Traveling small-scale Josephson vortices

K. Hirata, Shuuichi Ooi, T. Mochiku

Likai Yang, Jennifer Wang, Daryoush Shiri +6 more

The development of large-scale superconducting quantum computers critically depends on the accurate readout of qubit states. To enable this, cryogenic parametric amplifiers have been proposed and developed, and have since found widespread use in small-signal detection applications. Among various amplifier architectures, Josephson traveling-wave parametric amplifiers (JTWPAs) are widely adopted due to their broad bandwidth, high saturation power, and compatibility with superconducting qubits. In this work, we present a design workflow of JTWPAs based on a co-simulation approach combining electromagnetic (EM) and circuit-level simulation. We apply this method to a uniform JTWPA, comparing our simulation results with experimental data. To mitigate the challenge of constructing lumped element circuit models usually used in conventional simulation method, we directly perform full EM analysis of the device to accurately determine its linear response. The extracted S parameters are then fed into a nonlinear harmonic balance simulator with Josephson junctions modeled as circuit elements. By doing so, the linear and gain properties of the JTWPA are extracted. Different factors that impact device performance including gain compression, phase matching, and fabrication imperfections are further investigated and modeled. A good agreement between simulation and measurements is identified, establishing the effectiveness of our approach in designing quantum parametric amplifiers.

Andreas J. Wallraff, Alexander V. Lukashenko, Jürgen Lisenfeld +5 more

Rosario Fazio, Ulrich Geigenmüller, Gerd Schön

K. H. Lee

We present numerically the static vortex configurations in an annular Josephson ladder subject to a transverse magnetic field. When the applied magnetic field is small, the supercurrents flow only along the angular direction. In that case, the phases of the inner and outer rings are coupled each other with the same winding numbers of vorticity. Such a phase-locking remains until the magnetic field reaches a certain high value over which Josephson vortices begin to appear between the two rings. The flux imbalance of the inner and outer rings leads to the different winding numbers of vorticity and the appearance of vortices in some plaquettes. The vortex configurations for a given magnetic field are found not to be unique but to exhibit other excited states of vortices with the higher energies and lower probabilities of occurrence.