Realization of fast all-microwave CZ gates with a tunable coupler

  1. Shaowei Li,
  2. Daojin Fan,
  3. Ming Gong,
  4. Yangsen Ye,
  5. Xiawei Chen,
  6. Yulin Wu,
  7. Huijie Guan,
  8. Hui Deng,
  9. Hao Rong,
  10. He-Liang Huang,
  11. Chen Zha,
  12. Kai Yan,
  13. Shaojun Guo,
  14. Haoran Qian,
  15. Haibin Zhang,
  16. Fusheng Chen,
  17. Qingling Zhu,
  18. Youwei Zhao,
  19. Shiyu Wang,
  20. Chong Ying,
  21. Sirui Cao,
  22. Jiale Yu,
  23. Futian Liang,
  24. Yu Xu,
  25. Jin Lin,
  26. Cheng Guo,
  27. Lihua Sun,
  28. Na Li,
  29. Lianchen Han,
  30. Cheng-Zhi Peng,
  31. Xiaobo Zhu,
  32. and Jian-Wei Pan
The development of high-fidelity two-qubit quantum gates is essential for digital quantum computing. Here, we propose and realize an all-microwave parametric Controlled-Z (CZ) gates
by coupling strength modulation in a superconducting Transmon qubit system with tunable couplers. After optimizing the design of the tunable coupler together with the control pulse numerically, we experimentally realized a 100 ns CZ gate with high fidelity of 99.38%±0.34% and the control error being 0.1%. We note that our CZ gates are not affected by pulse distortion and do not need pulse correction, {providing a solution for the real-time pulse generation in a dynamic quantum feedback circuit}. With the expectation of utilizing our all-microwave control scheme to reduce the number of control lines through frequency multiplexing in the future, our scheme draws a blueprint for the high-integrable quantum hardware design.

Realization of high-fidelity CZ gates in extensible superconducting qubits design with a tunable coupler

  1. Yangsen Ye,
  2. Sirui Cao,
  3. Yulin Wu,
  4. Xiawei Chen,
  5. Qingling Zhu,
  6. Shaowei Li,
  7. Fusheng Chen,
  8. Ming Gong,
  9. Chen Zha,
  10. He-Liang Huang,
  11. Youwei Zhao,
  12. Shiyu Wang,
  13. Shaojun Guo,
  14. Haoran Qian,
  15. Futian Liang,
  16. Jin Lin,
  17. Yu Xu,
  18. Cheng Guo,
  19. Lihua Sun,
  20. Na Li,
  21. Hui Deng,
  22. Xiaobo Zhu,
  23. and Jian-Wei Pan
High-fidelity two-qubits gates are essential for the realization of large-scale quantum computation and simulation. Tunable coupler design is used to reduce the problem of parasitic
coupling and frequency crowding in many-qubit systems and thus thought to be advantageous. Here we design a extensible 5-qubit system in which center transmon qubit can couple to every four near-neighbor qubit via a capacitive tunable coupler and experimentally demonstrate high-fidelity controlled-phase (CZ) gate by manipulating center qubit and one near-neighbor qubit. Speckle purity benchmarking (SPB) and cross entrophy benchmarking (XEB) are used to assess the purity fidelity and the fidelity of the CZ gate. The average purity fidelity of the CZ gate is 99.69±0.04\% and the average fidelity of the CZ gate is 99.65±0.04\% which means the control error is about 0.04\%. Our work will help resovle many chanllenges in the implementation of large scale quantum systems.

Observation of thermalization and information scrambling in a superconducting quantum processor

  1. Qingling Zhu,
  2. Zheng-Hang Sun,
  3. Ming Gong,
  4. Fusheng Chen,
  5. Yu-Ran Zhang,
  6. Yulin Wu,
  7. Yangsen Ye,
  8. Chen Zha,
  9. Shaowei Li,
  10. Shaojun Guo,
  11. Haoran Qian,
  12. He-Liang Huang,
  13. Jiale Yu,
  14. Hui Deng,
  15. Hao Rong,
  16. Jin Lin,
  17. Yu Xu,
  18. Lihua Sun,
  19. Cheng Guo,
  20. Na Li,
  21. Futian Liang,
  22. Cheng-Zhi Peng,
  23. Heng Fan,
  24. Xiaobo Zhu,
  25. and Jian-Wei Pan
Understanding various phenomena in non-equilibrium dynamics of closed quantum many-body systems, such as quantum thermalization, information scrambling, and nonergodic dynamics, is
a crucial for modern physics. Using a ladder-type superconducting quantum processor, we perform analog quantum simulations of both the XX ladder and one-dimensional (1D) XX model. By measuring the dynamics of local observables, entanglement entropy and tripartite mutual information, we signal quantum thermalization and information scrambling in the XX ladder. In contrast, we show that the XX chain, as free fermions on a 1D lattice, fails to thermalize, and local information does not scramble in the integrable channel. Our experiments reveal ergodicity and scrambling in the controllable qubit ladder, and opens the door to further investigations on the thermodynamics and chaos in quantum many-body systems.

