K. Nakamura

Photolithography-Only Fabrication of Transmons Using Double-Oblique Evaporation

  1. K. Aoyanagi,
  2. S. Abe,
  3. S. Chen,
  4. T. Inada,
  5. C. Kawai,
  6. Y. Mino,
  7. K. Nakamura,
  8. K. Nakazono,
  9. T. Nitta,
  10. and K. Watanabe
We investigate a photolithography-only fabrication process for transmon Josephson junctions using a modified double-oblique evaporation geometry. Using a bilayer resist process and
Al shadow evaporation, we fabricate junction structures and confirm by optical and scanning electron microscopy that the resulting narrowed crossing region reaches a geometrical area on the order of 104 nm2, which lies in the size range relevant to qubit junction fabrication. Room-temperature resistance screening shows that the junction resistance falls within the target range for the present transmon design over a usable process window and exhibits a clear design dependence. We further implement fabricated junctions in transmon devices and evaluate them in a three-dimensional Al cavity at 20mK, where we observe basic transmon qubit operation with f01=4.865 GHz, T1∼9μs, and T∗2∼0.4μs. These results demonstrate the feasibility of realizing functional transmon devices in a photolithography-only process using double-oblique evaporation.
arxiv pdf

Towards a spin-ensemble quantum memory for superconducting qubits

  1. C. Grezes,
  2. Y. Kubo,
  3. B. Julsgaard,
  4. T. Umeda,
  5. J. Isoya,
  6. H. Sumiya,
  7. H. Abe,
  8. S. Onoda,
  9. T. Ohshima,
  10. K. Nakamura,
  11. I. Diniz,
  12. A. Auffeves,
  13. V. Jacques,
  14. J.-F. Roch,
  15. D. Vion,
  16. D. Esteve,
  17. K. Moelmer,
  18. and P. Bertet
This article reviews efforts to build a new type of quantum device, which combines an ensemble of electronic spins with long coherence times, and a small-scale superconducting quantum
processor. The goal is to store over long times arbitrary qubit states in orthogonal collective modes of the spin-ensemble, and to retrieve them on-demand. We first present the protocol devised for such a multi-mode quantum memory. We then describe a series of experimental results using NV center spins in diamond, which demonstrate its main building blocks: the transfer of arbitrary quantum states from a qubit into the spin ensemble, and the multi-mode retrieval of classical microwave pulses down to the single-photon level with a Hahn-echo like sequence. A reset of the spin memory is implemented in-between two successive sequences using optical repumping of the spins.
arxiv pdf

Storage and retrieval of microwave fields at the single-photon level in a spin ensemble

  1. C. Grezes,
  2. B. Julsgaard,
  3. Y. Kubo,
  4. W. L. Ma,
  5. M. Stern,
  6. A. Bienfait,
  7. K. Nakamura,
  8. J. Isoya,
  9. S. Onoda,
  10. T. Ohshima,
  11. V. Jacques,
  12. D. Vion,
  13. D. Esteve,
  14. R. B. Liu,
  15. K. Mølmer,
  16. and P. Bertet
We report the storage of microwave pulses at the single-photon level in a spin-ensemble memory consisting of 1010 NV centers in a diamond crystal coupled to a superconducting LC resonator.
The energy of the signal, retrieved 100μs later by spin-echo techniques, reaches 0.3% of the energy absorbed by the spins, and this storage efficiency is quantitatively accounted for by simulations. This figure of merit is sufficient to envision first implementations of a quantum memory for superconducting qubits.
arxiv pdf