Atharv Joshi

2D transmons with lifetimes and coherence times exceeding 1 millisecond

  1. Matthew P. Bland,
  2. Faranak Bahrami,
  3. Jeronimo G.C. Martinez,
  4. Paal H. Prestegaard,
  5. Basil M. Smitham,
  6. Atharv Joshi,
  7. Elizabeth Hedrick,
  8. Alex Pakpour-Tabrizi,
  9. Shashwat Kumar,
  10. Apoorv Jindal,
  11. Ray D. Chang,
  12. Ambrose Yang,
  13. Guangming Cheng,
  14. Nan Yao,
  15. Robert J. Cava,
  16. Nathalie P. de Leon,
  17. and Andrew A. Houck
Materials improvements are a powerful approach to reducing loss and decoherence in superconducting qubits because such improvements can be readily translated to large scale processors.
Recent work improved transmon coherence by utilizing tantalum (Ta) as a base layer and sapphire as a substrate. The losses in these devices are dominated by two-level systems (TLSs) with comparable contributions from both the surface and bulk dielectrics, indicating that both must be tackled to achieve major improvements in the state of the art. Here we show that replacing the substrate with high-resistivity silicon (Si) dramatically decreases the bulk substrate loss, enabling 2D transmons with time-averaged quality factors (Q) exceeding 1.5 x 10^7, reaching a maximum Q of 2.5 x 10^7, corresponding to a lifetime (T_1) of up to 1.68 ms. This low loss allows us to observe decoherence effects related to the Josephson junction, and we use improved, low-contamination junction deposition to achieve Hahn echo coherence times (T_2E) exceeding T_1. We achieve these material improvements without any modifications to the qubit architecture, allowing us to readily incorporate standard quantum control gates. We demonstrate single qubit gates with 99.994% fidelity. The Ta-on-Si platform comprises a simple material stack that can potentially be fabricated at wafer scale, and therefore can be readily translated to large-scale quantum processors.
arxiv pdf

Quantum information processing with bosonic qubits in circuit QED

  1. Atharv Joshi,
  2. Kyungjoo Noh,
  3. and Yvonne Y. Gao
The unique features of quantum theory offer a powerful new paradigm for information processing. Translating these mathematical abstractions into useful algorithms and applications requires
quantum systems with significant complexity and sufficiently low error rates. Such quantum systems must be made from robust hardware that can coherently store, process, and extract the encoded information, as well as possess effective quantum error correction (QEC) protocols to detect and correct errors. Circuit quantum electrodynamics (cQED) provides a promising hardware platform for implementing robust quantum devices. In particular, bosonic encodings in cQED that use multi-photon states of superconducting cavities to encode information have shown success in realizing hardware-efficient QEC. Here, we review recent developments in the theory and implementation of quantum error correction with bosonic codes and report the progress made towards realizing fault-tolerant quantum information processing with cQED devices.
arxiv pdf