Probing the quantum motion of a macroscopic mechanical oscillator with a radio-frequency superconducting qubit

  1. Kyrylo Gerashchenko,
  2. Remi Rousseau,
  3. Léo Balembois,
  4. Himanshu Patange,
  5. Paul Manset,
  6. W. Clarke Smith,
  7. Zaki Leghtas,
  8. Emmanuel Flurin,
  9. Thibaut Jacqmin,
  10. and Samuel Deléglise
Long-lived mechanical resonators like drums oscillating at MHz frequencies and operating in the quantum regime offer a powerful platform for quantum technologies and tests of fundamental
physics. Yet, quantum control of such systems remains challenging, particularly owing to their low energy scale and the difficulty of achieving efficient coupling to other well-controlled quantum devices. Here, we demonstrate repeated, and high-fidelity interactions between a 4 MHz suspended silicon nitride membrane and a resonant superconducting heavy-fluxonium qubit. The qubit is initialized at an effective temperature of 27~μK and read out in a single-shot with 77% fidelity. During the membrane’s 6~ms lifetime, the two systems swap excitations more than 300 times. After each interaction, a state-selective detection is performed, implementing a stroboscopic series of weak measurements that provide information about the mechanical state. The accumulated records reconstruct the membrane’s position noise-spectrum, revealing both its thermal occupation nth≈47 at 10~mK and the qubit-induced back-action. By preparing the qubit either in its ground or excited state before each interaction, we observe an imbalance between the emission and absorption spectra, proportional to nth and nth+1, respectively-a hallmark of the non-commutation of phonon creation and annihilation operators. Since the predicted Diósi-Penrose gravitational collapse time is comparable to the measured mechanical decoherence time, our architecture enters a regime where gravity-induced decoherence could be tested directly.

Hyperinductance based on stacked Josephson junctions

  1. Paul Manset,
  2. José Palomo,
  3. Aurélien Schmitt,
  4. Kyrylo Gerashchenko,
  5. Rémi Rousseau,
  6. Himanshu Patange,
  7. Patrick Abgrall,
  8. Emmanuel Flurin,
  9. Samuel Deléglise,
  10. Thibaut Jacqmin,
  11. and Léo Balembois
Superinductances are superconducting circuit elements that combine a large inductance with a low parasitic capacitance to ground, resulting in a characteristic impedance exceeding the
resistance quantum RQ=h/(2e)2≃6.45kΩ. In recent years, these components have become key enablers for emerging quantum circuit architectures. However, achieving high characteristic impedance while maintaining scalability and fabrication robustness remains a major challenge. In this work, we present two fabrication techniques for realizing superinductances based on vertically stacked Josephson junctions. Using a multi-angle Manhattan (MAM) process and a zero-angle (ZA) evaporation technique — in which junction stacks are connected pairwise using airbridges — we fabricate one-dimensional chains of stacks that act as high-impedance superconducting transmission lines. Two-tone microwave spectroscopy reveals the expected n‾√ scaling of the impedance with the number of junctions per stack. The chain fabricated using the ZA process, with nine junctions per stack, achieves a characteristic impedance of ∼16kΩ, a total inductance of 5.9μH, and a maximum frequency-dependent impedance of 50kΩ at 1.4 GHz. Our results establish junction stacking as a scalable, robust, and flexible platform for next-generation quantum circuits requiring ultra-high impedance environments.