Yu Zeng

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.
arxiv pdf

Emulating many-body localization with a superconducting quantum processor

  1. Kai Xu,
  2. Jin-Jun Chen,
  3. Yu Zeng,
  4. Yuran Zhang,
  5. Chao Song,
  6. Wuxin Liu,
  7. Qiujiang Guo,
  8. Pengfei Zhang,
  9. Da Xu,
  10. Hui Deng,
  11. Keqiang Huang,
  12. H. Wang,
  13. Xiaobo Zhu,
  14. Dongning Zheng,
  15. and Heng Fan
The law of statistical physics dictates that generic closed quantum many-body systems initialized in nonequilibrium will thermalize under their own dynamics. However, the emergence
of many-body localization (MBL) owing to the interplay between interaction and disorder, which is in stark contrast to Anderson localization that only addresses noninteracting particles in the presence of disorder, greatly challenges this concept because it prevents the systems from evolving to the ergodic thermalized state. One critical evidence of MBL is the long-time logarithmic growth of entanglement entropy, and a direct observation of it is still elusive due to the experimental challenges in multiqubit single-shot measurement and quantum state tomography. Here we present an experiment of fully emulating the MBL dynamics with a 10-qubit superconducting quantum processor, which represents a spin-1/2 XY model featuring programmable disorder and long-range spin-spin interactions. We provide essential signatures of MBL, such as the imbalance due to the initial nonequilibrium, the violation of eigenstate thermalization hypothesis, and, more importantly, the direct evidence of the long-time logarithmic growth of entanglement entropy. Our results lay solid foundations for precisely simulating the intriguing physics of quantum many-body systems on the platform of large-scale multiqubit superconducting quantum processors.
arxiv pdf