Matteo Fadel

Loss Mechanisms in High-coherence Multimode Mechanical Resonators Coupled to Superconducting Circuits

  1. Raquel Garcia Belles,
  2. Alexander Anferov,
  3. Lukas F. Deeg,
  4. Loris Colicchio,
  5. Arianne Brooks,
  6. Tom Schatteburg,
  7. Maxwell Drimmer,
  8. Ines C. Rodrigues,
  9. Rodrigo Benevides,
  10. Marco Liffredo,
  11. Jyotish Patidar,
  12. Oleksandr Pshyk,
  13. Matteo Fadel,
  14. Luis Guillermo Villanueva,
  15. Sebastian Siol,
  16. Gerhard Kirchmair,
  17. and Yiwen Chu
Circuit quantum acoustodynamics (cQAD) devices have a wide range of applications in quantum science, all of which depend crucially on the quantum coherence of the mechanical subsystem.
In this context, high-overtone bulk acoustic-wave resonators (HBARs) are particularly promising, since they have shown very high quality factors with negligible dephasing. However, the introduction of piezoelectric films, which are necessary for coupling to a superconducting circuit, can lead to additional loss channels, such as surface scattering and two-level systems (TLS). Here, we study the acoustic dissipation of HBAR resonators in cQAD systems and find that the defect density of the piezoelectric material and its interface with the bulk are limiting factors for the coherence. We measure acoustic modes with phonon lifetimes up to 400 μs and lifetime-limited coherence times approaching one millisecond in the quantum regime. When coupled to a superconducting qubit, this leads to a hybrid system with a large quantum coherence cooperativity of CT2=1.1×105. These results represent a new milestone for the performance of cQAD devices and offer concrete paths forward for further improvements.
arxiv pdf

Theory-independent monitoring of the decoherence of a superconducting qubit with generalized contextuality

  1. Albert Aloy,
  2. Matteo Fadel,
  3. Thomas D. Galley,
  4. Caroline L. Jones,
  5. and Markus P. Mueller
Characterizing the nonclassicality of quantum systems under minimal assumptions is an important challenge for quantum foundations and technology. Here we introduce a theory-independent
method of process tomography and perform it on a superconducting qubit. We demonstrate its decoherence without assuming quantum theory or trusting the devices by modelling the system as a general probabilistic theory. We show that the superconducting system is initially well-described as a quantum bit, but that its realized state space contracts over time, which in quantum terminology indicates its loss of coherence. The system is initially nonclassical in the sense of generalized contextuality: it does not admit of a hidden-variable model where statistically indistinguishable preparations are represented by identical hidden-variable distributions. In finite time, the system becomes noncontextual and hence loses its nonclassicality. Moreover, we demonstrate in a theory-independent way that the system undergoes non-Markovian evolution at late times. Our results extend theory-independent tomography to time-evolving systems, and show how important dynamical physical phenomena can be experimentally monitored without assuming quantum theory.
arxiv pdf