Verification of a resetting protocol for an uncontrolled superconducting qubit

  1. Ming Gong,
  2. Feihu Xu,
  3. Zheng-Da Li,
  4. Zizhu Wang,
  5. Yu-Zhe Zhang,
  6. Yulin Wu,
  7. Shaowei Li,
  8. Youwei Zhao,
  9. Shiyu Wang,
  10. Chen Zha,
  11. Hui Deng,
  12. Zhiguang Yan,
  13. Hao Rong,
  14. Futian Liang,
  15. Jin Lin,
  16. Yu Xu,
  17. Cheng Guo,
  18. Lihua Sun,
  19. Anthony D. Castellano,
  20. Chengzhi Peng,
  21. Yu-Ao Chen,
  22. Xiaobo Zhu,
  23. and Jian-Wei Pan
We experimentally verify the simplest non-trivial case of a quantum resetting protocol with five superconducting qubits, testing it with different types of free evolutions and target-probe
interactions. After post-selection, we obtained a reset state fidelity as high as 0.951, and the process fidelity was found to be 0.792. We also implemented 100 randomly-chosen interactions and demonstrated an average success probability of 0.323, experimentally confirmed the non-zeros probability of success for unknown interactions; the numerical simulated value is 0.384. We anticipate this protocol will have widespread applications in quantum information processing science, since it is able to combat any form of free evolution.

Demonstration of Adiabatic Variational Quantum Computing with a Superconducting Quantum Coprocessor

  1. Ming-Cheng Chen,
  2. Ming Gong,
  3. Xiao-Si Xu,
  4. Xiao Yuan,
  5. Jian-Wen Wang,
  6. Can Wang,
  7. Chong Ying,
  8. Jin Lin,
  9. Yu Xu,
  10. Yulin Wu,
  11. Shiyu Wang,
  12. Hui Deng,
  13. Futian Liang,
  14. Cheng-Zhi Peng,
  15. Simon C. Benjamin,
  16. Xiaobo Zhu,
  17. Chao-Yang Lu,
  18. and Jian-Wei Pan
Adiabatic quantum computing enables the preparation of many-body ground states. This is key for applications in chemistry, materials science, and beyond. Realisation poses major experimental
challenges: Direct analog implementation requires complex Hamiltonian engineering, while the digitised version needs deep quantum gate circuits. To bypass these obstacles, we suggest an adiabatic variational hybrid algorithm, which employs short quantum circuits and provides a systematic quantum adiabatic optimisation of the circuit parameters. The quantum adiabatic theorem promises not only the ground state but also that the excited eigenstates can be found. We report the first experimental demonstration that many-body eigenstates can be efficiently prepared by an adiabatic variational algorithm assisted with a multi-qubit superconducting coprocessor. We track the real-time evolution of the ground and exited states of transverse-field Ising spins with a fidelity up that can reach about 99%.

Deterministic entanglement swapping in a superconducting circuit

  1. Wen Ning,
  2. Xin-Jie Huang,
  3. Pei-Rong Han,
  4. Hekang Li,
  5. Hui Deng,
  6. Zhen-Biao Yang,
  7. Zhi-Rong Zhong,
  8. Yan Xia,
  9. Kai Xu,
  10. Dongning Zheng,
  11. and Shi-Biao Zheng
Entanglement swapping allows two particles that have never been coupled directly or indirectly to be nonlocally correlated. Besides fundamental interest, this procedure has applications
in complex entanglement manipulation and quantum communication. Entanglement swapping for qubits has been demonstrated in optical experiments, but where the process was conditional on detection of preset photon coincidence events, which succeeded with only a small probability. Here we report an unconditional entanglement swapping experiment with superconducting qubits. Using controllable qubit-qubit couplings mediated by a resonator, we prepare two highly entangled qubit pairs and then perform the Bell state measurement on two qubits coming from different entangled pairs, projecting the remaining two qubits to one of four Bell states. The measured concurrences for these Bell states are above 0.75,demonstrating the quantum nature of entanglement swapping. With this setup, we further demonstrate delayed-choice entanglement swapping, confirming whether two qubits behaved as in an entangled state or as in a separate state is determined by a later choice of the type of measurement on their partners. This is the first demonstration of entanglement-separability duality in a deterministic way, closing the detection loophole the previous experiments suffer from.

Genuine 12-qubit entanglement on a superconducting quantum processor

  1. Ming Gong,
  2. Ming-Cheng Chen,
  3. Yarui Zheng,
  4. Shiyu Wang,
  5. Chen Zha,
  6. Hui Deng,
  7. Zhiguang Yan,
  8. Hao Rong,
  9. Yulin Wu,
  10. Shaowei Li,
  11. Fusheng Chen,
  12. Youwei Zhao,
  13. Futian Liang,
  14. Jin Lin,
  15. Yu Xu,
  16. Cheng Guo,
  17. Lihua Sun,
  18. Anthony D. Castellano,
  19. Haohua Wang,
  20. Chengzhi Peng,
  21. Chao-Yang Lu,
  22. Xiaobo Zhu,
  23. and Jian-Wei Pan
We report the preparation and verification of a genuine 12-qubit entanglement in a superconducting processor. The processor that we designed and fabricated has qubits lying on a 1D
chain with relaxation times ranging from 29.6 to 54.6 μs. The fidelity of the 12-qubit entanglement was measured to be above 0.5544±0.0025, exceeding the genuine multipartite entanglement threshold by 21 standard deviations. Our entangling circuit to generate linear cluster states is depth-invariant in the number of qubits and uses single- and double-qubit gates instead of collective interactions. Our results are a substantial step towards large-scale random circuit sampling and scalable measurement-based quantum computing.

