Yongxin Song

Resource-Efficient Cross-Platform Verification with Modular Superconducting Devices

  1. Kieran Dalton,
  2. Johannes Knörzer,
  3. Finn Hoehne,
  4. Yongxin Song,
  5. Alexander Flasby,
  6. Dante Colao Zanuz,
  7. Mohsen Bahrami Panah,
  8. Ilya Besedin,
  9. Jean-Claude Besse,
  10. and Andreas Wallraff
Large-scale quantum computers are expected to benefit from modular architectures. Validating the capabilities of modular devices requires benchmarking strategies that assess performance
within and between modules. In this work, we evaluate cross-platform verification protocols, which are critical for quantifying how accurately different modules prepare the same quantum state — a key requirement for modular scalability and system-wide consistency. We demonstrate these algorithms using a six-qubit flip-chip superconducting quantum device consisting of two three-qubit modules on a single carrier chip, with connectivity for intra- and inter-module entanglement. We examine how the resource requirements of protocols relying solely on classical communication between modules scale exponentially with qubit number, and demonstrate that introducing an inter-module two-qubit gate enables sub-exponential scaling in cross-platform verification. This approach reduces the number of repetitions required by a factor of four for three-qubit states, with greater reductions projected for larger and higher-fidelity devices.
arxiv pdf

Realization of Constant-Depth Fan-Out with Real-Time Feedforward on a Superconducting Quantum Processor

  1. Yongxin Song,
  2. Liberto Beltrán,
  3. Ilya Besedin,
  4. Michael Kerschbaum,
  5. Marek Pechal,
  6. François Swiadek,
  7. Christoph Hellings,
  8. Dante Colao Zanuz,
  9. Alexander Flasby,
  10. Jean-Claude Besse,
  11. and Andreas Wallraff
When using unitary gate sequences, the growth in depth of many quantum circuits with output size poses significant obstacles to practical quantum computation. The quantum fan-out operation,
which reduces the circuit depth of quantum algorithms such as the quantum Fourier transform and Shor’s algorithm, is an example that can be realized in constant depth independent of the output size. Here, we demonstrate a quantum fan-out gate with real-time feedforward on up to four output qubits using a superconducting quantum processor. By performing quantum state tomography on the output states, we benchmark our gate with input states spanning the entire Bloch sphere. We decompose the output-state error into a set of independently characterized error contributions. We extrapolate our constant-depth circuit to offer a scaling advantage compared to the unitary fan-out sequence beyond 25 output qubits with feedforward control, or beyond 17 output qubits if the classical feedforward latency is negligible. Our work highlights the potential of mid-circuit measurements combined with real-time conditional operations to improve the efficiency of complex quantum algorithms.
arxiv pdf