Direct evidence for cosmic-ray-induced correlated errors in superconducting qubit array

  1. Xue-Gang Li,
  2. Jun-Hua Wang,
  3. Yao-Yao Jiang,
  4. Guang-Ming Xue,
  5. Xiao-Xia Cai,
  6. Jun Zhou,
  7. Ming Gong,
  8. Zhao-Feng Liu,
  9. Shuang-Yu Zheng,
  10. Deng-Ke Ma,
  11. Mo Chen,
  12. Wei-Jie Sun,
  13. Shuang Yang,
  14. Fei Yan,
  15. Yi-Rong Jin,
  16. Xue-Feng Ding,
  17. and Hai-Feng Yu
Correlated errors can significantly impact the quantum error correction, which challenges the assumption that errors occur in different qubits independently in both space and time.
Superconducting qubits have been found to suffer correlated errors across multiple qubits, which could be attributable to ionizing radiations and cosmic rays. Nevertheless, the direct evidence and a quantitative understanding of this relationship are currently lacking. In this work, we propose to continuously monitor multi-qubit simultaneous charge-parity jumps to detect correlated errors and find that occur more frequently than multi-qubit simultaneous bit flips. Then, we propose to position two cosmic-ray muon detectors directly beneath the sample box in a dilution refrigerator and successfully observe the correlated errors in a superconducting qubit array triggered by muons. By introducing a lead shielding layer on the refrigerator, we also reveal that the majority of other correlated errors are primarily induced by gamma rays. Furthermore, we find the superconducting film with a higher recombination rate of quasiparticles used in the qubits is helpful in reducing the duration of correlated errors. Our results provide experimental evidence of the impact of gamma rays and muons on superconducting quantum computation and offer practical insights into mitigation strategies for quantum error correction. In addition, we observe the average occurrence rate of muon-induced correlated errors in our processor is approximately 0.40 min−1cm−2, which is comparable to the muon event rate detected by the muon detector with 0.506 min−1cm−2. This demonstrates the potential applications of superconducting qubit arrays as low-energy threshold sensors in the field of high-energy physics.

Multiplexed control scheme for scalable quantum information processing with superconducting qubits

  1. Pan Shi,
  2. Jiahao Yuan,
  3. Fei Yan,
  4. and Haifeng Yu
The advancement of scalable quantum information processing relies on the accurate and parallel manipulation of a vast number of qubits, potentially reaching into the millions. Superconducting
qubits, traditionally controlled through individual circuitry, currently face a formidable scalability challenge due to the excessive use of wires. This challenge is nearing a critical point where it might soon surpass the capacities of on-chip routing, I/O packaging, testing platforms, and economically feasible solutions. Here we introduce a multiplexed control scheme that efficiently utilizes shared control lines for operating multiple qubits and couplers. By integrating quantum hardware-software co-design, our approach utilizes advanced techniques like frequency multiplexing and individual tuning. This enables simultaneous and independent execution of single- and two-qubit gates with significantly simplified wiring. This scheme has the potential to diminish the number of control lines by one to two orders of magnitude in the near future, thereby substantially enhancing the scalability of superconducting quantum processors.

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.

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.

Engineering superconducting qubits to reduce quasiparticles and charge noise

  1. Xianchuang Pan,
  2. Haolan Yuan,
  3. Yuxuan Zhou,
  4. Libo Zhang,
  5. Jian Li,
  6. Song Liu,
  7. Zhi Hao Jiang,
  8. Gianluigi Catelani,
  9. Ling Hu,
  10. and Fei Yan
In any physical realization of a qubit, identifying, quantifying, and suppressing mechanisms of decoherence are important steps towards the goal of engineering a universal quantum computeror a quantum simulator. Superconducting circuits based on Josephson junctions offer flexibility in qubit design; however, their performance is adversely affected by quasiparticles (broken Cooper pairs) whose density, as observed in various systems, is considerably higher than that expected in thermal equilibrium. A full understanding of the generation mechanism and a mitigation strategy that is compatible with scalable, high-coherence devices are therefore highly desirable. Here we experimentally demonstrate how to control quasiparticle generation by downsizing the qubit structure, capping it with a metallic cover, and equipping it with suitable quasiparticle traps. We achieve record low charge-parity switching rate (<1Hz) in our aluminium devices. At the same time, the devices display improved stability with respect to discrete charging events. Our findings support the hypothesis that the generation of quasiparticles is dominated by the breaking of Cooper pairs at the junction, as a result of photon absorption mediated by the antenna-like qubit structure. We thus demonstrate a convenient approach to shape the electromagnetic environment of superconducting circuits in the sub-terahertz regime, inhibiting decoherence from quasiparticle poisoning.[/expand]

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.

Realization of Super-Robust Geometric Control in a Superconducting Circuit

  1. Sai Li,
  2. Bao-Jie Liu,
  3. Zhongchu Ni,
  4. Libo Zhang,
  5. Zheng-Yuan Xue,
  6. Jian Li,
  7. Fei Yan,
  8. Yuanzhen Chen,
  9. Song Liu,
  10. Man-Hong Yung,
  11. Yuan Xu,
  12. and Dapeng Yu
Geometric phases accompanying adiabatic quantum evolutions can be used to construct robust quantum control for quantum information processing due to their noise-resilient feature. A
significant development along this line is to construct geometric gates using nonadiabatic quantum evolutions to reduce errors due to decoherence. However, it has been shown that nonadiabatic geometric gates are not necessarily more robust than dynamical ones, in contrast to an intuitive expectation. Here we experimentally investigate this issue for the case of nonadiabatic holonomic quantum computation~(NHQC) and show that conventional NHQC schemes cannot guarantee the expected robustness due to a cross coupling to the states outside the computational space. We implement a new set of constraints for gate construction in order to suppress such cross coupling to achieve an enhanced robustness. Using a superconducting quantum circuit, we demonstrate high-fidelity holonomic gates whose infidelity against quasi-static transverse errors can be suppressed up to the fourth order, instead of the second order in conventional NHQC and dynamical gates. In addition, we explicitly measure the accumulated dynamical phase due to the above mentioned cross coupling and verify that it is indeed much reduced in our NHQC scheme. We further demonstrate a protocol for constructing two-qubit NHQC gates also with an enhanced robustness.