Bao-Jie Liu

Observation of discrete charge states of a coherent two-level system in a superconducting qubit

  1. Bao-Jie Liu,
  2. Ying-Ying Wang,
  3. Tal Sheffer,
  4. and Chen Wang
We report observations of discrete charge states of a coherent dielectric two-level system (TLS) that is strongly coupled to an offset-charge-sensitive superconducting transmon qubit.
We measure an offset charge of 0.072e associated with the two TLS eigenstates, which have a transition frequency of 2.9 GHz and a relaxation time exceeding 3 ms. Combining measurements in the strong dispersive and resonant regime, we quantify both transverse and longitudinal couplings of the TLS-qubit interaction. We further perform joint tracking of TLS transitions and quasiparticle tunneling dynamics but find no intrinsic correlations. This study demonstrates microwave-frequency TLS as a source of low-frequency charge noise.
arxiv pdf

Realization of Super-Robust Geometric Control in a Superconducting Circuit

  1. Sai Li,
  2. Bao-Jie Liu,
  3. Zhongchu Ni,
  4. Libo Zhang,
  5. Zheng-Yuan Xue,
  6. Jian Li,
  7. Fei Yan,
  8. Yuanzhen Chen,
  9. Song Liu,
  10. Man-Hong Yung,
  11. Yuan Xu,
  12. and Dapeng Yu
Geometric phases accompanying adiabatic quantum evolutions can be used to construct robust quantum control for quantum information processing due to their noise-resilient feature. A
significant development along this line is to construct geometric gates using nonadiabatic quantum evolutions to reduce errors due to decoherence. However, it has been shown that nonadiabatic geometric gates are not necessarily more robust than dynamical ones, in contrast to an intuitive expectation. Here we experimentally investigate this issue for the case of nonadiabatic holonomic quantum computation~(NHQC) and show that conventional NHQC schemes cannot guarantee the expected robustness due to a cross coupling to the states outside the computational space. We implement a new set of constraints for gate construction in order to suppress such cross coupling to achieve an enhanced robustness. Using a superconducting quantum circuit, we demonstrate high-fidelity holonomic gates whose infidelity against quasi-static transverse errors can be suppressed up to the fourth order, instead of the second order in conventional NHQC and dynamical gates. In addition, we explicitly measure the accumulated dynamical phase due to the above mentioned cross coupling and verify that it is indeed much reduced in our NHQC scheme. We further demonstrate a protocol for constructing two-qubit NHQC gates also with an enhanced robustness.
arxiv pdf

Implementing universal nonadiabatic holonomic quantum gates with transmons

  1. Zhuo-Ping Hong,
  2. Bao-Jie Liu,
  3. Jia-Qi Cai,
  4. Xin-Ding Zhang,
  5. Yong Hu,
  6. Z. D. Wang,
  7. and Zheng-Yuan Xue
Geometric phases are well known to be noise-resilient in quantum evolutions/operations. Holonomic quantum gates provide us with a robust way towards universal quantum computation, as
these quantum gates are actually induced by nonabelian geometric phases. Here we propose and elaborate how to efficiently implement universal nonadiabatic holonomic quantum gates on simpler superconducting circuits, with a single transmon serving as a qubit. In our proposal, an arbitrary single-qubit holonomic gate can be realized in a single-loop scenario, by varying the amplitudes and phase difference of two microwave fields resonantly coupled to a transmon, while nontrivial two-qubit holonomic gates may be generated with a transmission-line resonator being simultaneously coupled to the two target transmons in an effective resonant way. Moreover, our scenario may readily be scaled up to a two-dimensional lattice configuration, which is able to support large scalable quantum computation, paving the way for practically implementing universal nonadiabatic holonomic quantum computation with superconducting circuits.
arxiv pdf