Valla Fatemi

Distinguishing parity-switching mechanisms in a superconducting qubit

  1. Spencer Diamond,
  2. Valla Fatemi,
  3. Max Hays,
  4. Heekun Nho,
  5. Pavel D. Kurilovich,
  6. Thomas Connolly,
  7. Vidul R. Joshi,
  8. Kyle Serniak,
  9. Luigi Frunzio,
  10. Leonid I. Glazman,
  11. and Michel H. Devoret
Single-charge tunneling is a decoherence mechanism affecting superconducting qubits, yet the origin of excess quasiparticle excitations (QPs) responsible for this tunneling in superconducting
devices is not fully understood. We measure the flux dependence of charge-parity (or simply, „parity“) switching in an offset-charge-sensitive transmon qubit to identify the contributions of photon-assisted parity switching and QP generation to the overall parity-switching rate. The parity-switching rate exhibits a qubit-state-dependent peak in the flux dependence, indicating a cold distribution of excess QPs which are predominantly trapped in the low-gap film of the device. Moreover, we find that the photon-assisted process contributes significantly to both parity switching and the generation of excess QPs by fitting to a model that self-consistently incorporates photon-assisted parity switching as well as inter-film QP dynamics.
arxiv pdf

Weyl Josephson Circuits

  1. Valla Fatemi,
  2. Anton R. Akhmerov,
  3. and Landry Bretheau
We introduce Weyl Josephson circuits: small Josephson junction circuits that simulate Weyl band structures. We first formulate a general approach to design circuits that are analogous
to Bloch Hamiltonians of a desired dimensionality and symmetry class. We then construct and analyze a six-junction device that produces a 3D Weyl Hamiltonian with broken inversion symmetry and in which topological phase transitions can be triggered \emph{in situ}. We argue that currently available superconducting circuit technology allows experiments that probe topological properties inaccessible in condensed matter systems.
arxiv pdf