Carles Altimiras

Absence of a dissipative quantum phase transition in Josephson junctions

  1. Anil Murani,
  2. Nicolas Bourlet,
  3. Hélène le Sueur,
  4. Fabien Portier,
  5. Carles Altimiras,
  6. Daniel Esteve,
  7. Hermann Grabert,
  8. Jürgen Stockburger,
  9. and Philippe Joyez
Half a century after its discovery, the Josephson junction has become the most important nonlinear quantum electronic component at our disposal. It has helped reshaping the SI system
around quantum effects and is used in scores of quantum devices. By itself, the use of Josephson junctions in the Volt metrology seems to imply an exquisite understanding of the component in every aspects. Yet, surprisingly, there have been long-standing subtle issues regarding the modeling of the interaction of a junction with its electromagnetic environment which has generated broadly accepted misconceptions and paradoxical predictions. Here, we invalidate experimentally one such prediction, namely that a Josephson junction connected to a resistor becomes insulating beyond a given value of the resistance, due to a dissipative quantum phase transition. Our work clarifies how this key quantum component should be modeled and resolves contradictions in the theory.
arxiv pdf

Tunable microwave impedance matching to a high impedance source using a Josephson metamaterial

  1. Carles Altimiras,
  2. Olivier Parlavecchio,
  3. Philippe Joyez,
  4. Denis Vion,
  5. Patrice Roche,
  6. Daniel Esteve,
  7. and Fabien Portier
We report the efficient coupling of a 50Ω microwave circuit to a high impedance conductor. We use an impedance transformer consisting of a λ/4 co-planar resonator whose inner conductor
contains an array of superconducting quantum interference devices (SQUIDs), providing the resonator with a large and tunable lineic inductance ∼80μ0, resulting in a large characteristic impedance ZC∼1kΩ. The impedance matching efficiency is characterized by measuring the shot noise power emitted by a dc biased high resistance tunnel junction connected to the resonator. We demonstrate matching to impedances in the 15 to 35kΩ range with bandwidths above 100MHz around a resonant frequency tunable in the 4 to 6GHz range.
arxiv pdf