Two-level approximation of transmons in quantum quench experiments

  1. H. S. Yan,
  2. Yong-Yi Wang,
  3. S. K. Zhao,
  4. Z. H. Yang,
  5. Z. T. Wang,
  6. Kai Xu,
  7. Ye Tian,
  8. H. F. Yu,
  9. Heng Fan,
  10. and S. P. Zhao
Quantum quench is a typical protocol in the study of nonequilibrium dynamics of quantum many-body systems. Recently a number of experiments with superconducting transmon qubits are
reported, in which the celebrated spin and hard-core Bose-Hubbard models with two energy levels on individual sites are used. The transmons have nonequidistant energy levels, among which the two lowest levels form the computational subspace. In this work, we numerically simulate realistic experiments of quantum quench dynamics and discuss the applicability of the two-level approximation for the multilevel transmons. We calculate the fidelity decay (i.e., the time-dependent overlap of evolving wave functions) due to the state leakage to transmon high energy levels for two kinds of quantum quench experiments with time reversal and time evolution in one direction, respectively. We present the results of the fidelity decay for different system Hamiltonians with various initial state, qubit coupling strength, and external driving. The extent to which the spin and hard-core Bose-Hubbard models can be applied under various circumstances is discussed and compared with experimental observations. Our work provides a precise way to assess the two-level approximation of transmons in quantum quench experiments and shows that good approximation is reachable using the present-day superconducting circuit architecture.

Observation of critical phase transition in a generalized Aubry-André-Harper model on a superconducting quantum processor with tunable couplers

  1. Hao Li,
  2. Yong-Yi Wang,
  3. Yun-Hao Shi,
  4. Kaixuan Huang,
  5. Xiaohui Song,
  6. Gui-Han Liang,
  7. Zheng-Yang Mei,
  8. Bozhen Zhou,
  9. He Zhang,
  10. Jia-Chi Zhang,
  11. Shu Chen,
  12. Shiping Zhao,
  13. Ye Tian,
  14. Zhan-Ying Yang,
  15. Zhongcheng Xiang,
  16. Kai Xu,
  17. Dongning Zheng,
  18. and Heng Fan
Quantum simulation enables study of many-body systems in non-equilibrium by mapping to a controllable quantum system, providing a new tool for computational intractable problems. Here,
using a programmable quantum processor with a chain of 10 superconducting qubits interacted through tunable couplers, we simulate the one-dimensional generalized Aubry-André-Harper model for three different phases, i.e., extended, localized and critical phases. The properties of phase transitions and many-body dynamics are studied in the presence of quasi-periodic modulations for both off-diagonal hopping coefficients and on-site potentials of the model controlled respectively by adjusting strength of couplings and qubit frequencies. We observe the spin transport for initial single- and multi-excitation states in different phases, and characterize phase transitions by experimentally measuring dynamics of participation entropies. Our experimental results demonstrate that the newly developed tunable coupling architecture of superconducting processor extends greatly the simulation realms for a wide variety of Hamiltonians, and may trigger further investigations on various quantum and topological phenomena.

Entanglement-interference complementarity and experimental demonstration in a superconducting circuit

  1. Xin-Jie Huang,
  2. Pei-Rong Han,
  3. Wen Ning,
  4. Shou-Ban Yang,
  5. Xin Zhu,
  6. Jia-Hao Lü,
  7. Ri-Hua Zheng,
  8. Hekang Li,
  9. Zhen-Biao Yang,
  10. Qi-Cheng Wu,
  11. Kai Xu,
  12. Chui-Ping Yang,
  13. Dongning Zheng,
  14. Heng Fan,
  15. and Shi-Biao Zheng
Quantum entanglement between an interfering particle and a detector for acquiring the which-path information plays a central role for enforcing Bohr’s complementary principle,
but the quantitative relation between this entanglement and the fringe visibility remains untouched upon. Here we find an equality for quantifying this relation. Our equality characterizes how well the interference pattern can be preserved when an interfering particle, initially carrying a definite amount of coherence, is entangled with a which-path detector to a certain degree. This equality provides a connection between entanglement and interference in the unified framework of coherence, revealing the quantitative entanglement-interference complementarity for the first time. We experimentally demonstrate this relation with a superconducting circuit, where a resonator serves as a which-path detector for an interfering qubit. The results demonstrate quantum entanglement is the mechanism for prohibiting any detector from acquiring which-path information without deteriorating the interference pattern, which was not confirmed previously.

