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.

Experimental Realization of Two Qutrits Gate with Tunable Coupling in Superconducting Circuits

  1. Kai Luo,
  2. Wenhui Huang,
  3. Ziyu Tao,
  4. Libo Zhang,
  5. Yuxuan Zhou,
  6. Ji Chu,
  7. Wuxin Liu,
  8. Biying Wang,
  9. Jiangyu Cui,
  10. Song Liu,
  11. Fei Yan,
  12. Man-Hong Yung,
  13. Yuanzhen Chen,
  14. Tongxing Yan,
  15. and Dapeng Yu
Gate-based quantum computation has been extensively investigated using quantum circuits based on qubits. In many cases, such qubits are actually made out of multilevel systems but with
only two states being used for computational purpose. While such a strategy has the advantage of being in line with the common binary logic, it in some sense wastes the ready-for-use resources in the large Hilbert space of these intrinsic multi-dimensional systems. Quantum computation beyond qubits (e.g., using qutrits or qudits) has thus been discussed and argued to be more efficient than its qubit counterpart in certain scenarios. However, one of the essential elements for qutrit-based quantum computation, two-qutrit quantum gate, remains a major challenge. In this work, we propose and demonstrate a highly efficient and scalable two-qutrit quantum gate in superconducting quantum circuits. Using a tunable coupler to control the cross-Kerr coupling between two qutrits, our scheme realizes a two-qutrit conditional phase gate with fidelity 89.3% by combining simple pulses applied to the coupler with single-qutrit operations. We further use such a two-qutrit gate to prepare an EPR state of two qutrits with a fidelity of 95.5%. Our scheme takes advantage of a tunable qutrit-qutrit coupling with a large on/off ratio. It therefore offers both high efficiency and low cross talk between qutrits, thus being friendly for scaling up. Our work constitutes an important step towards scalable qutrit-based quantum computation.

Cancelling microwave crosstalk with fixed-frequency qubits

  1. Wuerkaixi Nuerbolati,
  2. Zhikun Han,
  3. Ji Chu,
  4. Yuxuan Zhou,
  5. Xinsheng Tan,
  6. Yang Yu,
  7. Song Liu,
  8. and Fei Yan
Scalable quantum information processing requires that modular gate operations can be executed in parallel. The presence of crosstalk decreases the individual addressability, causing
erroneous results during simultaneous operations. For superconducting qubits which operate in the microwave regime, electromagnetic isolation is often limited due to design constraints, leading to signal crosstalk that can deteriorate the quality of simultaneous gate operations. Here, we propose and demonstrate a method based on AC Stark effect for calibrating the microwave signal crosstalk. The method is suitable for processors based on fixed-frequency qubits which are known for high coherence and simple control. The optimal compensation parameters can be reliably identified from a well-defined interference pattern. We implement the method on an array of 7 superconducting qubits, and show its effectiveness in removing the majority of crosstalk errors.

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.

Scalable method for eliminating residual ZZ interaction between superconducting qubits

  1. Zhongchu Ni,
  2. Sai Li,
  3. Libo Zhang,
  4. Ji Chu,
  5. Jingjing Niu,
  6. Tongxing Yan,
  7. Xiuhao Deng,
  8. Ling Hu,
  9. Jian Li,
  10. Youpeng Zhong,
  11. Song Liu,
  12. Fei Yan,
  13. Yuan Xu,
  14. and Dapeng Yu
Unwanted ZZ interaction is a quantum-mechanical crosstalk phenomenon which correlates qubit dynamics and is ubiquitous in superconducting qubit system. It adversely affects the quality
of quantum operations and can be detrimental in scalable quantum information processing. Here we propose and experimentally demonstrate a practically extensible approach for complete cancellation of residual ZZ interaction between fixed-frequency transmon qubits, which are known for long coherence and simple control. We apply to the intermediate coupler that connects the qubits a weak microwave drive at a properly chosen frequency in order to noninvasively induce ac Stark shift for ZZ cancellation. We verify the cancellation performance by measuring vanishing two-qubit entangling phases and ZZ correlations. In addition, we implement randomized benchmarking experiment to extract the idling gate fidelity which shows good agreement with the coherence limit, demonstrating the effectiveness of ZZ cancellation. Our method allows independent addressability of each qubit-qubit connection, and is applicable to both non-tunable and tunable coupler, promising better compatibility with future large-scale quantum processors.

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.

Experimental Realization of Universal Time-optimal non-Abelian Geometric Gates

  1. Zhikun Han,
  2. Yuqian Dong,
  3. Baojie Liu,
  4. Xiaopei Yang,
  5. Shuqing Song,
  6. Luqing Qiu,
  7. Danyu Li,
  8. Ji Chu,
  9. Wen Zheng,
  10. Jianwen Xu,
  11. Tianqi Huang,
  12. Zhimin Wang,
  13. Xiangmin Yu,
  14. Xinsheng Tan,
  15. Dong Lan,
  16. Man-Hong Yung,
  17. and Yang Yu
Based on the geometrical nature of quantum phases, non-adiabatic holonomic quantum control (NHQC) has become a standard technique for enhancing robustness in constructing quantum gates.
However, the conventional approach of NHQC is sensitive to control instability, as it requires the driving pulses to cover a fixed pulse area. Furthermore, even for small-angle rotations, all operations need to be completed with the same duration of time. Here we experimentally demonstrate a time-optimal and unconventional approach of NHQC (called TOUNHQC), which can optimize the operation time of any holonomic gate. Compared with the conventional approach, TOUNHQC provides an extra layer of robustness to decoherence and control errors. The experiment involves a scalable architecture of superconducting circuit, where we achieved a fidelity of 99.51% for a single qubit gate using interleaved randomized benchmarking. Moreover, a two-qubit holonomic control-phase gate has been implemented where the gate error can be reduced by as much as 18% compared with NHQC.

Realization of Superadiabatic Two-qubit Gates Using Parametric Modulation in Superconducting Circuits

  1. Ji Chu,
  2. Danyu Li,
  3. Xiaopei Yang,
  4. Shuqing Song,
  5. Zhikun Han,
  6. Zhen Yang,
  7. Yuqian Dong,
  8. Wen Zheng,
  9. Zhimin Wang,
  10. Xiangmin Yu,
  11. Dong Lan,
  12. Xinsheng Tan,
  13. and Yang Yu
We propose a protocol to realize parametric control of two-qubit coupling, where the amplitude and phase are tuned by a longitudinal field. Based on the tunable Hamiltonian, we demonstrate
the superadiabatic two-qubit quantum gate using superconducting quantum circuits. Our experimental results show that the state of qubits evolves adiabatically during the gate operation even though the processing time is close to the quantum limit. In addition, the quantum state transition is insensitive to the variation of control parameters, and the fidelity of a SWAP gate achieved 98.5%. This robust parametric two-qubit gate can alleviate the tension of frequency crowding for quantum computation with multiple qubits.