Luca Hofele

Realizing Lattice Surgery on Two Distance-Three Repetition Codes with Superconducting Qubits

  1. Ilya Besedin,
  2. Michael Kerschbaum,
  3. Jonathan Knoll,
  4. Ian Hesner,
  5. Lukas Bödeker,
  6. Luis Colmenarez,
  7. Luca Hofele,
  8. Nathan Lacroix,
  9. Christoph Hellings,
  10. François Swiadek,
  11. Alexander Flasby,
  12. Mohsen Bahrami Panah,
  13. Dante Colao Zanuz,
  14. Markus Müller,
  15. and Andreas Wallraff
Quantum error correction is needed for quantum computers to be capable of fault-tolerantly executing algorithms using hundreds of logical qubits. Recent experiments have demonstrated
subthreshold error rates for state preservation of a single logical qubit. In addition, the realization of universal quantum computation requires the implementation of logical entangling gates. Lattice surgery offers a practical approach for implementing such gates, particularly in planar quantum processor layouts. In this work, we demonstrate lattice surgery between two distance-three repetition-code qubits by splitting a single distance-three surface-code qubit. Using a quantum circuit fault-tolerant to bit-flip errors, we achieve an improvement in the value of the decoded ZZ logical two-qubit observable compared to a similar non-encoded circuit. By preparing the surface-code qubit in initial states parametrized by a varying polar angle, we evaluate the performance of the lattice surgery operation for non-cardinal states on the logical Bloch sphere and employ logical two-qubit tomography to reconstruct the Pauli transfer matrix of the operation. In this way, we demonstrate the functional building blocks needed for lattice surgery operations on larger-distance codes based on superconducting circuits.
arxiv pdf

Fast Flux-Activated Leakage Reduction for Superconducting Quantum Circuits

  1. Nathan Lacroix,
  2. Luca Hofele,
  3. Ants Remm,
  4. Othmane Benhayoune-Khadraoui,
  5. Alexander McDonald,
  6. Ross Shillito,
  7. Stefania Lazar,
  8. Christoph Hellings,
  9. Francois Swiadek,
  10. Dante Colao Zanuz,
  11. Alexander Flasby,
  12. Mohsen Bahrami Panah,
  13. Michael Kerschbaum,
  14. Graham J. Norris,
  15. Alexandre Blais,
  16. Andreas Wallraff,
  17. and Sebastian Krinner
Quantum computers will require quantum error correction to reach the low error rates necessary for solving problems that surpass the capabilities of conventional computers. One of the
dominant errors limiting the performance of quantum error correction codes across multiple technology platforms is leakage out of the computational subspace arising from the multi-level structure of qubit implementations. Here, we present a resource-efficient universal leakage reduction unit for superconducting qubits using parametric flux modulation. This operation removes leakage down to our measurement accuracy of 7⋅10−4 in approximately 50ns with a low error of 2.5(1)⋅10−3 on the computational subspace, thereby reaching durations and fidelities comparable to those of single-qubit gates. We demonstrate that using the leakage reduction unit in repeated weight-two stabilizer measurements reduces the total number of detected errors in a scalable fashion to close to what can be achieved using leakage-rejection methods which do not scale. Our approach does neither require additional control electronics nor on-chip components and is applicable to both auxiliary and data qubits. These benefits make our method particularly attractive for mitigating leakage in large-scale quantum error correction circuits, a crucial requirement for the practical implementation of fault-tolerant quantum computation.
arxiv pdf