We report on the observation of spatially-localized excitations in a ladder of small Josephson junctions. The excitations are whirling states which persist under a spatially-homogeneous force due to the bias current. These states of the ladder are visualized using a low temperature scanning laser microscopy. We also compute breather solutions with high accuracy in corresponding model equations. The stability analysis of these solutions is used to interpret the measured patterns in the I − V characteristics.The present decade has been marked by an intense theoretical research on dynamical localization phenomena in spatially discrete systems, namely on discrete breathers (DB). These exact solutions of the underlying equations of motion are characterized by periodicity in time and localization in space. Away from the DB center the system approaches a stable (typically static) equilibrium. (For reviews see [1], [2]). These solutions are robust to changes of the equations of motion, exist in translationally invariant systems and any lattice dimension. Discrete breathers have been discussed in connection with a variety of physical systems such as large molecules, molecular crystals [3], spin lattices [4,5], to name just a few.For a localized excitation such as a DB, the excitation of plane waves which might carry the energy away from the DB does not occur due to the spatial discreteness of the system. The discreteness provides a cutoff for the wave length of plane waves and thus allows to avoid resonances of all temporal DB harmonics with the plane waves. The nonlinearity of the equations of motion is needed to allow for the tuning of the DB frequency [1].Though the DB concept was initially developed for conservative systems, it can be easily extended to dissipative systems [6]. There discrete breathers become timeperiodic spatially localized attractors, competing with other (perhaps nonlocal) attractors in phase space. The characteristic property of DBs in dissipative systems is that these localized excitations are predicted to persist under the influence of a spatially homogeneous driving force. This is due to the fact, that the driving force compensates the dissipative losses of the DB.So far research in this field was predominantly theoretical. Identifying and analyzing of experimental systems for the direct observation of DBs thus becomes a very actual and challenging problem. Experiments on localization of light propagating in weakly coupled optical waveguides [7], low-dimensional crystals [8] and antiferromagnetic materials [9] have been recently reported. In this work we realize the theoretical proposal [10] to observe DB-like localized excitations in arrays of coupled Josephson junctions. A Josephson junction is formed between two superconducting islands. Each island is characterized by a macroscopic wave function Ψ ∼ e iθ of the superconducting state. The dynamics of the junction is described by the time evolution of the gauge-invariant phase difference ϕ = θ 2 − θ 1 − 2π Φ0A · ds between adjacent islands. H...

Interfacing photonic and solid-state qubits within a hybrid quantum architecture offers a promising route towards large scale distributed quantum computing. Ideal candidates for coherent qubit interconversion are optically active spins, magnetically coupled to a superconducting resonator. We report on an on-chip cavity QED experiment with magnetically anisotropic Er(3+)∶Y2SiO5 crystals and demonstrate collective strong coupling of rare-earth spins to a lumped element resonator. Moreover, the electron spin resonance and relaxation dynamics of the erbium spins are detected via direct microwave absorption, without the aid of a cavity.

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The difference between the phases of superconducting order parameter plays in superconducting circuits the role similar to that played by the electrostatic potential difference required to drive a current in conventional circuits. This fundamental property can be altered by inserting in a superconducting circuit a particular type of weak link, the so-called Josephson π-junction having inverted current-phase relation and enabling a shift of the phase by π. We demonstrate the operation of three superconducting circuits -two of them are classical and one quantum -which all utilize such π-phase shifters realized using superconductor-ferromagnet-superconductor sandwich technology. The classical circuits are based on single-flux-quantum cells, which are shown to be scalable and compatible with conventional niobium-based superconducting electronics. The quantum circuit is a π-phase biased qubit, for which we observe coherent Rabi oscillations and compare the measured coherence time with that of conventional superconducting phase qubits. 1 arXiv:1005.1581v1 [cond-mat.supr-con]

The ability to switch the coupling between quantum bits (qubits) on and off is essential for implementing many quantum-computing algorithms. We demonstrated such control with two flux qubits coupled together through their mutual inductances and through the dc superconducting quantum interference device (SQUID) that reads out their magnetic flux states. A bias current applied to the SQUID in the zero-voltage state induced a change in the dynamic inductance, reducing the coupling energy controllably to zero and reversing its sign.