Magnetic dipole induced guided vortex motion

We present evidence of magnetically controlled guided vortex motion in a hybrid superconductor/ferromagnet nanosystem consisting of an Al film on top of a square array of permalloy square rings. When the rings are magnetized with an in-plane external field H, an array of point-like dipoles with moments antiparallel to H, is formed. The resulting magnetic template generates a strongly anisotropic pinning potential landscape for vortices in the superconducting layer. Transport measurements show that this anisotropy is able to confine the flux motion along the high symmetry axes of the square lattice of dipoles. This guided vortex motion can be either re-routed by 90 degrees by simply changing the dipole orientation or even strongly suppressed by inducing a flux-closure magnetic state with very low stray fields in the rings.Comment: 5 pages, 3 figure

Conversion of bright magneto-optical resonances into dark at fixed laser frequency for D2 excitation of atomic rubidium

Nonlinear magneto-optical resonances on the hyperfine transitions belonging to the D2 line of rubidium were changed from bright to dark resonances by changing the laser power density of the single exciting laser field or by changing the vapor temperature in the cell. In one set of experiments atoms were excited by linearly polarized light from an extended cavity diode laser with polarization vector perpendicular to the light's propagation direction and magnetic field, and laser induced fluorescence (LIF) was observed along the direction of the magnetic field, which was scanned. A low-contrast bright resonance was observed at low laser power densities when the laser was tuned to the Fg=2 --> Fe=3 transition of Rb-87 and near to the Fg=3 --> Fe=4 transition of Rb-85. The bright resonance became dark as the laser power density was increased above 0.6mW/cm2 or 0.8 mW/cm2, respectively. When the Fg=2 --> Fe=3 transition of Rb-87 was excited with circularly polarized light in a second set of experiments, a bright resonance was observed, which became dark when the temperature was increased to around 50C. The experimental observations at room temperature could be reproduced with good agreement by calculations based on a theoretical model, although the theoretical model was not able to describe measurements at elevated temperatures, where reabsorption was thought to play a decisive role. The model was derived from the optical Bloch equations and included all nearby hyperfine components, averaging over the Doppler profile, mixing of magnetic sublevels in the external magnetic field, and a treatment of the coherence properties of the exciting radiation field.Comment: 9 pages, 7 figure

Optical response of a misaligned and suspended Fabry-Perot cavity

The response to a probe laser beam of a suspended, misaligned and detuned optical cavity is examined. A five degree of freedom model of the fluctuations of the longitudinal and transverse mirror coordinates is presented. Classical and quantum mechanical effects of radiation pressure are studied with the help of the optical stiffness coefficients and the signals provided by an FM sideband technique and a quadrant detector, for generic values of the product $\varpi \tau$ of the fluctuation frequency times the cavity round trip. A simplified version is presented for the case of small misalignments. Mechanical stability, mirror position entanglement and ponderomotive squeezing are accommodated in this model. Numerical plots refer to cavities under test at the so-called Pisa LF facility.Comment: 14 pages (4 figures) submitted to Phys. Rev.

Transformation of electromagnetically induced transparency into absorption in a thermal potassium optical cell with spin preserving coating

We report a new experimental approach where an order of magnitude enhancement of the electromagnetically induced absorption (EIA) resonance contrast, thus making it similar to that of the EIT resonance contrast is observed under the same conditions. The EIA signal results from the interaction of a weak probe beam with a ground state that has been driven by the pump (counter-propagating) beam. Probe absorption spectra are presented where the laser frequency is slowly detuned over the D 1 line of 39 K vapor contained in a cell with a PDMS antirelaxation coating. In addition to the frequency detuning, a magnetic field orthogonal to the laser beams is scanned around zero value at a higher rate. With both laser beams linearly polarized, an EIT resonance is observed. However, changing the pump beam polarization from linear to circular reverses the resonance signal from EIT to EIA

Stationary entanglement between two movable mirrors in a classically driven Fabry-Perot cavity

We consider a Fabry-Perot cavity made by two moving mirrors and driven by an intense classical laser field. We show that stationary entanglement between two vibrational modes of the mirrors, with effective mass of the order of micrograms, can be generated by means of radiation pressure. The resulting entanglement is however quite fragile with respect to temperature.Comment: 15 pages, 3 figure