Fanming Qu

Procedure of tuning up a three-site artificial Kitaev chain based on transmon measurements

  1. Xiaozhou Yang,
  2. Zhaozheng Lyu,
  3. Xiang Wang,
  4. Enna Zhuo,
  5. Yunxiao Zhang,
  6. Duolin Wang,
  7. Yukun Shi,
  8. Yuyang Huang,
  9. Bing Li,
  10. Xiaohui Song,
  11. Peiling Li,
  12. Bingbing Tong,
  13. Ziwei Dou,
  14. Jie Shen,
  15. Guangtong Liu,
  16. Fanming Qu,
  17. and Li Lu
Artificial Kitaev chains (AKCs), formed of quantum dot-superconductor linear arrays, provide a promising platform for hosting Majorana bound states (MBSs) and implementing topological
quantum computing. The main challenges along this research direction would include the tuning up of AKCs for hosting MBSs and the readout of the parity of the chains. In this work, we present a step-by-step procedure for tuning up a three-site AKC to its sweet spots based on the spectra of a transmon circuit which is integrated with the chain for the purpose of reading out the parity of the chain. The signatures of the transmon’s plasma modes in each step, particular those related to the appearance of MBSs in the chain, will be given. We find that the sweet spots in a three-site AKC can be classified into three types based on the relative strengths of elastic cotunneling (ECT) and crossed Andreev reflection (CAR): ECT-dominated sweet spots, genuine sweet spots and CAR-dominated sweet spots. We show that the ECT-dominated and CAR-dominated sweet spots can be more conveniently accessed and utilized in transmon-based measurements.
arxiv pdf

Read out the fermion parity of a potential artificial Kitaev chain utilizing a transmon qubit

  1. Enna Zhuo,
  2. Xiaozhou Yang,
  3. Yuyang Huang,
  4. Zhaozheng Lyu,
  5. Ang Li,
  6. Bing Li,
  7. Yunxiao Zhang,
  8. Xiang Wang,
  9. Duolin Wang,
  10. Yukun Shi,
  11. Anqi Wang,
  12. E. P. A. M. Bakkers,
  13. Xiaodong Han,
  14. Xiaohui Song,
  15. Peiling Li,
  16. Bingbing Tong,
  17. Ziwei Dou,
  18. Guangtong Liu,
  19. Fanming Qu,
  20. Jie Shen,
  21. and Li Lu
Artificial Kitaev chains have emerged as a promising platform for realizing topological quantum computing. Once the chains are formed and the Majorana zero modes are braided/fused,
reading out the parity of the chains is essential for further verifying the non-Abelian property of the Majorana zero modes. Here we demonstrate the feasibility of using a superconducting transmon qubit, which incorporates an end of a four-site quantum dot-superconductor chain based on a Ge/Si nanowire, to directly detect the singlet/doublet state, and thus the parity of the entire chain. We also demonstrate that for multiple-dot chains there are two types of 0-{\pi} transitions between different charging states: the parity-flip 0-{\pi} transition and the parity-preserved 0-{\pi} transition. Furthermore, we show that the inter-dot coupling, hence the strengths of cross Andreev reflection and elastic cotunneling of electrons, can be adjusted by local electrostatic gating in chains fabricated on Ge/Si core-shell nanowires. Our exploration would be helpful for the ultimate realization of topological quantum computing based on artificial Kitaev chains.
arxiv pdf

Homointerface planar Josephson junction based on inverse proximity effect

  1. Juewen Fan,
  2. Bingyan Jiang,
  3. Jiaji Zhao,
  4. Ran Bi,
  5. Jiadong Zhou,
  6. Zheng Liu,
  7. Ning Kang,
  8. Fanming Qu,
  9. Li Lu,
  10. and Xiaosong Wu
The quality of a superconductor-normal metal-superconductor (SNS) Josephson junction (JJ) depends crucially on the transparency of the superconductor-normal metal (S/N) interface. We
demonstrate a technique for fabricating planar JJs with perfect interfaces. The technique utilizes a strong inverse proximity effect (IPE) discovered in Al/V5S8 bilayers, by which Al is driven into the normal state. The highly transparent homointerface enables the flow of Josephson supercurrent across a 2.9 μm long weak link. Moreover, our JJ exhibits a giant critical current and a large product of the critical current and the normal state resistance. The latter exceeds the theoretical bound, which is probably related to the unusual normal metal weak link.
arxiv pdf