M. W. Beekman

Wafer-scale uniformity of Dolan-bridge and bridgeless Manhattan-style Josephson junctions for superconducting quantum processors

  1. N. Muthusubramanian,
  2. P. Duivestein,
  3. C. Zachariadis,
  4. M. Finkel,
  5. S. L. M. van der Meer,
  6. H. M. Veen,
  7. M. W. Beekman,
  8. T. Stavenga,
  9. A. Bruno,
  10. and L. DiCarlo
We investigate die-level and wafer-scale uniformity of Dolan-bridge and bridgeless Manhattan Josephson junctions, using multiple substrates with and without through-silicon vias (TSVs).
Dolan junctions fabricated on planar substrates have the highest yield and lowest room-temperature conductance spread, equivalent to ~100 MHz in transmon frequency. In TSV-integrated substrates, Dolan junctions suffer most in both yield and disorder, making Manhattan junctions preferable. Manhattan junctions show pronounced conductance decrease from wafer centre to edge, which we qualitatively capture using a geometric model of spatially-dependent resist shadowing during junction electrode evaporation. Analysis of actual junction overlap areas using scanning electron micrographs supports the model, and further points to a remnant spatial dependence possibly due to contact resistance.
arxiv pdf

Protecting quantum entanglement from qubit errors and leakage via repetitive parity measurements

  1. C. C. Bultink,
  2. T. E. O'Brien,
  3. R. Vollmer,
  4. N. Muthusubramanian,
  5. M. W. Beekman,
  6. M. A. Rol,
  7. X. Fu,
  8. B. Tarasinski,
  9. V. Ostroukh,
  10. B. Varbanov,
  11. A. Bruno,
  12. and L. DiCarlo
Protecting quantum information from errors is essential for large-scale quantum computation. Quantum error correction (QEC) encodes information in entangled states of many qubits, and
performs parity measurements to identify errors without destroying the encoded information. However, traditional QEC cannot handle leakage from the qubit computational space. Leakage affects leading experimental platforms, based on trapped ions and superconducting circuits, which use effective qubits within many-level physical systems. We investigate how two-transmon entangled states evolve under repeated parity measurements, and demonstrate the use of hidden Markov models to detect leakage using only the record of parity measurement outcomes required for QEC. We show the stabilization of Bell states over up to 26 parity measurements by mitigating leakage using postselection, and correcting qubit errors using Pauli-frame transformations. Our leakage identification method is computationally efficient and thus compatible with real-time leakage tracking and correction in larger quantum processors.
arxiv pdf