Probing a transmon qubit via the ultra-strong coupling to a Josephson waveguide

  1. Javier Puertas Martinez,
  2. Sebastien Leger,
  3. Nicolas Gheereart,
  4. Remy Dassonneville,
  5. Luca Planat,
  6. Farshad Foroughi,
  7. Yuriy Krupko,
  8. Olivier Buisson,
  9. Cecile Naud,
  10. Wiebke Guichard,
  11. Serge Florens,
  12. Izak Snyman,
  13. and Nicolas Roch
Exploring the quantum world often starts by drawing a sharp boundary between a microscopic subsystem and the bath to which it is invariably coupled. In most cases, knowledge of the
physical processes occuring in the bath is not required in great detail. However, recent developments in circuit quantum electrodynamics are presenting regimes where the actual dynamics of engineered baths, such as microwave photon resonators, becomes relevant. Here we take a major technological step forward, by tailoring a centimeter-scale on-chip bath from a very long metamaterial made of 4700 tunable Josephson junctions. By monitoring how each measurable bosonic resonance of the circuit acquires a phase-shift due to its interaction with a transmon qubit, one can indirectly measure qubit properties, such as transition frequency, linewidth and non-linearity. This new platform also demonstrates the ultra-strong coupling regime for the first time in the context of Josephson waveguides. Our device combines a large number of modes (up to 10 in the present setup) that are simultaneously hybridised with the two-level system, and a broadening dominated by the artificial environment that is a sizeable fraction of the qubit transition frequency. Finally, we provide a quantitative and parameter-free model of this large quantum system, and show that the finite environment seen by the qubit is equivalent to a truly macroscopic bath.

Unexpectedly allowed transition in two inductively coupled transmons

  1. Étienne Dumur,
  2. Bruno Küng,
  3. Alexey Feofanov,
  4. Thomas Weißl,
  5. Yuriy Krupko,
  6. Nicolas Roch,
  7. Cécile Naud,
  8. Wiebke Guichard,
  9. and Olivier Buisson
We present experimental results in which the unexpected zero-two transition of a circuit composed of two inductively coupled transmons is observed. This transition shows an unusual
magnetic flux dependence with a clear disappearance at zero magnetic flux. In a transmon qubit the symmetry of the wave functions prevents this transition to occur due to selection rule. In our circuit the Josephson effect introduces strong couplings between the two normal modes of the artificial atom. This leads to a coherent superposition of states from the two modes enabling such transitions to occur.