Characterization of loss mechanisms in a fluxonium qubit

  1. Hantao Sun,
  2. Feng Wu,
  3. Hsiang-Sheng Ku,
  4. Xizheng Ma,
  5. Jin Qin,
  6. Zhijun Song,
  7. Tenghui Wang,
  8. Gengyan Zhang,
  9. Jingwei Zhou,
  10. Yaoyun Shi,
  11. Hui-Hai Zhao,
  12. and Chunqing Deng
Using a fluxonium qubit with in situ tunability of its Josephson energy, we characterize its energy relaxation at different flux biases as well as different Josephson energy values.
The relaxation rate at qubit energy values, ranging more than one order of magnitude around the thermal energy kBT, can be quantitatively explained by a combination of dielectric loss and 1/f flux noise with a crossover point. The amplitude of the 1/f flux noise is consistent with that extracted from the qubit dephasing measurements at the flux sensitive points. In the dielectric loss dominant regime, the loss is consistent with that arises from the electric dipole interaction with two-level-system (TLS) defects. In particular, as increasing Josephson energy thus decreasing qubit frequency at the flux insensitive spot, we find that the qubit exhibits increasingly weaker coupling to TLS defects thus desirable for high-fidelity quantum operations.

Titanium Nitride Film on Sapphire Substrate with Low Dielectric Loss for Superconducting Qubits

  1. Hao Deng,
  2. Zhijun Song,
  3. Ran Gao,
  4. Tian Xia,
  5. Feng Bao,
  6. Xun Jiang,
  7. Hsiang-Sheng Ku,
  8. Zhisheng Li,
  9. Xizheng Ma,
  10. Jin Qin,
  11. Hantao Sun,
  12. Chengchun Tang,
  13. Tenghui Wang,
  14. Feng Wu,
  15. Wenlong Yu,
  16. Gengyan Zhang,
  17. Xiaohang Zhang,
  18. Jingwei Zhou,
  19. Xing Zhu,
  20. Yaoyun Shi,
  21. Hui-Hai Zhao,
  22. and Chunqing Deng
Dielectric loss is one of the major decoherence sources of superconducting qubits. Contemporary high-coherence superconducting qubits are formed by material systems mostly consisting
of superconducting films on substrate with low dielectric loss, where the loss mainly originates from the surfaces and interfaces. Among the multiple candidates for material systems, a combination of titanium nitride (TiN) film and sapphire substrate has good potential because of its chemical stability against oxidization, and high quality at interfaces. In this work, we report a TiN film deposited onto sapphire substrate achieving low dielectric loss at the material interface. Through the systematic characterizations of a series of transmon qubits fabricated with identical batches of TiN base layers, but different geometries of qubit shunting capacitors with various participation ratios of the material interface, we quantitatively extract the loss tangent value at the substrate-metal interface smaller than 8.9×10−4 in 1-nm disordered layer. By optimizing the interface participation ratio of the transmon qubit, we reproducibly achieve qubit lifetimes of up to 300 μs and quality factors approaching 8 million. We demonstrate that TiN film on sapphire substrate is an ideal material system for high-coherence superconducting qubits. Our analyses further suggest that the interface dielectric loss around the Josephson junction part of the circuit could be the dominant limitation of lifetimes for state-of-the-art transmon qubits.

Fluxonium: an alternative qubit platform for high-fidelity operations

  1. Feng Bao,
  2. Hao Deng,
  3. Dawei Ding,
  4. Ran Gao,
  5. Xun Gao,
  6. Cupjin Huang,
  7. Xun Jiang,
  8. Hsiang-Sheng Ku,
  9. Zhisheng Li,
  10. Xizheng Ma,
  11. Xiaotong Ni,
  12. Jin Qin,
  13. Zhijun Song,
  14. Hantao Sun,
  15. Chengchun Tang,
  16. Tenghui Wang,
  17. Feng Wu,
  18. Tian Xia,
  19. Wenlong Yu,
  20. Fang Zhang,
  21. Gengyan Zhang,
  22. Xiaohang Zhang,
  23. Jingwei Zhou,
  24. Xing Zhu,
  25. Yaoyun Shi,
  26. Jianxin Chen,
  27. Hui-Hai Zhao,
  28. and Chunqing Deng
Superconducting qubits provide a promising path toward building large-scale quantum computers. The simple and robust transmon qubit has been the leading platform, achieving multiple
milestones. However, fault-tolerant quantum computing calls for qubit operations at error rates significantly lower than those exhibited in the state of the art. Consequently, alternative superconducting qubits with better error protection have attracted increasing interest. Among them, fluxonium is a particularly promising candidate, featuring large anharmonicity and long coherence times. Here, we engineer a fluxonium-based quantum processor that integrates high qubit-coherence, fast frequency-tunability, and individual-qubit addressability for reset, readout, and gates. With simple and fast gate schemes, we achieve an average single-qubit gate fidelity of 99.97% and a two-qubit gate fidelity of up to 99.72%. This performance is comparable to the highest values reported in the literature of superconducting circuits. Thus our work, for the first time within the realm of superconducting qubits, reveals an approach toward fault-tolerant quantum computing that is alternative and competitive to the transmon system.

Suppression of Qubit Crosstalk in a Tunable Coupling Superconducting Circuit

  1. Gengyan Zhang,
  2. Pranav S. Mundada,
  3. and Andrew A. Houck
We report the suppression of static ZZ crosstalk in a two-qubit, two-coupler superconducting circuit, where the ZZ interaction between the two qubits can be tuned to near zero. Characterization
of qubit crosstalk is performed using randomized benchmarking and a two-qubit iSWAP gate is implemented using parametric modulation. We observe the dependence of single-qubit gate fidelity on ZZ interaction strength and identify effective thermalization of the tunable coupler as a crucial prerequisite for high fidelity two-qubit gates.

Suppression of photon shot noise dephasing in a tunable coupling superconducting qubit

  1. Gengyan Zhang,
  2. Yanbing Liu,
  3. James J. Raftery,
  4. and Andrew A. Houck
We demonstrate the suppression of photon shot noise dephasing in a superconducting qubit by eliminating its dispersive coupling to the readout cavity. This is achieved in a tunable
coupling qubit, where the qubit frequency and coupling rate can be controlled independently. We observe that the coherence time approaches twice the relaxation time and becomes less sensitive to thermal photon noise when the dispersive coupling rate is tuned from several MHz to 22 kHz. This work provides a promising building block in circuit quantum electrodynamics that can hold high coherence and be integrated into larger systems.