Yi-Cong Zheng

Tantalum airbridges for scalable superconducting quantum processors

  1. Kunliang Bu,
  2. Sainan Huai,
  3. Zhenxing Zhang,
  4. Dengfeng Li,
  5. Yuan Li,
  6. Jingjing Hu,
  7. Xiaopei Yang,
  8. Maochun Dai,
  9. Tianqi Cai,
  10. Yi-Cong Zheng,
  11. and Shengyu Zhang
The unique property of tantalum (Ta), particularly its long coherent lifetime in superconducting qubits and its exceptional resistance to both acid and alkali, makes it promising for
superconducting quantum processors. It is a notable advantage to achieve high-performance quantum processors with neat and unified fabrication of all circuit elements, including coplanar waveguides (CPW), qubits, and airbridges, on the tantalum film-based platform. Here, we propose a reliable tantalum airbridges with separate or fully-capped structure fabricated via a novel lift-off method, where a barrier layer with aluminium (Al) film is first introduced to separate two layers of photoresist and then etched away before the deposition of tantalum film, followed by cleaning with piranha solution to remove the residual photoresist on the chip. We characterize such tantalum airbridges as the control line jumpers, the ground plane crossovers and even coupling elements. They exhibit excellent connectivity, minimal capacitive loss, effectively suppress microwave and flux crosstalk and offer high freedom of coupling. Besides, by presenting a surface-13 tunable coupling superconducting quantum processor with median T1 reaching above 100 μs, the overall adaptability of tantalum airbridges is verified. The median single-qubit gate fidelity shows a tiny decrease from about 99.95% for the isolated Randomized Benchmarking to 99.94% for the simultaneous one. This fabrication method, compatible with all known superconducting materials, requires mild conditions of film deposition compared with the commonly used etching and grayscale lithography. Meanwhile, the experimental achievement of non-local coupling with controlled-Z (CZ) gate fidelity exceeding 99.2% may further facilitate qLDPC codes, laying a foundation for scalable quantum computation and quantum error correction with entirely tantalum elements.
arxiv pdf

Suppressing ZZ Crosstalk of Quantum Computers through Pulse and Scheduling Co-Optimization

  1. Lei Xie,
  2. Jidong Zhai,
  3. Zhenxing Zhang,
  4. Jonathan Allcock,
  5. Shengyu Zhang,
  6. and Yi-Cong Zheng
Noise is a significant obstacle to quantum computing, and ZZ crosstalk is one of the most destructive types of noise affecting superconducting qubits. Previous approaches to suppressing
ZZ crosstalk have mainly relied on specific chip design that can complicate chip fabrication and aggravate decoherence. To some extent, special chip design can be avoided by relying on pulse optimization to suppress ZZ crosstalk. However, existing approaches are non-scalable, as their required time and memory grow exponentially with the number of qubits involved. To address the above problems, we propose a scalable approach by co-optimizing pulses and scheduling. We optimize pulses to offer an ability to suppress ZZ crosstalk surrounding a gate, and then design scheduling strategies to exploit this ability and achieve suppression across the whole circuit. A main advantage of such co-optimization is that it does not require special hardware support. Besides, we implement our approach as a general framework that is compatible with different pulse optimization methods. We have conducted extensive evaluations by simulation and on a real quantum computer. Simulation results show that our proposal can improve the fidelity of quantum computing on 4∼12 qubits by up to 81× (11× on average). Ramsey experiments on a real quantum computer also demonstrate that our method can eliminate the effect of ZZ crosstalk to a great extent.
arxiv pdf