H. le Sueur

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

Emission of photon multiplets by a dc-biased superconducting circuit

  1. G. C. Ménard,
  2. A. Peugeot,
  3. C. Padurariu,
  4. C. Rolland,
  5. B. Kubala,
  6. Y. Mukharsky,
  7. Z. Iftikhar,
  8. C. Altimiras,
  9. P. Roche,
  10. H. le Sueur,
  11. P. Joyez,
  12. D. Esteve,
  13. J. Ankerhold,
  14. and F. Portier
We observe the emission of bunches of k⩾1 photons by a circuit made of a microwave resonator in series with a voltage-biased tunable Josephson junction. The bunches are emitted at
specific values Vk of the bias voltage, for which each Cooper pair tunneling across the junction creates exactly k photons in the resonator. The latter is a micro-fabricated spiral coil which resonates and leaks photons at 4.4~GHz in a measurement line. Its characteristic impedance of 1.97~kΩ is high enough to reach a strong junction-resonator coupling and a bright emission of the k-photon bunches. We show that a RWA treatment of the system accounts quantitatively for the observed radiation intensity, from k=1 to 6, and over three orders of magnitude when varying the Josephson energy EJ. We also measure the second order correlation function of the radiated microwave to determine its Fano factor Fk, which in the low EJ limit, confirms with Fk=k the emission of k photon bunches. At larger EJ, a more complex behavior is observed in quantitative agreement with numerical simulations.
arxiv pdf

Antibunched photons emitted by a dc biased Josephson junction

  1. C. Rolland,
  2. A. Peugeot,
  3. S. Dambach,
  4. M. Westig,
  5. B. Kubala,
  6. C. Altimiras,
  7. H. le Sueur,
  8. P. Joyez,
  9. D. Vion,
  10. P. Roche,
  11. D. Esteve,
  12. J. Ankerhold,
  13. and F. Portier
We show experimentally that a dc biased Josephson junction in series with a high-enough impedance microwave resonator emits antibunched photons. Our resonator is made of a simple micro-fabricated
spiral coil that resonates at 4.4 GHz and reaches a 1.97 kΩ characteristic impedance. The second order correlation function of the power leaking out of the resonator drops down to 0.3 at zero delay, which demonstrates the antibunching of the photons emitted by the circuit at a rate of 6 10^7 photons per second. Results are found in quantitative agreement with our theoretical predictions. This simple scheme could offer an efficient and bright single-photon source in the microwave domain.
arxiv pdf