Dephasing-insensitive quantum information storage and processing with superconducting qubits

  1. Qiujiang Guo,
  2. Shi-Biao Zheng,
  3. Jianwen Wang,
  4. Chao Song,
  5. Pengfei Zhang,
  6. Kemin Li,
  7. Wuxin Liu,
  8. Hui Deng,
  9. Keqiang Huang,
  10. Dongning Zheng,
  11. Xiaobo Zhu,
  12. H. Wang,
  13. C.-Y. Lu,
  14. and Jian-Wei Pan
A central task towards building a practical quantum computer is to protect individual qubits from decoherence while retaining the ability to perform high-fidelity entangling gates involving
arbitrary two qubits. Here we propose and demonstrate a dephasing-insensitive procedure for storing and processing quantum information in an all-to-all connected superconducting circuit involving multiple frequency-tunable qubits, each of which can be controllably coupled to any other through a central bus resonator. Although it is generally believed that the extra frequency tunability enhances the control freedom but induces more dephasing impact for superconducting qubits, our results show that any individual qubit can be dynamically decoupled from dephasing noise by applying a weak continuous and resonant driving field whose phase is reversed in the middle of the pulse. More importantly, we demonstrate a new method for realizing two-qubit phase gate with inherent dynamical decoupling via the combination of continuous driving and qubit-qubit swapping coupling. We find that the weak continuous driving fields not only enable the conditional dynamics essential for quantum information processing, but also protect both qubits from dephasing during the gate operation.

Observation of dynamical quantum phase transition by a superconducting qubit simulation

  1. Xue-Yi Guo,
  2. Chao Yang,
  3. Yu Zeng,
  4. Yi Peng,
  5. He-Kang Li,
  6. Hui Deng,
  7. Yi-Rong Jin,
  8. Shu Chen,
  9. Dongning Zheng,
  10. and Heng Fan
A dynamical quantum phase transition can occur in time evolution of sudden quenched quantum systems across phase transition. It corresponds to nonanalytic behavior at a critical time
for rate function of quantum state return amplitude, analogous to nonanalyticity of the free energy density at the critical temperature in macroscopic systems. A variety of many-body systems can be represented in momentum space as a spin-1/2 state evolving in Bloch sphere, where each momentum mode is decoupled and thus can be simulated independently by a single qubit. Here, we report the observation of dynamical quantum phase transition by a superconducting qubit simulation of the quantum quench dynamics of many-body systems. We take the Ising model with transverse field as an example. In experiment, the spin state initially polarized longitudinally evolves based on Hamiltonian with adjustable parameters depending on momentum and strength of the transverse magnetic field. The time evolved quantum state will be readout by state tomography. Evidences of dynamical quantum phase transition such as paths of time evolution state on Bloch sphere, the non-analytic behavior in dynamical free energy and the emergence of Skyrmion lattice in momentum-time space are provided. The experiment data agrees well with theoretical and numerical calculations. The experiment demonstrates for the first time explicitly the topological invariant, both topological trivial and non-trivial, for dynamical quantum phase transition. Our experiment results show that the quantum phase transition of many-body systems can be successfully simulated by a single qubit by varying control parameter over the range of momentum.

Parallel quantum operations with chiral spin states in a superconducting circuit

  1. Da-Wei Wang,
  2. Chao Song,
  3. Wei Feng,
  4. Han Cai,
  5. Da Xu,
  6. Hui Deng,
  7. Dongning Zheng,
  8. Xiaobo Zhu,
  9. H. Wang,
  10. Shiyao Zhu,
  11. and Marlan O. Scully
Recently it was shown that mesoscopic superpositions of photonic states can be prepared based on a spin-gated chiral photon rotation in a Fock-state lattice of three cavities coupled
to a spin (two-level atom). By exchanging the roles of the cavities and the spin, we have performed parallel operations on chiral spin states based on an antisymmetric spin exchange interaction (ASI) in a superconducting circuit. The ASI, which is also called Dzyaloshinskii-Moriya interaction, plays an important role in the formation of topological spin textures such as skyrmions. By periodically modulating the transition frequencies of three superconducting qubits interacting with a bus resonator, we synthesize a chiral ASI Hamiltonian with spin-gated chiral dynamics, which allow us to demonstrate a three-spin chiral logic gate and entangle up to five qubits in Greenberger-Horne-Zeilinger states. Our results pave the way for quantum simulation of magnetism with ASI and quantum computation with chiral spin states.