Observation of Emergent ℤ2 Gauge Invariance in a Superconducting Circuit

  1. Zhan Wang,
  2. Zi-Yong Ge,
  3. Zhongcheng Xiang,
  4. Xiaohui Song,
  5. Rui-Zhen Huang,
  6. Pengtao Song,
  7. Xue-Yi Guo,
  8. Luhong Su,
  9. Kai Xu,
  10. Dongning Zheng,
  11. and Heng Fan
Lattice gauge theory (LGT) is one of the most fundamental subjects in modern quantum many-body physics, and has recently attracted many research interests in quantum simulations. Here
we experimentally investigate the emergent ℤ2 gauge invariance in a 1D superconducting circuit with 10 transmon qubits. By precisely adjusting the staggered longitude and transverse fields to each qubit, we construct an effective Hamiltonian containing a LGT and gauge-broken terms. The corresponding matter sector can exhibit localization, and there also exist a 3-qubit operator, of which the expectation value can retain nonzero for long time in a low-energy regime. The above localization can be regarded as confinement of the matter field, and the 3-body operator is the ℤ2 gauge generator. Thus, these experimental results demonstrate that, despite the absent of gauge structure in the effective Hamiltonian, ℤ2 gauge invariance can still emerge in the low-energy regime. Our work paves the way for both theoretically and experimentally studying the rich physics in quantum many-body system with an emergent gauge invariance.

Probing Operator Spreading via Floquet Engineering in a Superconducting Circuit

  1. S. K. Zhao,
  2. Zi-Yong Ge,
  3. Zhongcheng Xiang,
  4. G. M. Xue,
  5. H. S. Yan,
  6. Z. T. Wang,
  7. Zhan Wang,
  8. H. K. Xu,
  9. F. F. Su,
  10. Z. H. Yang,
  11. He Zhang,
  12. Yu-Ran Zhang,
  13. Xue-Yi Guo,
  14. Kai Xu,
  15. Ye Tian,
  16. H. F. Yu,
  17. D. N. Zheng,
  18. Heng Fan,
  19. and S. P. Zhao
Operator spreading, often characterized by out-of-time-order correlators (OTOCs), is one of the central concepts in quantum many-body physics. However, measuring OTOCs is experimentally
challenging due to the requirement of reversing the time evolution of the system. Here we apply Floquet engineering to investigate operator spreading in a superconducting 10-qubit chain. Floquet engineering provides an effective way to tune the coupling strength between nearby qubits, which is used to demonstrate quantum walks with tunable coupling, dynamic localization, reversed time evolution, and the measurement of OTOCs. A clear light-cone-like operator propagation is observed in the system with multiphoton excitations, and the corresponding spreading velocity is equal to that of quantum walk. Our results indicate that the method has a high potential for simulating a variety of quantum many-body systems and their dynamics, which is also scalable to more qubits and higher dimensional circuits.

Stark many-body localization transitions in superconducting circuits

  1. Yong-Yi Wang,
  2. Zheng-Hang Sun,
  3. and Heng Fan
Recent numerical and experimental works have revealed a disorder-free many-body localization (MBL) in an interacting system subjecting to a linear potential, known as the Stark MBL.The conventional MBL, induced by disorder, has been widely studied by using quantum simulations based on superconducting circuits. Here, we consider the Stark MBL in two types of superconducting circuits, i.e., the 1D array of superconducting qubits, and the circuit where non-local interactions between qubits are mediated by a resonator bus. We calculate the entanglement entropy and participate entropy of the highly-excited eigenstates, and obtain the lower bound of the critical linear potential γc, using the finite-size scaling collapse. Moreover, we study the non-equilibrium properties of the Stark MBL. In particular, we observe an anomalous relaxation of the imbalance, dominated by the power-law decay t−ξ. The exponent ξ satisfies ξ∝|γ−γc|ν when γ<γc, and vanishes for γ≥γc, which can be employed to estimate the γc. Our work indicates that superconducting circuits are a promising platform for investigating the critical properties of the Stark MBL transition.[/expand]

Metrological characterisation of non-Gaussian entangled states of superconducting qubits

