Pauli Kehayias

Elimination of Flux Trapping in Superconducting Circuits in Ambient Magnetic Fields

  1. Rohan T. Kapur,
  2. Alex Wynn,
  3. Sergey K. Tolpygo,
  4. Neel Parmar,
  5. Anil Mankame,
  6. Adam A. Libson,
  7. Rabindra Das,
  8. Michele Kelley,
  9. Pauli Kehayias,
  10. Nathaniel J. O'Connor,
  11. Collin N. Muniz,
  12. Justin L. Mallek,
  13. and Jennifer M. Schloss
Superconductor digital electronics and quantum computing with superconducting qubits are promising next-generation computing technologies. When cooled down or operated in the presence
of a nonzero background magnetic field Br, superconducting thin films comprising the circuits can trap magnetic vortices that can degrade circuit or qubit performance. In this work, we report a practical solution for eliminating flux trapped during cooldown in ambient magnetic fields, Br≤60 $\upmu$T, based on controlled local thermal gradients and moats, etched holes in the superconducting films of the circuit. Thermal gradients created by integrated on-chip resistive heaters move vortices towards the moats, where they become trapped away from circuitry regions and pinning sites. Using magnetic imaging and electrical circuit readout, we demonstrate that this approach is capable of removing magnetic flux trapped during field cooling and magnetic flux nucleated by circuit operation. If used in an environment with basic magnetic shielding, this solution is capable of suppressing all magnetic flux in a large-scale circuit, overcoming one of the long-standing challenges preventing high-performance scalable computing using superconductors.
arxiv pdf

Mitigation of Magnetic Flux Trapping in Superconducting Electronics Using Moats

  1. Rohan T. Kapur,
  2. Sergey K. Tolpygo,
  3. Alex Wynn,
  4. Pauli Kehayias,
  5. Adam A. Libson,
  6. Collin N. Muniz,
  7. Michael J. Gold,
  8. Justin L. Mallek,
  9. Danielle A. Braje,
  10. and Jennifer M. Schloss
Magnetic flux (vortex) trapping remains a major obstacle to very large scale integration in superconducting electronics. Moats — etched regions in circuit layers placed in groundplanes and around critical circuitry — offer a simple passive approach to sequester flux. Here, we systematically examine the effectiveness of moat arrays in superconducting niobium films as a function of geometry (size, shape, and density) and background magnetic field. By measuring the vortex expulsion field, we estimate the flux saturation number and flux trapping temperature for a range of geometries. We find that many moat designs effectively sequester flux in magnetically shielded environments (< 1 μT), with high-aspect-ratio rectangular "slit" moats providing the strongest mitigation at minimal area cost. However, our measurements show that moats alone do not eliminate flux trapping in non-ideal films, as vortices can preferentially pin at material defects. These results provide design guidance for flux mitigation in superconducting integrated circuits and highlight the need for combined optimization of circuit geometries and materials.[/expand]
arxiv pdf