M. Field

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

High-fidelity optical readout of a superconducting qubit using a scalable piezo-optomechanical transducer

  1. T.C. van Thiel,
  2. M.J. Weaver,
  3. F. Berto,
  4. P. Duivestein,
  5. M. Lemang,
  6. K. Schuurman,
  7. M. Žemlička,
  8. F. Hijazi,
  9. A.C. Bernasconi,
  10. E. Lachman,
  11. M. Field,
  12. Y. Mohan,
  13. F. de Vries,
  14. N. Bultink,
  15. J. van Oven,
  16. J. Y. Mutus,
  17. R. Stockill,
  18. and S. Gröblacher
Superconducting quantum processors have made significant progress in size and computing potential. As a result, the practical cryogenic limitations of operating large numbers of superconductingqubits are becoming a bottleneck for further scaling. Due to the low thermal conductivity and the dense optical multiplexing capacity of telecommunications fiber, converting qubit signal processing to the optical domain using microwave-to-optics transduction would significantly relax the strain on cryogenic space and thermal budgets. Here, we demonstrate high-fidelity multi-shot optical readout through an optical fiber of a superconducting transmon qubit connected via a coaxial cable to a fully integrated piezo-optomechanical transducer. Using a demolition readout technique, we achieve a multi-shot readout fidelity of >99% at 6 μW of optical power transmitted into the cryostat with as few as 200 averages, without the use of a quantum-limited amplifier. With improved frequency matching between the transducer and the qubit readout resonator, we anticipate that single-shot optical readout is achievable. Due to the small footprint (<0.15mm2) and the modular fiber-based architecture, this device platform has the potential to scale towards use with thousands of qubits. Our results illustrate the potential of piezo-optomechanical transduction for low-dissipation operation of large quantum processors.[/expand]
arxiv pdf