Vortex qubit based on an annular Josephson junction containing a microshort

We report theoretical and experimental work on the development of a vortex qubit based on a microshort in an annular Josephson junction. The microshort creates a potential barrier for the vortex, which produces a double-well potential under the application of an in-plane magnetic field; The field strength tunes the barrier height. A one-dimensional model for this system is presented, from which we calculate the vortex depinning current and attempt frequency as well as the interwell coupling. Implementation of an effective microshort is achieved via a section of insulating barrier that is locally wider in the junction plane. Using a junction with this geometry we demonstrate classical state preparation and readout. The vortex is prepared in a given potential well by sending a series of "shaker" bias current pulses through the junction. Readout is accomplished by measuring the vortex depinning current.Comment: Submitted to Physical Review B (13 pages, 10 figures). Changed content to include more explanatio

High resolution measurements of the switching current in a Josephson tunnel junction: Thermal activation and macroscopic quantum tunneling

We have developed a scheme for a high resolution measurement of the switching current distribution of a current biased Josephson tunnel junction using a timing technique. The measurement setup is implemented such that the digital control and read-out electronics are optically decoupled from the analog bias electronics attached to the sample. We have successfully used this technique to measure the thermal activation and the macroscopic quantum tunneling of the phase in a small Josephson tunnel junction with a high experimental resolution. This technique may be employed to characterize current-biased Josephson tunnel junctions for applications in quantum information processing.Comment: 10 pages, 8 figures, 1 tabl

Spontaneous creation of discrete breathers in Josephson arrays

We report on the experimental generation of discrete breather states (intrinsic localized modes) in frustrated Josephson arrays. Our experiments indicate the formation of discrete breathers during the transition from the static to the dynamic (whirling) system state, induced by a uniform external current. Moreover, spatially extended resonant states, driven by a uniform current, are observed to evolve into localized breather states. Experiments were performed on single Josephson plaquettes as well as open-ended Josephson ladders with 10 and 20 cells. We interpret the breather formation as the result of the penetration of vortices into the system.Comment: 5 pages, 5 figure

Mode structure in superconducting metamaterial transmission-line resonators

FUNDAÇÃO DE AMPARO À PESQUISA E INOVAÇÃO DO ESTADO DE SANTA CATARINACNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOSuperconducting metamaterials are a promising resource for quantum-information science. In the context of circuit QED, they provide a means to engineer on-chip dispersion relations and a band structure that could ultimately be utilized for generating complex entangled states of quantum circuitry, for quantum-reservoir engineering, and as an element for quantum-simulation architectures. Here we report on the development and measurement at millikelvin temperatures of a particular type of circuit metamaterial resonator composed of planar superconducting lumped-element reactances in the form of a discrete left-handed transmission line that is compatible with circuit QED architectures. We discuss the details of the design, fabrication, and circuit properties of this system. As well, we provide an extensive characterization of the dense mode spectrum in these metamaterial resonators, which we conduct using both microwave-transmission measurements and laser-scanning microscopy. Results are observed to be in good quantitative agreement with numerical simulations and also an analytical model based upon current-voltage relationships for a discrete transmission line. In particular, we demonstrate that the metamaterial mode frequencies, spatial profiles of current and charge densities, and damping due to external loading can be readily modeled and understood, making this system a promising tool for future use in quantum-circuit applications and for studies of complex quantum systems.115120FUNDAÇÃO DE AMPARO À PESQUISA E INOVAÇÃO DO ESTADO DE SANTA CATARINACNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFUNDAÇÃO DE AMPARO À PESQUISA E INOVAÇÃO DO ESTADO DE SANTA CATARINACNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOSem informaçãoSem informaçãoAgências de fomento estrangeiras apoiaram essa pesquisa, mais informações acesse artig

Terahertz superlattice parametric oscillator

We report a GaAs/AlAs superlattice parametric oscillator. It was pumped by a microwave field (power few mW) and produced 3rd harmonic radiation (frequency near 300 GHz). The nonlinearity of the active superlattice was due to Bragg reflections of conduction electrons at the superlattice planes. A theory of the nonlinearity indicates that parametric oscillation should be possible up to frequencies above 10 THz. The active superlattice may be the object of further studies of predicted extraordinary nonlinearities for THz fields.Comment: 10 pages, 4 figure

Elliptic Solitons and Groebner Bases

We consider the solution of spectral problems with elliptic coefficients in the framework of the Hermite ansatz. We show that the search for exactly solvable potentials and their spectral characteristics is reduced to a system of polynomial equations solvable by the Gr\"obner bases method and others. New integrable potentials and corresponding solutions of the Sawada-Kotera, Kaup-Kupershmidt, Boussinesq equations and others are found.Comment: 18 pages, no figures, LaTeX'2

Reducing the impact of radioactivity on quantum circuits in a deep-underground facility

As quantum coherence times of superconducting circuits have increased from nanoseconds to hundreds of microseconds, they are currently one of the leading platforms for quantum information processing. However, coherence needs to further improve by orders of magnitude to reduce the prohibitive hardware overhead of current error correction schemes. Reaching this goal hinges on reducing the density of broken Cooper pairs, so-called quasiparticles. Here, we show that environmental radioactivity is a significant source of nonequilibrium quasiparticles. Moreover, ionizing radiation introduces time-correlated quasiparticle bursts in resonators on the same chip, further complicating quantum error correction. Operating in a deep-underground lead-shielded cryostat decreases the quasiparticle burst rate by a factor fifty and reduces dissipation up to a factor four, showcasing the importance of radiation abatement in future solid-state quantum hardware

Macroscopic Quantum Tunneling of a Fluxon in a Long Josephson Junction

Macroscopic quantum tunneling (MQT) for a single fluxon moving along a long Josephson junction is studied theoretically. To introduce a fluxon-pinning force, we consider inhomogeneities made by modifying thickness of an insulating layer locally. Two different situations are studied: one is the quantum tunneling from a metastable state caused by a single inhomogeneity, and the other is the quantum tunneling in a two-state system made by two inhomogeneities. In the quantum tunneling from a metastable state, the decay rate is estimated within the WKB approximation. Dissipation effects on a fluxon dynamics are taken into account by the Caldeira-Leggett theory. We propose a device to observe quantum tunneling of a fluxon experimentally. Required experimental resolutions to observe MQT of a fluxon seem attainable within the presently available micro-fabrication technique. For the two-state system, we study quantum resonance between two stable states, i.e., macroscopic quantum coherence (MQC). From the estimate for dissipation coefficients due to quasiparticle tunneling, the observation of MQC appears to be possible within the Caldeira-Leggett theory.Comment: 30 pages LaTeX including 11 PS figures, using jpsj.sty. To be published on J. Phys. Soc. Jpn. Overestimates for damping amplitude is correcte

Spatiotemporal dynamics of discrete sine-Gordon lattices with sinusoidal couplings

The spatiotemporal dynamics of a damped sine-Gordon chain with sinusoidal nearest-neighbor couplings driven by a constant uniform force are discussed. The velocity characteristics of the chain versus the external force is shown. Dynamics in the high- and low-velocity regimes are investigated. It is found that in the high-velocity regime, the dynamics is dominated by rotating modes, the velocity shows a branching bifurcation feature, while in the low-velocity regime, the velocity exhibits step-like dynamical transitions, broken by the destruction of strong resonances.Comment: 10 Revtex pages, 8 Eps figures, to appear in Phys. Rev.E 57(1998