Ambroise Peugeot

Engineering multi-photon dissipation with a dc-voltage-biased Josephson junction

  1. Marco Paradina,
  2. Ambroise Peugeot,
  3. Roberto Negrin,
  4. Oscar Novat,
  5. Tristan Villain,
  6. Anil Murani,
  7. Jean-Loup Ville,
  8. Sébastien Jezouin,
  9. Raphaël Lescanne,
  10. Audrey Bienfait,
  11. and Benjamin Huard
Multi-photon dissipation — a key resource for bosonic qubits — is usually realized by parametrically pumping a Josephson coupler at the cost of spurious nonlinear terms.
Here we instead engineer it using a dc-voltage-biased SQUID, such that these parasitic terms average out. We activate the conversion of one, two, or four photons of a high-Q mode into a single photon of a lossy mode. We characterize the two-photon dissipation by Wigner tomography, establishing dc-biased junctions as a resource for reservoir engineering and a viable route to cat-qubit stabilization.
arxiv pdf

Absence of Charge Offset Drift in a Transmon Qubit

  1. Adria Rospars,
  2. Hector Hutin,
  3. Yannick Seis,
  4. Cristóbal Lledó,
  5. Réouven Assouly,
  6. Romain Cazali,
  7. Rémy Dassonneville,
  8. Ambroise Peugeot,
  9. Alexandre Blais,
  10. Audrey Bienfait,
  11. and Benjamin Huard
Superconducting quantum circuits are sensitive to their electrostatic environment: uncontrolled charges accumulating on the electrodes of a Josephson junction shift the energy levels
of a qubit, perturbing its operation and restricting their design. This effect is captured by a single parameter – the charge offset – whose slow, unpredictable drift has proven difficult to eliminate in practice. Here, we report a tantalum-based transmon qubit in which the charge offset remains pinned at zero over nearly three months of measurements, including two thermal cycles, with no observable compromise to the qubit lifetime. This exceptional stability disappears in later cooldowns, indicating a fragile mechanism at play. We attribute it to the inductance of a thin superconducting layer inadvertently formed in parallel with the Josephson junction during fabrication. X-ray surface spectroscopy suggests this layer arises from an incomplete wet-etch of tantalum on sapphire. Deliberately engineering such a layer offers a route to eliminating charge-offset drift in superconducting circuits more broadly.
arxiv pdf