David Pahl

Placing and Routing Non-Local Quantum Error Correcting Codes in Multi-Layer Superconducting Qubit Hardware

  1. Melvin Mathews,
  2. Lukas Pahl,
  3. David Pahl,
  4. Vaishnavi L. Addala,
  5. Catherine Tang,
  6. William D. Oliver,
  7. and Jeffrey A. Grover
Quantum error correcting codes (QECCs) with asymptotically lower overheads than the surface code require non-local connectivity. Leveraging multi-layer routing and long-range coupling
capabilities in superconducting qubit hardware, we develop Hardware-Aware Layout, HAL: a robust, runtime-efficient heuristic algorithm that automates and optimizes the placement and routing of arbitrary QECCs. Using HAL, we perform a comparative study of hardware cost across various families of QECCs, including the bivariate bicycle codes, the open-boundary tile codes, and the constant-depth-decodable radial codes. The layouts produced by HAL confirm that open boundaries significantly reduce the hardware cost, while incurring reductions in logical efficiency. Among the best-performing codes were low-weight radial codes, despite lacking topological structure. Overall, HAL provides a valuable framework for evaluating the hardware feasibility of existing QECCs and guiding the discovery of new codes compatible with realistic hardware constraints.
arxiv pdf

Improved Parameter Targeting in {3D}-Integrated Superconducting Circuits through a Polymer Spacer Process

  1. Graham J. Norris,
  2. Laurent Michaud,
  3. David Pahl,
  4. Michael Kerschbaum,
  5. Christopher Eichler,
  6. Jean-Claude Besse,
  7. and Andreas Wallraff
Three-dimensional device integration facilitates the construction of superconducting quantum information processors with more than several tens of qubits by distributing elements such
as control wires, qubits, and resonators between multiple layers. The frequencies of resonators and qubits in flip-chip-bonded multi-chip modules depend on the details of their electromagnetic environment defined by the conductors and dielectrics in their vicinity. Accurate frequency targeting therefore requires precise control of the separation between chips and minimization of their relative tilt. Here, we describe a method to control the inter-chip separation by using polymer spacers. Compared to an identical process without spacers, we reduce the measured planarity error by a factor of 3.5, to a mean tilt of 76(35) μrad, and the deviation from the target inter-chip separation by a factor of ten, to a mean of 0.4(8) μm. We apply this process to coplanar waveguide resonator samples and observe chip-to-chip resonator frequency variations below 50 MHz (≈ 1 %). We measure internal quality factors of 5×105 at the single-photon level, suggesting that the added spacers are compatible with low-loss device fabrication.
arxiv pdf