We report on the observation of vortex formation in a Bose-Einstein condensate of ${}^{87}\text{R}\text{b}$ atoms. Vortices are generated by superimposing an oscillating excitation to the trapping potential introduced by an external magnetic field. For small amplitudes of the external excitation field we observe a bending of the cloud axis. Increasing the amplitude we observe formation of a growing number of vortices in the sample. Shot-to-shot variations in both vortex number and position within the condensed cloud are observed, probably due to the intrinsic vortex nucleation dynamics. We discuss the possible formation of vortices and antivortices in the sample as well as possible mechanisms for vortex nucleation.

We prepare diffractive planar arrays of metal nanoparticles that are chiral because of either the shape of individual particles (“molecular” chirality) or the orientation of achiral particles in the array (“structural” chirality). Both sorts of samples are shown to lead to comparable polarization changes in the diffracted light. For the case of structural chirality, one might assume that these effects can occur only through interparticle interactions, as would be the case for transmission measurements (zero-order diffraction). However, we show that the results can be explained by a simple model in which the polarization effects are based on independent scattering by individual particles, with no interparticle coupling, and with the array structure simply determining the direction of the diffraction maximum. We thus conclude that structural and molecular chiralities are indistinguishable in diffraction experiments.

An exchange-correlation energy functional involving fractional power of the one-body reduced density matrix [S. Sharma, J. K. Dewhurst, N. N. Lathiotakis, and E. K. U. Gross, Phys. Rev. B 78, 201103(R) (2008)] is applied to finite systems and to the homogeneous electron gas. The performance of the functional is assessed for the correlation and atomization energies of a large set of molecules and for the correlation energy of the homogeneous electron gas. High accuracy is found for these two very different types of systems.

An optical comb is shown to arise from a whispering gallery mode resonator pumped by two optical frequencies. Two externally excited modes couple due to Kerr nonlinearity to initially empty modes and give rise to new frequency components. This thresholdless process is much more efficient than the previously reported single-pump four-wave mixing. As a result, a few milliwatt pump is sufficient to generate strong secondary fields that efficiently generate higher-order frequency components and so on in a cascade process leading to an optical comb.

A quantum pattern recognition scheme is presented, which combines the idea of a classic Hopfield neural network with adiabatic quantum computation. Both the input and the memorized patterns are represented by means of the problem Hamiltonian. In contrast to classic neural networks, the algorithm can return a quantum superposition of multiple recognized patterns. A proof of principle for the algorithm for two qubits is provided using a liquid-state NMR quantum computer.

Recently a conditional quantum teleportation protocol has been proposed by Chen et al. [New J. Phys. 7, 172 (2005)] which is based on the collective spontaneous emission of a photon from a pair of quantum dots. We formulate a similar protocol for collective emission from a pair of atoms, one of which is entangled with a single mode of an optical cavity. We focus on the performance of the protocol as characterized by the fidelity of the teleported state and the overall success probability. We consider a strategy employing temporally resolved photodetection of the emitted photon in order to distinguish super-radiant from subradiant emission on the basis of a single detected photon. We find the fidelity approaches unity as the spacing of the atoms becomes much smaller than the emission wavelength with a success probability of 0.25 assuming perfect collection and detection of the emitted light. Incorporating finite collection and detection efficiency we find the protocol will still exceed a fidelity of 2/3 provided that the combined collection and detection efficiency is at least 3/4. We describe a cavity QED realization which could allow these requirements to be met using current experimental techniques.

The link between a quantum spin 1/2 and its associated su(2) algebra of Pauli spin matrices with Clifford algebra and quaternions is well known. A pair of spins or qubits, which are important throughout the field of quantum information for describing logic gates and entangled states, has similarly an su(4) algebra. We develop connections between this algebra and its subalgebras with the projective plane of seven elements (also related to octonions) and other entities in projective geometry and design theory.

The stopping power of antiprotons in atomic and molecular hydrogen as well as helium was calculated in an impact-energy range from 1 keV to 6.4 MeV. In the case of ${\text{H}}_2$ and He the targets were described with a single-active electron model centered on the target. The collision process was treated with the close-coupling formulation of the impact-parameter method. An extensive comparison of the present results with theoretical and experimental literature data was performed in order to evaluate which of the partly disagreeing theoretical and experimental data are most reliable. Furthermore, the size of the corrections to the first-order stopping number, the average energy transferred to the target electrons, and the relative importance of the excitation and the ionization process for the energy loss of the projectile was determined. Finally, the stopping powers of the H, ${\text{H}}_2$, and He targets were directly compared revealing specific similarities and differences of the three targets.

A modified model based on the Coulomb explosion model [H. Li et al., Phys. Rev. A 74, 023201 (2006)] is proposed to roughly estimate the nuclear fusion yields produced in the Coulomb explosion of deuterium cluster jet with irradiation of the intense laser pulses by taking the attenuation of laser energy absorbed by the clusters with a logarithmic-normal size distribution into account. The neutron yield generated inside the heated plasma filament, as the sum of the intercluster fusion yield and beam-target fusion yield, is calculated as a function of laser-cluster parameters such as the cluster size, the laser energy, and the focus spot radius and position. Only if these parameters match with each other can the neutron yield or the neutron conversion efficiency be maximized. The rough qualitative comparison about the variation tendency of fusion yield with laser-cluster parameters is made between our simulations and the reported measurements.

