Geometric resonance cooling of polarizable particles in an optical waveguide

In the radiation field of an optical waveguide, the Rayleigh scattering of photons is shown to result in a strongly velocity-dependent force on atoms. The pump field, which is injected in the fundamental branch of the waveguide, is favorably scattered by a moving atom into one of the transversely excited branches of propagating modes. All fields involved are far detuned from any resonances of the atom. For a simple polarizable particle, a linear friction force coefficient comparable to that of cavity cooling can be achieved.Comment: 4 page

Order by disorder in a four flavor Mott-insulator on the fcc lattice

The classical ground states of the SU(4) Heisenberg model on the face centered cubic lattice constitute a highly degenerate manifold. We explicitly construct all the classical ground states of the model. To describe quantum fluctuations above these classical states, we apply linear flavor-wave theory. At zero temperature, the bosonic flavor waves select the simplest of these SU(4) symmetry breaking states, the four-sublattice ordered state defined by the cubic unit cell of the fcc lattice. Due to geometrical constraints, flavor waves interact along specific planes only, thus rendering the system effectively two dimensional and forbidding ordering at finite temperatures. We argue that longer range interactions generated by quantum fluctuations can shift the transition to finite temperatures

Damping of quasiparticles in a Bose-Einstein condensate coupled to an optical cavity

We present a general theory for calculating the damping rate of elementary density wave excitations in a Bose-Einstein condensate strongly coupled to a single radiation field mode of an optical cavity. Thereby we give a detailed derivation of the huge resonant enhancement in the Beliaev damping of a density wave mode, predicted recently by K\'onya et al., Phys.~Rev.~A 89, 051601(R) (2014). The given density-wave mode constitutes the polariton-like soft mode of the self-organization phase transition. The resonant enhancement takes place, both in the normal and ordered phases, outside the critical region. We show that the large damping rate is accompanied by a significant frequency shift of this polariton mode. Going beyond the Born-Markov approximation and determining the poles of the retarded Green's function of the polariton, we reveal a strong coupling between the polariton and a collective mode in the phonon bath formed by the other density wave modes

The Dicke model phase transition in the quantum motion of a Bose-Einstein condensate in an optical cavity

We show that the motion of a laser-driven Bose-Einstein condensate in a high-finesse optical cavity realizes the spin-boson Dicke-model. The quantum phase transition of the Dicke-model from the normal to the superradiant phase corresponds to the self-organization of atoms from the homogeneous into a periodically patterned distribution above a critical driving strength. The fragility of the ground state due to photon measurement induced back action is calculated.Comment: 5 pages, 2 figure

Photonic tuning of quasi-particle decay in a superfluid

We show that the damping rate of elementary excitations of hybrid systems close to a phase transition can undergo a remarkable resonance like enhancement before mode softening takes place. In particular, we consider the friction of a collective density wave in a homogeneous superfluid of weakly interacting bosonic atoms coupled to the electromagnetic field of a single mode optical resonator. Here the Beliaev damping can thus be controlled by an external laser drive and be enhanced by several orders of magnitude

Photon-induced Self Trapping and Entanglement of a Bosonic Josephson Junction Inside an Optical Resonator

We study the influence of photons on the dynamics and the ground state of the atoms in a Bosonic Josephson junction inside an optical resonator. The system is engineered in such a way that the atomic tunneling can be tuned by changing the number of photons in the cavity. In this setup the cavity photons are a new means of control, which can be utilized both in inducing self-trapping solutions and in driving the crossover of the ground state from an atomic coherent state to a Schr\"odinger's cat state. This is achieved, for suitable setup configurations, with interatomic interactions weaker than those required in the absence of cavity. This is corroborated by the study of the entanglement entropy. In the presence of a laser, this quantum indicator attains its maximum value (which marks the formation of the cat-like state and, at a semiclassical level, the onset of self-trapping) for attractions smaller than those of the bare junction.Comment: 5 page

The betatron and its medical application

It is well known, that high-energy electrons can be used for tumor therapy. The so-called conventionel therapy with 100 through 250keV x· rays causes a great part of the x.rays to be scattered and absorbed in the sane tissue. In spite of the medicamental radiation prophylaxis additional radiation diseaes result by those compton scattered rays. By application of fast electrons and hard x.rays (so called gamma. rays) one tries to diminish those undesired side-effects and at the same time to increase the therapeutical effect of the ray treatment. As radiation source for fast electrons and hard gamma.rays one uses the Betatron, which was developed by NBRST in 1941 after preliminary operation of SLEPIAN, WALTON, WIDEROE and STEENDECK. The following statements are based on the references (1) through (6).</p

Structure of the perturbation series of the spin-1 Bose gas at low temperatures

The properties of Green's functions and various correlation functions of density and spin operators are considered in a homogeneous spin-1 Bose gas in different phases. The dielectric formalism is worked out and the partial coincidence of the one-particle and collective spectra is pointed out below the temperature of Bose-Einstein condensation. As an application the formalism is used to give two approximations for the propagators and the correlation functions and the spectra of excitations including shifts and widths due to the thermal cloud.Comment: 34 pages, 17 figure

SU(N) quantum spin models: A variational wavefunction study

The study of SU(N) quantum spin models is relevant to a variety of physical systems including ultracold atoms in optical lattices, and also leads to insights into novel quantum phases and phase transitions of SU(2) spin models. We use Gutzwiller projected fermionic variational wavefunctions to explore the phase diagram and correlation functions of SU(N) spin models in the self-conjugate representation, with Heisenberg bilinear and biquadratic interactions. In 1D, the variational phase diagram of the SU(4) spin chain is constructed by examining instabilities of the Gutzwiller projected free fermion ground state to various broken symmetries, and it agrees well with exact results.The spin and dimer correlations of the Gutzwiller projected free fermion state with N flavors of fermions are also in good agreement with exact and 1/N calculations for the critical points of SU(N) spin chains. In 2D, the variational phase diagram on the square lattice is obtained by studying instabilities of the Gutzwiller projected pi-flux state. The variational ground state of the pure Heisenberg model is found to exhibit long range Neel order for N=2,4 and spin Peierls order for N > 4. For N=4 and 6, biquadratic interactions lead to a complex phase diagram which includes an extended valence bond crystal in both cases, as well as a stable pi-flux phase for N=6. The spin correlations of the projected pi-flux state at N=4 are in good agreement with 1/N calculations. We find that this state also shows strongly enhanced dimer correlations, in qualitative accord with the large-N results. We compare our results with a recent QMC study of the SU(4) Heisenberg model.Comment: 22 pages, 7 figs, added references to arxiv versio