Hélène le Sueur

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

Manipulating Fock states of a harmonic oscillator while preserving its linearity

  1. Kristinn Juliusson,
  2. Simon Bernon,
  3. Xin Zhou,
  4. Vivien Schmitt,
  5. Hélène le Sueur,
  6. Patrice Bertet,
  7. Denis Vion,
  8. Mazyar Mirahimi,
  9. Pierre Rouchon,
  10. and Daniel Esteve
We present a new scheme for controlling the quantum state of a harmonic oscillator by coupling it to an anharmonic multilevel system (MLS) with first to second excited state transition
frequency on-resonance with the oscillator. In this scheme that we call „ef-resonant“, the spurious oscillator Kerr non-linearity inherited from the MLS is very small, while its Fock states can still be selectively addressed via an MLS transition at a frequency that depends on the number of photons. We implement this concept in a circuit-QED setup with a microwave 3D cavity (the oscillator, with frequency 6.4 GHz and quality factor QO=2E-6) embedding a frequency tunable transmon qubit (the MLS). We characterize the system spectroscopically and demonstrate selective addressing of Fock states and a Kerr non-linearity below 350 Hz. At times much longer than the transmon coherence times, a non-linear cavity response with driving power is also observed and explained.
arxiv pdf