Coupler-Assisted Leakage Reduction for Scalable Quantum Error Correction with Superconducting Qubits

  1. Xiaohan Yang,
  2. Ji Chu,
  3. Zechen Guo,
  4. Wenhui Huang,
  5. Yongqi Liang,
  6. Jiawei Liu,
  7. Jiawei Qiu,
  8. Xuandong Sun,
  9. Ziyu Tao,
  10. Jiawei Zhang,
  11. Jiajian Zhang,
  12. Libo Zhang,
  13. Yuxuan Zhou,
  14. Weijie Guo,
  15. Ling Hu,
  16. Ji Jiang,
  17. Yang Liu,
  18. Xiayu Linpeng,
  19. Tingyong Chen,
  20. Yuanzhen Chen,
  21. Jingjing Niu,
  22. Song Liu,
  23. Youpeng Zhong,
  24. and Dapeng Yu
Superconducting qubits are a promising platform for building fault-tolerant quantum computers, with recent achievement showing the suppression of logical error with increasing code
size. However, leakage into non-computational states, a common issue in practical quantum systems including superconducting circuits, introduces correlated errors that undermine QEC scalability. Here, we propose and demonstrate a leakage reduction scheme utilizing tunable couplers, a widely adopted ingredient in large-scale superconducting quantum processors. Leveraging the strong frequency tunability of the couplers and stray interaction between the couplers and readout resonators, we eliminate state leakage on the couplers, thus suppressing space-correlated errors caused by population propagation among the couplers. Assisted by the couplers, we further reduce leakage to higher qubit levels with high efficiency (98.1%) and low error rate on the computational subspace (0.58%), suppressing time-correlated errors during QEC cycles. The performance of our scheme demonstrates its potential as an indispensable building block for scalable QEC with superconducting qubits.

Deterministic quantum teleportation between distant superconducting chips

  1. Jiawei Qiu,
  2. Yang Liu,
  3. Jingjing Niu,
  4. Ling Hu,
  5. Yukai Wu,
  6. Libo Zhang,
  7. Wenhui Huang,
  8. Yuanzhen Chen,
  9. Jian Li,
  10. Song Liu,
  11. Youpeng Zhong,
  12. Luming Duan,
  13. and Dapeng Yu
Quantum teleportation~cite{Bennett1993} is of both fundamental interest and great practical importance in quantum information science. To date, quantum teleportation has been implemented
in various physical systems~\cite{Pirandola2015}, among which superconducting qubits are of particular practical significance as they emerge as a leading system to realize large-scale quantum computation~\cite{Arute2019,Wu2021}. Nevertheless, the number of superconducting qubits on the same chip is severely limited by the available chip size, the cooling power, and the wiring complexity. Realization of quantum teleportation and remote computation over qubits on distant superconducting chips is a key quantum communication technology to scaling up the system through a distributed quantum computational network~\cite{Gottesman1999,Eisert2000,Jiang2007,Kimble2008,Monroe2014}. However, this goal has not been realized yet in experiments due to the technical challenge of making a quantum interconnect between distant superconducting chips and the inefficient transfer of flying microwave photons over the lossy interconnects~\cite{Kurpiers2018,Axline2018,Campagne2018,Magnard2020}. Here we demonstrate deterministic teleportation of quantum states and entangling gates between distant superconducting chips connected by a 64-meter-long cable bus featuring an ultralow loss of 0.32~dB/km at cryogenic temperatures, where high fidelity remote entanglement is generated via flying microwave photons utilizing time-reversal-symmetry~\cite{Cirac1997,Korotkov2011}. Apart from the fundamental interest of teleporting macroscopic superconducting qubits over a long distance, our work lays a foundation to realization of large-scale superconducting quantum computation through a distributed computational network~\cite{Gottesman1999,Eisert2000,Jiang2007,Kimble2008,Monroe2014}.

Low-loss interconnects for modular superconducting quantum processors

  1. Jingjing Niu,
  2. Libo Zhang,
  3. Yang Liu,
  4. Jiawei Qiu,
  5. Wenhui Huang,
  6. Jiaxiang Huang,
  7. Hao Jia,
  8. Jiawei Liu,
  9. Ziyu Tao,
  10. Weiwei Wei,
  11. Yuxuan Zhou,
  12. Wanjing Zou,
  13. Yuanzhen Chen,
  14. Xiaowei Deng,
  15. Xiuhao Deng,
  16. Changkang Hu,
  17. Ling Hu,
  18. Jian Li,
  19. Dian Tan,
  20. Yuan Xu,
  21. Fei Yan,
  22. Tongxing Yan,
  23. Song Liu,
  24. Youpeng Zhong,
  25. Andrew N. Cleland,
  26. and Dapeng Yu
Scaling is now a key challenge in superconducting quantum computing. One solution is to build modular systems in which smaller-scale quantum modules are individually constructed and
calibrated, and then assembled into a larger architecture. This, however, requires the development of suitable interconnects. Here, we report low-loss interconnects based on pure aluminium coaxial cables and on-chip impedance transformers featuring quality factors up to 8.1×105, which is comparable to the performance of our transmon qubits fabricated on single-crystal sapphire substrate. We use these interconnects to link five quantum modules with inter-module quantum state transfer and Bell state fidelities up to 99\%. To benchmark the overall performance of the processor, we create maximally-entangled, multi-qubit Greenberger-Horne-Zeilinger (GHZ) states. The generated inter-module four-qubit GHZ state exhibits 92.0\% fidelity. We also entangle up to 12 qubits in a GHZ state with 55.8±1.8% fidelity, which is above the genuine multipartite entanglement threshold of 1/2. These results represent a viable modular approach for large-scale superconducting quantum processors.

