A. Bacher

Spin Environment of a Superconducting Qubit in High Magnetic Fields

  1. S. Günzler,
  2. J. Beck,
  3. D. Rieger,
  4. N. Gosling,
  5. N. Zapata,
  6. M. Field,
  7. S. Geisert,
  8. A. Bacher,
  9. J. K. Hohmann,
  10. M. Spiecker,
  11. W. Wernsdorfer,
  12. and I. M. Pop
Superconducting qubits equipped with quantum non-demolishing readout and active feedback can be used as information engines to probe and manipulate microscopic degrees of freedom, whether
intentionally designed or naturally occurring in their environment. In the case of spin systems, the required magnetic field bias presents a challenge for superconductors and Josephson junctions. Here we demonstrate a granular aluminum nanojunction fluxonium qubit (gralmonium) with spectrum and coherence resilient to fields beyond one Tesla. Sweeping the field reveals a paramagnetic spin-1/2 ensemble, which is the dominant gralmonium loss mechanism when the electron spin resonance matches the qubit. We also observe a suppression of fast flux noise in magnetic field, suggesting the freezing of surface spins. Using an active state stabilization sequence, the qubit hyperpolarizes long-lived two-level systems (TLSs) in its environment, previously speculated to be spins. Surprisingly, the coupling to these TLSs is unaffected by magnetic fields, leaving the question of their origin open. The robust operation of gralmoniums in Tesla fields offers new opportunities to explore unresolved questions in spin environment dynamics and facilitates hybrid architectures linking superconducting qubits with spin systems.
arxiv pdf

Gralmonium: Granular Aluminum Nano-Junction Fluxonium Qubit

  1. D. Rieger,
  2. S. Günzler,
  3. M. Spiecker,
  4. P. Paluch,
  5. P. Winkel,
  6. L. Hahn,
  7. J. K. Hohmann,
  8. A. Bacher,
  9. W. Wernsdorfer,
  10. and I. M. Pop
Mesoscopic Josephson junctions (JJs), consisting of overlapping superconducting electrodes separated by a nanometer thin oxide layer, provide a precious source of nonlinearity for superconducting
quantum circuits and are at the heart of state-of-the-art qubits, such as the transmon and fluxonium. Here, we show that in a fluxonium qubit the role of the JJ can also be played by a lithographically defined, self-structured granular aluminum (grAl) nano-junction: a superconductor-insulator-superconductor (SIS) JJ obtained in a single layer, zero-angle evaporation. The measured spectrum of the resulting qubit, which we nickname gralmonium, is indistinguishable from the one of a standard fluxonium qubit. Remarkably, the lack of a mesoscopic parallel plate capacitor gives rise to an intrinsically large grAl nano-junction charging energy in the range of 10−100GHz, comparable to its Josephson energy EJ. We measure average energy relaxation times of T1=10μs and Hahn echo coherence times of Techo2=9μs. The exponential sensitivity of the gralmonium to the EJ of the grAl nano-junction provides a highly susceptible detector. Indeed, we observe spontaneous jumps of the value of EJ on timescales from milliseconds to days, which offer a powerful diagnostics tool for microscopic defects in superconducting materials.
arxiv pdf