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.

Photon-Dressed Bloch-Siegert Shift in an Ultrastrongly Coupled Circuit Quantum Electrodynamical System

  1. Shuai-Peng Wang,
  2. Guo-Qiang Zhang,
  3. Yimin Wang,
  4. Zhen Chen,
  5. Tiefu Li,
  6. J. S. Tsai,
  7. Shi-Yao Zhu,
  8. and J. Q. You
A cavity quantum electrodynamical (QED) system beyond the strong-coupling regime is expected to exhibit intriguing quantum phenomena. Here we report a direct measurement of the photon-dressed
qubit transition frequencies up to four photons by harnessing the same type of state transitions in an ultrastrongly coupled circuit-QED system realized by inductively coupling a superconducting flux qubit to a coplanar-waveguide resonator. This demonstrates a convincing observation of the photon-dressed Bloch-Siegert shift in the ultrastrongly coupled quantum system. Moreover, our results show that the photon-dressed Bloch-Siegert shift becomes more pronounced as the photon number increases, which is a characteristic of the quantum Rabi model.

Simultaneous excitation of two noninteracting atoms with time-frequency correlated photon pairs in a superconducting circuit

  1. Wenhui Ren,
  2. Wuxin Liu,
  3. Chao Song,
  4. Hekang Li,
  5. Qiujiang Guo,
  6. Zhen Wang,
  7. Dongning Zheng,
  8. Girish S. Agarwal,
  9. Marlan O. Scully,
  10. Shi-Yao Zhu,
  11. H. Wang,
  12. and Da-Wei Wang
Here we report the first observation of simultaneous excitation of two noninteracting atoms by a pair of time-frequency correlated photons in a superconducting circuit. The strong coupling
regime of this process enables the synthesis of a three-body interaction Hamiltonian, which allows the generation of the tripartite Greenberger-Horne-Zeilinger state in a single step with a fidelity as high as 0.95. We further demonstrate the quantum Zeno effect of inhibiting the simultaneous two-atom excitation by continuously measuring whether the first photon is emitted. This work provides a new route in synthesizing many-body interaction Hamiltonian and coherent control of entanglement.

Multiqubit Greenberger-Horne-Zeilinger state generated by synthetic magnetic field in circuit QED

  1. Wei Feng,
  2. Da-Wei Wang,
  3. Han Cai,
  4. and Shi-Yao Zhu
We propose a scheme to generate Greenberger-Horne-Zeilinger (GHZ) state for N superconducting qubits in a circuit QED system. By sinusoidally modulating the qubit-qubit coupling, a
synthetic magnetic field has been made which broken the time-reversal symmetry of the system. Directional rotation of qubit excitation can be realized in a three-qubit loop under the artificial magnetic field. Based on the special quality that the rotation of qubit excitation has different direction for single- and double-excitation loops, we can generate three-qubit GHZ state and extend this preparation method to arbitrary multiqubit GHZ state. Our analysis also shows that the scheme is robust to various operation errors and environmental noise.