M. Rosticher

High-sensitivity AC-charge detection with a MHz-frequency fluxonium qubit

  1. B.-L. Najera-Santos,
  2. R. Rousseau,
  3. K. Gerashchenko,
  4. H. Patange,
  5. A. Riva,
  6. M. Villiers,
  7. T. Briant,
  8. P.-F. Cohadon,
  9. A. Heidmann,
  10. J. Palomo,
  11. M. Rosticher,
  12. H. le Sueur,
  13. A. Sarlette,
  14. W. C. Smith,
  15. Z. Leghtas,
  16. E. Flurin,
  17. T. Jacqmin,
  18. and S. Deléglise
Owing to their strong dipole moment and long coherence times, superconducting qubits have demonstrated remarkable success in hybrid quantum circuits. However, most qubit architectures
are limited to the GHz frequency range, severely constraining the class of systems they can interact with. The fluxonium qubit, on the other hand, can be biased to very low frequency while being manipulated and read out with standard microwave techniques. Here, we design and operate a heavy fluxonium with an unprecedentedly low transition frequency of 1.8 MHz. We demonstrate resolved sideband cooling of the „hot“ qubit transition with a final ground state population of 97.7 %, corresponding to an effective temperature of 23 μK. We further demonstrate coherent manipulation with coherence times T1=34 μs, T∗2=39 μs, and single-shot readout of the qubit state. Importantly, by directly addressing the qubit transition with a capacitively coupled waveguide, we showcase its high sensitivity to a radio-frequency field. Through cyclic qubit preparation and interrogation, we transform this low-frequency fluxonium qubit into a frequency-resolved charge sensor. This method results in a charge sensitivity of 33 μe/Hz‾‾‾√, or an energy sensitivity (in joules per hertz) of 2.8 ℏ. This method rivals state-of-the-art transport-based devices, while maintaining inherent insensitivity to DC charge noise. The high charge sensitivity combined with large capacitive shunt unlocks new avenues for exploring quantum phenomena in the 1−10 MHz range, such as the strong-coupling regime with a resonant macroscopic mechanical resonator.
arxiv pdf

One hundred second bit-flip time in a two-photon dissipative oscillator

  1. C. Berdou,
  2. A. Murani,
  3. U. Reglade,
  4. W. C. Smith,
  5. M. Villiers,
  6. J. Palomo,
  7. M. Rosticher,
  8. A. Denis,
  9. P. Morfin,
  10. M. Delbecq,
  11. T. Kontos,
  12. N. Pankratova,
  13. F. Rautschke,
  14. T. Peronnin,
  15. L.-A. Sellem,
  16. P. Rouchon,
  17. A. Sarlette,
  18. M. Mirrahimi,
  19. P. Campagne-Ibarcq,
  20. S. Jezouin,
  21. R. Lescanne,
  22. and Z. Leghtas
Current implementations of quantum bits (qubits) continue to undergo too many errors to be scaled into useful quantum machines. An emerging strategy is to encode quantum information
in the two meta-stable pointer states of an oscillator exchanging pairs of photons with its environment, a mechanism shown to provide stability without inducing decoherence. Adding photons in these states increases their separation, and macroscopic bit-flip times are expected even for a handful of photons, a range suitable to implement a qubit. However, previous experimental realizations have saturated in the millisecond range. In this work, we aim for the maximum bit-flip time we could achieve in a two-photon dissipative oscillator. To this end, we design a Josephson circuit in a regime that circumvents all suspected dynamical instabilities, and employ a minimally invasive fluorescence detection tool, at the cost of a two-photon exchange rate dominated by single-photon loss. We attain bit-flip times of the order of 100 seconds for states pinned by two-photon dissipation and containing about 40 photons. This experiment lays a solid foundation from which the two-photon exchange rate can be gradually increased, thus gaining access to the preparation and measurement of quantum superposition states, and pursuing the route towards a logical qubit with built-in bit-flip protection.
arxiv pdf