With a theoretical model including both the single-atom and collective responses to the two-color laser field, we investigate the influence of macroscopic effects on the generation of isolated attosecond pulse. It is shown that broadband supercontinuous high harmonics can be produced by controlling the ionization or acceleration step of high-order harmonic generation (HHG). Utilizing the broadband supercontinuum, isolated sub-100 attosecond pulse can be generated. We also investigate the influence of atomic density on the enhancement of HHG. Moreover, the distortion and dephasing of two-color field in the propagation of ionized gas medium are also discussed.

We study quantum dynamics of a Bose-Einstein condensate in a one-dimensional optical lattice in the limit of weak atom-atom interactions within the truncated Wigner approximation for a system with one dynamically unstable mode. This extends previous classical studies of a pulsating dynamical instability in which atoms periodically collect together and subsequently disperse back into the initial homogeneous state. We observe that the quasiperiodic behavior still persists in the presence of quantum fluctuations for a single realization that represents a typical experimental outcome, but ensemble averages show various manifestations of quantum statistics. Quantum effects become more prominent when the effective interaction strength is increased.

We describe the implementation of a system for studying light-matter interactions using an ensemble of ${10}^6$ cold rubidium 87 atoms trapped in a single-beam optical dipole trap. In this configuration the elongated shape of the atomic cloud increases the strength of the collective light-atom coupling. Trapping all optically allows for long storage times in a low decoherence environment. We are able to perform several thousands of measurements on one atomic ensemble with little destruction. We report results on paramagnetic Faraday rotations from a macroscopically polarized atomic ensemble. Our results confirm that strong light-atom coupling is achievable in this system which makes it attractive for single-pass quantum information protocols.

We consider the motion of a solitary wave, a nematicon, in a finite cell filled with a nematic liquid crystal. A modulation theory is developed to describe the boundary-induced bouncing of a nematicon in a one-dimensional cell and it is found to give predictions in very good agreement with numerical solutions. The boundary-induced motion is then considered numerically for a two-dimensional cell and a simple extension of the modulation theory from one to two space dimensions is then made, with good agreement being found with numerical solutions for the nematicon trajectory. The role of nematicon shape and relative position to the boundaries in its evolution is discussed.

This paper demonstrates the engineering of spatially entangled two-photon states behind a double slit by tailoring the incident pure two-photon state. We experimentally characterize many different two-photon states by measuring their complete two-photon interference patterns in the far field of the double slit. Spatial entanglement right behind the double slit can reside in either the modulus or the phase of the two-photon field. The balance between these two types of entanglement is fully controlled by experimentally utilizing the phase-front curvatures of the pump beam and the phase-matching profile. We project either a far-field image or a magnified near-field image of the two-photon source onto the double slit. Our theoretical analysis shows how the two-photon interference pattern behind the double slit effectively acts as a phase-sensitive probe of the incident two-photon field profile. We thus present phase-sensitive measurements of the generated two-photon field profile probed in an image plane of the two-photon source.

We investigate the nonlinear response of GaAs-based photonic crystal cavities at time scales which are much faster than the typical thermal relaxation rate in photonic devices. We demonstrate a strong interplay between thermally and carrier-induced nonlinear effects. We have introduced a dynamical model entailing two thermal relaxation constants which is in very good agreement with experiments. These results will be very important for photonic crystal-based nonlinear devices intended to deal with practical high repetition rate optical signals.

A growing expertise to engineer, manipulate, and probe different cold-atom analogs of electronic condensed-matter systems allows to probe properties of exotic pairing. We study paired states of spin-imbalanced ultracold atomic system of fermions with attractive short-range interactions in one-dimensional traps. Calculations are done using the Bethe ansatz technique, and the trap is incorporated into the solution via a local-density approximation. The thermodynamic Bethe ansatz equations are solved numerically and different local-density profiles are calculated for zero and finite temperatures. A procedure to identify the homogeneous-system phase diagram using local-density profiles in the trap is also proposed. Such scheme would be immediately useful for the experiments.

The entanglement properties of a multiparty pure state are invariant under local unitary transformations. The stabilizer dimension of a multiparty pure state characterizes how many types of such local unitary transformations exist for the state. We find that the stabilizer dimension of an $n$-qubit $\left(n\ge 2\right)$ graph state is associated with three specific configurations in its graph. We further show that the stabilizer dimension of an $n$-qubit $\left(n\ge 3\right)$ graph state is equal to the degree of irreducible two-qubit correlations in the state.

In this paper the effect of Dzyaloshinskii-Moriya interaction and anisotropy on the entanglement of Heisenberg chain has been studied. While the anisotropy suppresses the entanglement due to favoring of the alignment of spins, the DM interaction restores the spoiled entanglement via creation of the quantum fluctuations. Thermodynamic limit of the model and emerging of nonanalytic behavior of the entanglement have also been probed. The singularities of the entanglement correspond to the critical boundary separating different phases of the model. The singularity of the entanglement derivative approaches the critical point from the gapped phase and will be symmetric if both phases on the boundary are gapped.

We present an approach to simulating quantum computation based on a classical model that directly imitates discrete quantum systems. Qubits are represented as harmonic functions in a two-dimensional vector space. Multiplication of qubit representations of different frequencies results in exponential growth of the state space similar to the tensor-product composition of qubit spaces in quantum mechanics. Individual qubits remain accessible in a composite system, which is represented as a complex function of a single variable, though entanglement imposes a demand on resources that scales exponentially with the number of entangled qubits. We carry out a simulation of Shor’s algorithm and discuss a simpler implementation in this classical model.