Conditional coherent control with superconducting artificial atoms

  1. Chang-Kang Hu,
  2. Jiahao Yuan,
  3. Bruno A. Veloso,
  4. Jiawei Qiu,
  5. Yuxuan Zhou,
  6. Libo Zhang,
  7. Ji Chu,
  8. Orkesh Nurbolat,
  9. Ling Hu,
  10. Jian Li,
  11. Yuan Xu,
  12. Youpeng Zhong,
  13. Song Liu,
  14. Fei Yan,
  15. Dian Tan,
  16. R. Bachelard,
  17. Alan C. Santos,
  18. C. J. Villas-Boas,
  19. and Dapeng Yu
Controlling the flow of quantum information is a fundamental task for quantum computers, which is unpractical to realize on classical devices. Coherent devices which can process quantum
states are thus required to route the quantum states yielding the information. In this paper we demonstrate experimentally the smallest quantum transistor for superconducting processors, composed of collector and emitter qubits, and the coupler. The interaction strength between the collector and emitter is controlled by tuning the frequency and the state of the gate qubit, effectively implementing a quantum switch. From the truth-table measurement (open-gate fidelity 93.38%, closed-gate fidelity 98.77%), we verify the high performance of the quantum transistor. We also show that taking into account the third energy level of the qubits is critical to achieving a high-fidelity transistor. The presented device has a strong potential for quantum information processes in superconducting platforms.

Optimal charging of a superconducting quantum battery

  1. Chang-Kang Hu,
  2. Jiawei Qiu,
  3. Paulo J. P. Souza,
  4. Jiahao Yuan,
  5. Yuxuan Zhou,
  6. Libo Zhang,
  7. Ji Chu,
  8. Xianchuang Pan,
  9. Ling Hu,
  10. Jian Li,
  11. Yuan Xu,
  12. Youpeng Zhong,
  13. Song Liu,
  14. Fei Yan,
  15. Dian Tan,
  16. R. Bachelard,
  17. C. J. Villas-Boas,
  18. Alan C. Santos,
  19. and Dapeng Yu
Quantum batteries are miniature energy storage devices and play a very important role in quantum thermodynamics. In recent years, quantum batteries have been extensively studied, but
limited in theoretical level. Here we report the experimental realization of a quantum battery based on superconducting qubits. Our model explores dark and bright states to achieve stable and powerful charging processes, respectively. Our scheme makes use of the quantum adiabatic brachistochrone, which allows us to speed up the {battery ergotropy injection. Due to the inherent interaction of the system with its surrounding, the battery exhibits a self-discharge, which is shown to be described by a supercapacitor-like self-discharging mechanism. Our results paves the way for proposals of new superconducting circuits able to store extractable work for further usage.

Suppressing Coherent Two-Qubit Errors via Dynamical Decoupling

  1. Jiawei Qiu,
  2. Yuxuan Zhou,
  3. Chang-Kang Hu,
  4. Jiahao Yuan,
  5. Libo Zhang,
  6. Ji Chu,
  7. Wenhui Huang,
  8. Weiyang Liu,
  9. Kai Luo,
  10. Zhongchu Ni,
  11. Xianchuang Pan,
  12. Zhixuan Yang,
  13. Yimeng Zhang,
  14. Yuanzhen Chen,
  15. Xiu-Hao Deng,
  16. Ling Hu,
  17. Jian Li,
  18. Jingjing Niu,
  19. Yuan Xu,
  20. Tongxing Yan,
  21. Youpeng Zhong,
  22. Song Liu,
  23. Fei Yan,
  24. and Dapeng Yu
Scalable quantum information processing requires the ability to tune multi-qubit interactions. This makes the precise manipulation of quantum states particularly difficult for multi-qubit
interactions because tunability unavoidably introduces sensitivity to fluctuations in the tuned parameters, leading to erroneous multi-qubit gate operations. The performance of quantum algorithms may be severely compromised by coherent multi-qubit errors. It is therefore imperative to understand how these fluctuations affect multi-qubit interactions and, more importantly, to mitigate their influence. In this study, we demonstrate how to implement dynamical-decoupling techniques to suppress the two-qubit analogue of the dephasing on a superconducting quantum device featuring a compact tunable coupler, a trending technology that enables the fast manipulation of qubit–qubit interactions. The pure-dephasing time shows an up to ~14 times enhancement on average when using robust sequences. The results are in good agreement with the noise generated from room-temperature circuits. Our study further reveals the decohering processes associated with tunable couplers and establishes a framework to develop gates and sequences robust against two-qubit errors.

High-fidelity, high-scalability two-qubit gate scheme for superconducting qubits

  1. Yuan Xu,
  2. Ji Chu,
  3. Jiahao Yuan,
  4. Jiawei Qiu,
  5. Yuxuan Zhou,
  6. Libo Zhang,
  7. Xinsheng Tan,
  8. Yang Yu,
  9. Song Liu,
  10. Jian Li,
  11. Fei Yan,
  12. and Dapeng Yu
High-quality two-qubit gate operations are crucial for scalable quantum information processing. Often, the gate fidelity is compromised when the system becomes more integrated. Therefore,
a low-error-rate, easy-to-scale two-qubit gate scheme is highly desirable. Here, we experimentally demonstrate a new two-qubit gate scheme that exploits fixed-frequency qubits and a tunable coupler in a superconducting quantum circuit. The scheme requires less control lines, reduces crosstalk effect, simplifies calibration procedures, yet produces a controlled-Z gate in 30ns with a high fidelity of 99.5%. Error analysis shows that gate errors are mostly coherence-limited. Our demonstration paves the way for large-scale implementation of high-fidelity quantum operations.