Anil Murani

Autoparametric resonance extending the bit-flip time of a cat qubit up to 0.3 s

  1. Antoine Marquet,
  2. Antoine Essig,
  3. Joachim Cohen,
  4. Nathanaël Cottet,
  5. Anil Murani,
  6. Emanuele Abertinale,
  7. Simon Dupouy,
  8. Audrey Bienfait,
  9. Théau Peronnin,
  10. Sébastien Jezouin,
  11. Raphaël Lescanne,
  12. and Benjamin Huard
Cat qubits, for which logical |0⟩ and |1⟩ are coherent states |±α⟩ of a harmonic mode, offer a promising route towards quantum error correction. Using dissipation to our advantage
so that photon pairs of the harmonic mode are exchanged with single photons of its environment, it is possible to stabilize the logical states and exponentially increase the bit-flip time of the cat qubit with the photon number |α|2. Large two-photon dissipation rate κ2 ensures fast qubit manipulation and short error correction cycles, which are instrumental to correct the remaining phase-flip errors in a repetition code of cat qubits. Here we introduce and operate an autoparametric superconducting circuit that couples a mode containing the cat qubit to a lossy mode whose frequency is set at twice that of the cat mode. This passive coupling does not require a parametric pump and reaches a rate κ2/2π≈2 MHz. With such a strong two-photon dissipation, bit-flip errors of the autoparametric cat qubit are prevented for a characteristic time up to 0.3 s with only a mild impact on phase-flip errors. Besides, we illustrate how the phase of a quantum superposition between |α⟩ and |−α⟩ can be arbitrarily changed by driving the harmonic mode while keeping the engineered dissipation active.
arxiv pdf

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