiting the effect. In this paper, we prove, in principle, that by operating a specific nanophotonic device properly, one can conditionally “bunch” coincident input photons in a way that is more configurable than with an ordinary 50:50 beam splitter, while maintaining the inherent scalability of such an on-chip device.

Non-Markovian corrections to the Markovian quantum master equation of an open quantum system are investigated up to the second order of the interaction between the system of interest and a thermal reservoir. The concept of “natural correlation” is discussed. When the system is naturally correlated with a thermal reservoir, the time evolution of the reduced density matrix looks Markovian even in a short-time regime. If the total system was initially in an “unnatural” state, the natural correlation is established during the time evolution, and after that the time evolution becomes Markovian in a long-time regime. It is also shown that for a certain set of reduced density matrices, the naturally correlated state does not exist. If the initial reduced density matrix has no naturally correlated state, the time evolution is inevitably non-Markovian in a short-time regime.

The quantum-to-classical transition of a quantum state is a topic of great interest in fundamental and practical aspects. A coarse-graining in quantum measurement has recently been suggested as its possible account in addition to the usual decoherence model. We here investigate the reconstruction of a Gaussian state (single mode and two modes) by coarse-grained homodyne measurements. To this aim, we employ two methods, the direct reconstruction of the covariance matrix and the maximum likelihood estimation (MLE), respectively, and examine the reconstructed state under each scheme compared to the state interacting with a Gaussian (squeezed thermal) reservoir. We clearly demonstrate that the coarse-graining model, though applied equally to all quadrature amplitudes, is not compatible with the decoherence model by a thermal (phase-insensitive) reservoir. Furthermore, we compare the performance of the direct reconstruction and the MLE methods by investigating the fidelity and the nonclassicality of the reconstructed states and show that the MLE method can generally yield a more reliable reconstruction, particularly without information on a reference frame (phase of input state).

We give a full explanation of the noiseless subsystem that protects a single qubit against collective errors and the corresponding recursive scheme described by C.-K. Li et al. [Phys. Rev. A 84, 044301 (2011)] from a representation theory point of view. Furthermore, we extend the construction to qudits under the influence of collective SU($d$) errors. We find that under this recursive scheme, the asymptotic encoding rate is $1/d$.

In this work, we study the photoelectron spectra of the diatomic molecule O${}_2$ in the direction perpendicular to the laser polarization. Compared with atoms, molecules have additional parameters, e.g., the orientation of the molecule and the symmetry of the initial electronic state, and thus give rise to richer strong-field phenomena. Our simulations show that for parallel and perpendicular alignments of the laser polarization with respect to the molecular axis, the photoelectron spectra display above-threshold ionization (ATI) peaks with a specific $2\hslash \omega$ separation in which the absorption of an even number of photons is absent. However, for other alignment angles, the even-order ATI peaks appear. According to our analysis, the physical mechanism of the alignment effect of the even-order ATI peaks can be attributed to the symmetry of the molecular ground-state wave function with respect to the photoelectron emission direction.