Semifluxon molecule under control

Josephson junctions with a phase drop pi in the ground state allow to create vortices of supercurrent carrying only half of the magnetic flux quantum Phi_0~2.07*10^-15 Wb. Such semifluxons have two-fold degenerate ground states denoted up (with flux +Phi_0/2 and supercurrent circulating clockwise) and down (with flux -Phi_0/2 and supercurrent circulating counterclockwise). We investigate a molecule consisting of two coupled semifluxons in a 0-pi-0 long Josephson junction. The fluxes (polarities) of semifluxons are measured by two on-chip SQUIDs. By varying the dc bias current applied to the 0-pi-0 junction, we demonstrate controllable manipulation and switching between two states, up-down and down-up, of a semifluxon molecule. These results provide a major step towards employing semifluxons as bits or qubits for classical and quantum digital electronics

Improving the performance of superconducting microwave resonators in magnetic fields

The operation of superconducting coplanar waveguide cavities, as used for circuit quantum electrodynamics and kinetic inductance detectors, in perpendicular magnetic fields normally leads to a reduction of the device performance due to energy dissipating Abrikosov vortices. We experimentally investigate the vortex induced energy losses in such Nb resonators with different spatial distributions of micropatterned pinning sites (antidots) by transmission spectroscopy measurements at 4.2 K. In comparison to resonators without antidots we find a significant reduction of vortex induced losses and thus increased quality factors over a broad range of frequencies and applied powers in moderate fields

Magnetic hysteresis effects in superconducting coplanar microwave resonators

We performed transmission spectroscopy experiments on coplanar half wavelength niobium resonators at a temperature T=4.2 K. We observe not only a strong dependence of the quality factor Q and the resonance frequency f_res on an externally applied magnetic field but also on the magnetic history of our resonators, i.e. on the spatial distribution of trapped Abrikosov vortices in the device. We find these results to be valid for a broad range of frequencies and angles between the resonator plane and the magnetic field direction as well as for resonators with and without antidots near the edges of the center conductor and the ground planes. In a detailed analysis we show, that characteristic features of the experimental data can only be reproduced in calculations, if a highly inhomogeneous rf-current density and a flux density gradient with maxima at the edges of the superconductor is assumed. We furthermore demonstrate, that the hysteretic behaviour of the resonator properties can be used to considerably reduce the vortex induced losses and to fine-tune the resonance frequency by the proper way of cycling to a desired magnetic field

Polarization squeezing of intense pulses with a fiber Sagnac interferometer

We report on the generation of polarization squeezing of intense, short light pulses using an asymmetric fiber Sagnac interferometer. The Kerr nonlinearity of the fiber is exploited to produce independent amplitude squeezed pulses. The polarization squeezing properties of spatially overlapped amplitude squeezed and coherent states are discussed. The experimental results for a single amplitude squeezed beam are compared to the case of two phase-matched, spatially overlapped amplitude squeezed pulses. For the latter, noise variances of -3.4dB below shot noise in the S0 and the S1 and of -2.8dB in the S2 Stokes parameters were observed, which is comparable to the input squeezing magnitude. Polarization squeezing, that is squeezing relative to a corresponding polarization minimum uncertainty state, was generated in S1.Comment: v4: 2 small typos corrected v3: misc problems with Tex surmounted - mysteriously missing text returned to results - vol# for Korolkova et al. PRA v2: was a spelling change in author lis

All-fibre source of amplitude-squeezed light pulses

An all-fibre source of amplitude squeezed solitons utilizing the self-phase modulation in an asymmetric Sagnac interferometer is experimentally demonstrated. The asymmetry of the interferometer is passively controlled by an integrated fibre coupler, allowing for the optimisation of the noise reduction. We have carefully studied the dependence of the amplitude noise on the asymmetry and the power launched into the Sagnac interferometer. Qualitatively, we find good agreement between the experimental results, a semi-classical theory and earlier numerical calculations [Schmitt etl.al., PRL Vol. 81, p.2446, (1998)]. The stability and flexibility of this all-fibre source makes it particularly well suited to applications in quantum information science

