A. V. Ustinov

Hybrid quantum circuit with implanted erbium ions

  1. S. Probst,
  2. N. Kukharchyk,
  3. H. Rotzinger,
  4. A. Tkalcec,
  5. S. Wünsch,
  6. A. D. Wieck,
  7. M. Siegel,
  8. A. V. Ustinov,
  9. and P. A. Bushev
We report on hybrid circuit QED experiments with focused ion beam implanted Er3+ ions in Y2SiO5 coupled to an array of superconducting lumped element microwave resonators. The Y2SiO5
crystal is divided into several areas with distinct erbium doping concentrations, each coupled to a separate resonator. The coupling strength is varied from 5 MHz to 18.7 MHz, while the linewidth ranges between 50 MHz and 130 MHz. We confirm the paramagnetic properties of the implanted spin ensemble by evaluating the temperature dependence of the coupling. The efficiency of the implantation process is analyzed and the results are compared to a bulk doped Er:Y2SiO5 sample. We demonstrate the successful integration of these engineered erbium spin ensembles with superconducting circuits.
arxiv pdf

Broadband Sample Holder for Microwave Spectroscopy of Superconducting Qubits

  1. A.S. Averkin,
  2. A. Karpov,
  3. K. Shulga,
  4. E. Glushkov,
  5. N. Abramov,
  6. U. Huebner,
  7. E. Il'ichev,
  8. and A. V. Ustinov
We present a practical design and implementation of a broadband sample holder suitable for microwave experiments with superconducting integrated circuits at millikelvin temperatures.
Proposed design can be easily integrated in standard dilution cryostats, has flat pass band response in a frequency range from 0 to 32 GHz, allowing the RF testing of the samples with substrate size up to 4×4 mm. The parasitic higher modes interference in the holder structure is analyzed and prevented via design considerations. The developed setup can be used for characterization of superconducting parametric amplifiers, bolometers and qubits. We tested the designed sample holder by characterizing of a superconducting flux qubit at 20 mK temperature.
arxiv pdf

Probing the TLS Density of States in SiO Films using Superconducting Lumped Element Resonators

  1. S. T. Skacel,
  2. Ch. Kaiser,
  3. S. Wuensch,
  4. H. Rotzinger,
  5. A. Lukashenko,
  6. M. Jerger,
  7. G. Weiss,
  8. M. Siegel,
  9. and A. V. Ustinov
We have investigated dielectric losses in amorphous SiO thin films under operating conditions of superconducting qubits (mK temperatures and low microwave powers). For this purpose,
we have developed a broadband measurement setup employing multiplexed lumped element resonators using a broadband power combiner and a low-noise amplifier. The measured temperature and power dependences of the dielectric losses are in good agreement with those predicted for atomic two-level tunneling systems (TLS). By measuring the losses at different frequencies, we found that the TLS density of states is energy dependent. This had not been seen previously in loss measurements. These results contribute to a better understanding of decoherence effects in superconducting qubits and suggest a possibility to minimize TLS-related decoherence by reducing the qubit operation frequency.
arxiv pdf

Fabrication and measurements of hybrid Nb/Al Josephson junctions and flux qubits with pi-shifters

  1. A. V. Shcherbakova,
  2. K. G. Fedorov,
  3. K. V. Shulga,
  4. V. V. Ryazanov,
  5. V. V. Bolginov,
  6. V. A Oboznov,
  7. S. V. Egorov,
  8. V. O. Shkolnikov,
  9. M. J. Wolf,
  10. D. Beckmann,
  11. and A. V. Ustinov
We describe fabrication and testing of composite flux qubits combining Nb- and Al-based superconducting circuit technology. This hybrid approach to making qubits allows for employing
pi-phase shifters fabricated using well-established Nb-based technology of superconductor-ferromagnet-superconductor Josephson junctions. The important feature here is to obtain high interface transparency between Nb and Al layers without degrading sub-micron shadow mask. We achieve this by in-situ Ar etching using e-beam gun. Shadow-evaporated Al/AlOx/Al Josephson junctions with Nb bias pads show the expected current-voltage characteristics with reproducible critical currents. Using this technique, we fabricated composite Nb/Al flux qubits with Nb/CuNi/Nb pi-shifters and measured their magnetic field response. The observed offset between the field responses of the qubits with and without pi-junction is attributed to the pi phase shift. The reported approach can be used for implementing a variety of hybrid Nb/Al superconducting quantum circuits.
arxiv pdf

Ultra-compact tunable split-ring resonators

  1. A. Vidiborskiy,
  2. V. P. Koshelets,
  3. L. V. Filippenko,
  4. S. V. Shitov,
  5. and A. V. Ustinov
We propose tunable superconducting split-ring resonators (SRRs) employing nonlinear Josephson inductance. A fraction of SRR is replaced by Nb-AlOx-Nb Josephson tunnel junctions connected
in parallel and forming a superconducting quantum interference device (SQUID), whose inductance is sensitive to the external dc magnetic field. Due to the lumped nature of the Josephson inductance, the SRR can be made very compact and its resonance frequency can be tuned by applying magnetic field. We present the model, results of extensive EM-simulation and experimental data for the SRR weakly coupled to a transmission line within frequency range 11-13 GHz.
arxiv pdf

Anisotropic rare-earth spin ensemble strongly coupled to a superconducting resonator

  1. S. Probst,
  2. H. Rotzinger,
  3. S. Wünsch,
  4. P. Jung,
  5. M. Jerger,
  6. M. Siegel,
  7. A. V. Ustinov,
  8. and P. A. Bushev
Interfacing photonic and solid-state qubits within a hybrid quantum architecture offers a promising route towards large scale distributed quantum computing. Ideal candidates for coherent
qubit interconversion are optically active spins magnetically coupled to a superconducting resonator. We report on a cavity QED experiment with magnetically anisotropic Er3+:Y2SiO5 crystals and demonstrate strong coupling of rare-earth spins to a lumped element resonator. In addition, the electron spin resonance and relaxation dynamics of the erbium spins are detected via direct microwave absorption, without aid of a cavity.
arxiv pdf