Jens Koch

Symmetries and collective excitations in large superconducting circuits

  1. David G. Ferguson,
  2. A. A. Houck,
  3. and Jens Koch
The intriguing appeal of circuits lies in their modularity and ease of fabrication. Based on a toolbox of simple building blocks, circuits present a powerful framework for achieving
new functionality by combining circuit elements into larger networks. It is an open question to what degree modularity also holds for quantum circuits — circuits made of superconducting material, in which electric voltages and currents are governed by the laws of quantum physics. If realizable, quantum coherence in larger circuit networks has great potential for advances in quantum information processing including topological protection from decoherence. Here, we present theory suitable for quantitative modeling of such large circuits and discuss its application to the fluxonium device. Our approach makes use of approximate symmetries exhibited by the circuit, and enables us to obtain new predictions for the energy spectrum of the fluxonium device which can be tested with current experimental technology.
arxiv pdf

Low-Disorder Microwave Cavity Lattices for Quantum Simulation with Photons

  1. Devin L. Underwood,
  2. Will E. Shanks,
  3. Jens Koch,
  4. and Andrew A. Houck
We assess experimentally the suitability of coupled transmission line resonators for studies of quantum phase transitions of light. We have measured devices with low photon hopping
rates t/2pi = 0.8MHz to quantify disorder in individual cavity frequencies. The observed disorder is consistent with small imperfections in fabrication. We studied the dependence of the disorder on transmission line geometry and used our results to fabricate devices with disorder less than two parts in 10^4. The normal mode spectrum of devices with a high photon hopping rate t/2pi = 31MHz shows little effect of disorder, rendering resonator arrays a good backbone for the study of condensed matter physics with photons.
arxiv pdf