Amber M. King

Efficient n-qubit entangling operations via a superconducting quantum router

  1. Xuntao Wu,
  2. Haoxiong Yan,
  3. Gustav Andersson,
  4. Alexander Anferov,
  5. Christopher R. Conner,
  6. Yash J. Joshi,
  7. Bayan Karimi,
  8. Amber M. King,
  9. Shiheng Li,
  10. Howard L. Malc,
  11. Jacob M. Miller,
  12. Harsh Mishra,
  13. Hong Qiao,
  14. Minseok Ryu,
  15. Jian Shi,
  16. and Andrew N. Cleland
Quantum algorithms on near-term quantum processors are typically executed using shallow quantum circuits composed of one- and two-qubit gates. However, as circuit depth and gate number
increase, gate imperfections and qubit decoherence begin to dominate, limiting algorithmic complexity. An alternative approach is to explore gates involving more than two qubits. In previous work (X. Wu et al., Physical Review X 14, 041030 (2024)), we demonstrated a new superconducting qubit architecture with user-selectable two-qubit interactions via a reconfigurable router, used to connect pairs of qubits. Here, we leverage this novel architecture to realize programmable and efficient multi-qubit operations involving more than two qubits, resulting in faster preparation of multi-qubit entangled states with good fidelities. We also successfully apply model-free reinforcement learning to perform multi-qubit gates, including training a two-qubit controlled-Z gate as well as three-qubit controlled-SWAP and controlled-controlled-phase (Fredkin and Toffoli) gates. Higher nth-order gates may also be feasible, using our high-connectivity router design. This could provide a more efficient and higher-fidelity implementation of complex quantum algorithms and a more practical approach to quantum computation.
arxiv pdf

Mitigating cosmic ray-like correlated events with a modular quantum processor

  1. Xuntao Wu,
  2. Yash J. Joshi,
  3. Haoxiong Yan,
  4. Gustav Andersson,
  5. Alexander Anferov,
  6. Christopher R. Conner,
  7. Bayan Karimi,
  8. Amber M. King,
  9. Shiheng Li,
  10. Howard L. Malc,
  11. Jacob M. Miller,
  12. Harsh Mishra,
  13. Hong Qiao,
  14. Minseok Ryu,
  15. Siyuan Xing,
  16. Jian Shi,
  17. and Andrew N. Cleland
Quantum processors based on superconducting qubits are being scaled to larger qubit numbers, enabling the implementation of small-scale quantum error correction codes. However, catastrophic
chip-scale correlated errors have been observed in these processors, attributed to e.g. cosmic ray impacts, which challenge conventional error-correction codes such as the surface code. These events are characterized by a temporary but pronounced suppression of the qubit energy relaxation times. Here, we explore the potential for modular quantum computing architectures to mitigate such correlated energy decay events. We measure cosmic ray-like events in a quantum processor comprising a motherboard and two flip-chip bonded daughterboard modules, each module containing two superconducting qubits. We monitor the appearance of correlated qubit decay events within a single module and across the physically separated modules. We find that while decay events within one module are strongly correlated (over 85%), events in separate modules only display ∼2% correlations. We also report coincident decay events in the motherboard and in either of the two daughterboard modules, providing further insight into the nature of these decay events. These results suggest that modular architectures, combined with bespoke error correction codes, offer a promising approach for protecting future quantum processors from chip-scale correlated errors.
arxiv pdf