He-Kang Li

Deterministic generation and tomography of a macroscopic Bell state between a millimeter-sized spin system and a superconducting qubit

  1. Da Xu,
  2. Xu-Ke Gu,
  3. Yuan-Chao Weng,
  4. He-Kang Li,
  5. Yi-Pu Wang,
  6. Shi-Yao Zhu,
  7. and J. Q. You
Entanglement is a fundamental property in quantum mechanics that systems share inseparable quantum correlation regardless of their mutual distances. Owing to the fundamental significance
and versatile applications, the generation of quantum entanglement between {\it macroscopic} systems has been a focus of current research. Here we report on the deterministic generation and tomography of the macroscopically entangled Bell state in a hybrid quantum system containing a millimeter-sized spin system and a micrometer-sized superconducting qubit. The deterministic generation is realized by coupling the macroscopic spin system and the qubit via a microwave cavity. Also, we develop a joint tomography approach to confirming the deterministic generation of the Bell state, which gives a generation fidelity of 0.90±0.01. Our work makes the macroscopic spin system the largest system capable of generating the maximally entangled quantum state.
arxiv pdf

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