  1. Kai Xu,
  2. Yu-Ran Zhang,
  3. Zheng-Hang Sun,
  4. Hekang Li,
  5. Pengtao Song,
  6. Zhongcheng Xiang,
  7. Kaixuan Huang,
  8. Hao Li,
  9. Yun-Hao Shi,
  10. Chi-Tong Chen,
  11. Xiaohui Song,
  12. Dongning Zheng,
  13. Franco Nori,
  14. H. Wang,
  15. and Heng Fan
Multipartite entangled states are significant resources for both quantum information processing and quantum metrology. In particular, non-Gaussian entangled states are predicted to
achieve a higher sensitivity of precision measurements than Gaussian states. On the basis of metrological sensitivity, the conventional linear Ramsey squeezing parameter (RSP) efficiently characterises the Gaussian entangled atomic states but fails for much wider classes of highly sensitive non-Gaussian states. These complex non-Gaussian entangled states can be classified by the nonlinear squeezing parameter (NLSP), as a generalisation of the RSP with respect to nonlinear observables, and identified via the Fisher information. However, the NLSP has never been measured experimentally. Using a 19-qubit programmable superconducting processor, here we report the characterisation of multiparticle entangled states generated during its nonlinear dynamics. First, selecting 10 qubits, we measure the RSP and the NLSP by single-shot readouts of collective spin operators in several different directions. Then, by extracting the Fisher information of the time-evolved state of all 19 qubits, we observe a large metrological gain of 9.89[Math Processing Error] dB over the standard quantum limit, indicating a high level of multiparticle entanglement for quantum-enhanced phase sensitivity. Benefiting from high-fidelity full controls and addressable single-shot readouts, the superconducting processor with interconnected qubits provides an ideal platform for engineering and benchmarking non-Gaussian entangled states that are useful for quantum-enhanced metrology.

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.

Approximating Lattice Gauge Theories on Superconducting Circuits: Quantum Phase Transition and Quench Dynamics

  1. Zi-Yong Ge,
  2. Rui-Zhen Huang,
  3. Zi Yang Meng,
  4. and Heng Fan
We propose an implementation to approximate Z2 lattice gauge theory (LGT) on superconducting quantum circuits, where the effective theory is a mixture of a LGT and a gauge-broken term.
Using matrix product state based methods, both the ground state properties and quench dynamics are systematically investigated. With an increase of the transverse (electric) field, the system displays a quantum phase transition from a disordered phase to a translational symmetry breaking phase. In the ordered phase, an approximate Gaussian law of the Z2 LGT emerges in the ground state. Moreover, to shed light on the experiments, we also study the quench dynamics, where there is a dynamical signature of the spontaneous translational symmetry breaking. The spreading of the single particle of matter degree is diffusive under the weak transverse field, while it is ballistic with small velocity for the strong field. Furthermore, due to the existence of an approximate Gaussian law under the strong transverse field, the matter degree can also exhibit a confinement which leads to a strong suppression of the nearest-neighbor hopping. Our results pave the way for simulating the LGT on superconducting circuits, including the quantum phase transition and quench dynamics.

Demonstration of a non-Abelian geometric controlled-Not gate in a superconducting circuit

  1. Kai Xu,
  2. Wen Ning,
  3. Xin-Jie Huang,
  4. Pei-Rong Han,
  5. Hekang Li,
  6. Zhen-Biao Yang,
  7. Dongning Zheng,
  8. Heng Fan,
  9. and Shi-Biao Zheng
Holonomies, arising from non-Abelian geometric transformations of quantum states in Hilbert space, offer a promising way for quantum computation. The non-community of these holonomies
renders them suitable for realization of a universal set of quantum logic gates, while the global geometric feature may result in some noise-resilient advantages. Here we report on the first on-chip realization of the non-Abelian geometric controlled-Not gate, which is a buidling block for constructing a holonomic quantum computer. The conditional dynamics is achieved in an all-to-all connected architecture involving multiple frequency-tunable superconducting qubits controllably coupled to a resonator; a holonomic gate between any two qubits can be implemented by tuning their frequencies on resonance with the resonator and applying a two-tone drive to one of them. The combination of the present gate and previously demonstrated holonomic single-qubit operations represents an all-holonomic approach to scalable quantum computation on a superconducting platform.