Dynamics of fractional vortices in long Josephson junctions

Im Rahmen dieser Arbeit wurden die statischen und dynamischen Eigenschaften fraktionaler Flußwirbel in langen Josephsonkontakten untersucht. Im Gegensatz zu den in der Supraleitung seit langem bekannten Abrikosov- und Josephsonflußwirbeln tragen fraktionale Flußwirbel nicht ein sondern nur den Bruchteil eines magnetischen Flußquants. Sie sind ortsgebundene, zu einem gewissen Grad durch äußere Kräfte verformbare Objekte, die sich spontan an den Phasensprungstellen so genannter 0-kappa-Josephsonkontakte ausbilden können. 0-kappa-Kontakte und fraktionale Flußwirbel stellen dabei eine Verallgemeinerung der weitaus bekannteren 0-pi-Kontakte und den darin auftretenden Semifluxonen dar, wobei nun nicht nur pi- sondern beliebige, durch den Parameter kappa beschriebene Sprünge der Josephsonphase betrachtet werden. Durch die Verwendung sogenannter "künstlicher", auf Nb-AlOx-Nb Technologie basierender 0-kappa-Kontakte, in denen der Sprung der Josephsonphase durch einen extern steuerbaren Strom erzeugt wird und damit beliebig einstellbar ist, konnten erstmals experimentelle Untersuchungen der klassischen Dynamik fraktionaler Flußwirbel durchgeführt werden. Zum einen gelang der Nachweis halbzahliger Nullfeldstufen, die Ausdruck eines zeitlich periodischen Umklapp-/Hüpfprozesses fraktionaler Flußwirbel in Josephsonkontakten mittlerer Länge und geringer Dämpfung sind. Zum anderen konnte mit Hilfe der Resonanzspektroskopie die oszillatorische Eigenmode fraktionaler Flußwirbel untersucht werden. Im Gegensatz zu Josephsonfluxonen sind fraktionale Flußwirbel zu einer charakteristischen Eigenschwingung um ihre Ruhelage fähig, deren Frequenz innerhalb der so genannten Plasmabandlücke liegt und über die Größe des Phasensprungs kappa und einen extern angelegten Biasstrom kontinuierlich zwischen der Plasmafrequenz und Null durchgestimmt werden kann. Die Abhängigkeit der Eigenschwingung eines einzelnen fraktionalen Flußwirbels von Biasstrom und kappa konnte erstmals experimentell bestimmt werden.In this thesis static and dynamic properties of fractional vortices in long Josephson junctions are investigated. Fractional vortices are circulating supercurrents similar to the well-known Josephson fluxons. Yet, they show the distinguishing property of carrying only a fraction of the magnetic flux quantum. Fractional vortices are interesting non-linear objects. They spontaneously appear and are pinned at the phase discontinuity points of so called 0-kappa junctions but can be bend or flipped by external forces like bias currents or magnetic fields. 0-kappa junctions and fractional vortices are generalizations of the well-known 0-pi junctions and semifluxons, where not only phase jumps of pi but arbitrary values denoted by kappa are considered. By using so-called artificial 0-kappa junctions that are based on standard Nb-AlOx-Nb technology the classical dynamics of fractional vortices has been investigated experimentally for the very first time. Here, half-integer zero field steps could be observed. These voltage steps on the junction’s current-voltage characteristics correspond to the periodic flipping/hopping of fractional vortices. In addition, the oscillatory eigenmodes of fractional vortices were investigated. In contrast to fluxons fractional vortices have an oscillatory eigenmode with a frequency within the plasma gap. Using resonance spectroscopy the dependence of the eigenmode frequency on the flux carried by the vortex and an applied bias current was determined

Trapping of ultracold atoms in a He-3/He-4 dilution refrigerator

International audienceWe describe the preparation of ultracold atomic clouds in a dilution refrigerator. The closed-cycle 3He/4He cryostat was custom made to provide optical access for laser cooling, optical manipulation and detection of atoms. We show that the cryostat meets the requirements for cold atom experiments, specifically in terms of operating a magneto-optical trap, magnetic traps and magnetic transport under ultrahigh vacuum conditions. The presented system is a step toward the creation of a quantum hybrid system combining ultracold atoms and solid-state quantum devices