Unraveling the role of disorderness in superconducting materials on qubit coherence

  1. Ran Gao,
  2. Feng Wu,
  3. Hantao Sun,
  4. Jianjun Chen,
  5. Hao Deng,
  6. Xizheng Ma,
  7. Xiaohe Miao,
  8. Zhijun Song,
  9. Xin Wan,
  10. Fei Wang,
  11. Tian Xia,
  12. Make Ying,
  13. Chao Zhang,
  14. Yaoyun Shi,
  15. Hui-Hai Zhao,
  16. and Chunqing Deng
Introducing disorderness in the superconducting materials has been considered promising to enhance the electromagnetic impedance and realize noise-resilient superconducting qubits.
Despite a number of pioneering implementations, the understanding of the correlation between the material disorderness and the qubit coherence is still developing. Here, we demonstrate the first and a systematic characterization of fluxonium qubits with the superinductors made from titanium-aluminum-nitride with varied disorderness. From qubit noise spectroscopy, the flux noise and the dielectric loss are extracted as a measure of the coherence properties. Our results reveal that the 1/f flux noise dominates the qubit decoherence around the flux-frustration point, strongly correlated with the material disorderness; while the dielectric loss remains low under a wide range of material properties. From the flux-noise amplitudes, the areal density (σ) of the phenomenological spin defects and material disorderness are found to be approximately correlated by σ∝ρ3xx, or effectively (kFl)−3. This work has provided new insights on the origin of decoherence channels within superconductors, and could serve as a useful guideline for material design and optimization.

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.

Epitaxial titanium nitride microwave resonators: Structural, chemical, electrical, and microwave properties

  1. Ran Gao,
  2. Wenlong Yu,
  3. Hao Deng,
  4. Hsiang-Sheng Ku,
  5. Zhisheng Li,
  6. Minghua Wang,
  7. Xiaohe Miao,
  8. Yue Lin,
  9. and Chunqing Deng
Titanium nitride is an attractive material for a range of superconducting quantum-circuit applications owing to its low microwave losses, high surface inductance, and chemical stability.
The physical properties and device performance, nevertheless, depend strongly on the quality of the materials. Here we focus on the highly crystalline and epitaxial titanium nitride thin films deposited on sapphire substrates using magnetron sputtering at an intermediate temperature (300∘C). We perform a set of systematic and comprehensive material characterization to thoroughly understand the structural, chemical, and transport properties. Microwave losses at low temperatures are studied using patterned microwave resonators, where the best internal quality factor in the single-photon regime is measured to be 3.3×106, and >1.0×107 in the high-power regime. Adjusted with the material filling factor of the resonators, the microwave loss-tangent here compares well with the previously reported best values for superconducting resonators. This work lays the foundation of using epitaxial titanium nitride for low-loss superconducting quantum circuits.

Observation of Floquet states in a strongly driven artificial atom

  1. Chunqing Deng,
  2. Jean-Luc Orgiazzi,
  3. Feiruo Shen,
  4. Sahel Ashhab,
  5. and Adrian Lupascu
We present experiments on the driven dynamics of a two-level superconducting artificial atom. The driving strength reaches 4.78 GHz, significantly exceeding the transition frequency
of 2.288 GHz. The observed dynamics is described in terms of quasienergies and quasienergy states, in agreement with Floquet theory. In addition, we observe the role of pulse shaping in the dynamics, as determined by non-adiabatic transitions between Floquet states, and we implement subnanosecond single-qubit operations. These results pave the way to quantum control using strong driving with applications in quantum technologies.