Disentangling the sources of ionizing radiation in superconducting qubits

  1. L. Cardani,
  2. I. Colantoni,
  3. A. Cruciani,
  4. F. De Dominicis,
  5. G. D'Imperio,
  6. M. Laubenstein,
  7. A. Mariani,
  8. L. Pagnanini,
  9. S. Pirro,
  10. C. Tomei,
  11. N. Casali,
  12. F. Ferroni,
  13. D. Frolov,
  14. L. Gironi,
  15. A. Grassellino,
  16. M. Junker,
  17. C. Kopas,
  18. E. Lachman,
  19. C.R.H. McRae,
  20. J. Mutus,
  21. M. Nastasi,
  22. D. P. Pappas,
  23. R. Pilipenko,
  24. M. Sisti,
  25. V. Pettinacci,
  26. A. Romanenko,
  27. D. Van Zanten,
  28. M. Vignati,
  29. J. D. Withrow,
  30. and N. Z. Zhelev
Radioactivity was recently discovered as a source of decoherence and correlated errors for the real-world implementation of superconducting quantum processors. In this work, we measure
levels of radioactivity present in a typical laboratory environment (from muons, neutrons, and gamma’s emitted by naturally occurring radioactive isotopes) and in the most commonly used materials for the assembly and operation of state-of-the-art superconducting qubits. We develop a GEANT-4 based simulation to predict the rate of impacts and the amount of energy released in a qubit chip from each of the mentioned sources. We finally propose mitigation strategies for the operation of next-generation qubits in a radio-pure environment.

Cryogenic single-port calibration for superconducting microwave resonator measurements

  1. Haozhi Wang,
  2. S. Singh,
  3. C.R.H. McRae,
  4. J.C. Bardin,
  5. S.-X. Lin,
  6. N. Messaoudi,
  7. A.R. Castelli,
  8. Y. J. Rosen,
  9. E. T. Holland,
  10. D. P. Pappas,
  11. and J. Y. Mutus
Superconducting circuit testing and materials loss characterization requires robust and reliable methods for the extraction of internal and coupling quality factors of microwave resonators.
A common method, imposed by limitations on the device design or experimental configuration, is the single-port reflection geometry, i.e. reflection-mode. However, impedance mismatches in cryogenic systems must be accounted for through calibration of the measurement chain while it is at low temperatures. In this paper, we demonstrate a data-based, single-port calibration using commercial microwave standards and a vector network analyzer (VNA) with samples at millikelvin temperature in a dilution refrigerator, making this method useful for measurements of quantum phenomena. Finally, we cross reference our data-based, single-port calibration and reflection measurement with over-coupled 2D- and 3D-resonators against well established two-port techniques corroborating the validity of our method.

Dielectric loss in epitaxial Al/GaAs/Al trilayers for superconducting circuits

  1. C.R.H. McRae,
  2. A. McFadden,
  3. R. Zhao,
  4. H. Wang,
  5. J. L. Long,
  6. T. Zhao,
  7. S. Park,
  8. M. Bal,
  9. C.J. Palmstrøm,
  10. and D. P. Pappas
Epitaxially-grown superconductor/dielectric/superconductor trilayers have the potential to form high-performance superconducting quantum devices and may even allow scalable superconducting
quantum computing with low-surface-area qubits such as the merged-element transmon. In this work, we measure the power-independent loss and two-level-state (TLS) loss of epitaxial, wafer-bonded, and substrate-removed Al/GaAs/Al trilayers by measuring lumped element superconducting microwave resonators at millikelvin temperatures and down to single photon powers. The power-independent loss of the device is (4.8±0.1)×10−5 and resonator-induced intrinsic TLS loss is (6.4±0.2)×10−5. Dielectric loss extraction is used to determine a lower bound of the intrinsic TLS loss of the trilayer of 7.2×10−5. The unusually high power-independent loss is attributed to GaAs’s intrinsic piezoelectricity.

A merged-element transmon

  1. R. Zhao,
  2. S. Park,
  3. T. Zhao,
  4. M. Bal,
  5. C.R.H. McRae,
  6. J. Long,
  7. and D. P. Pappas
Transmon qubits are ubiquitous in the pursuit of quantum computing using superconducting circuits. However, they have some drawbacks that still need to be addressed. Most importantly,
the scalability of transmons is limited by the large device footprint needed to reduce the participation of the lossy capacitive parts of the circuit. In this work, we investigate and evaluate losses in a novel device geometry, namely, the merged-element transmon (mergemon). To this end, we replace the large external shunt capacitor of a traditional transmon with the intrinsic capacitance of a Josephson junction (JJ) and achieve an approximately 100 times reduction in qubit dimensions. We report the implementation of the mergemon using a sputtered Nb/amorphous Si (a-Si)/Nb trilayer film. In an experiment below 10 mK, the frequency of the readout resonator, capacitively coupled to the mergemon, exhibits a qubit-state dependent shift in the low power regime. The device also demonstrates the single- and multi-photon transitions that symbolize a weakly anharmonic system in the two-tone spectroscopy. The transition spectra are explained well with master-equation simulations. A participation ratio analysis identifies the dielectric loss of the a-Si tunnel barrier and its interfaces as the dominant source for qubit relaxation. We expect the mergemon to achieve high coherence in relatively small device dimensions when implemented using a low-loss, epitaxially-grown, and lattice-matched trilayer.

Dielectric loss extraction for superconducting microwave resonators

  1. C.R.H. McRae,
  2. R.E. Lake,
  3. J. L. Long,
  4. M. Bal,
  5. X. Wu,
  6. B. Jugdersuren,
  7. T.H. Metcalf,
  8. X. Liu,
  9. and D. P. Pappas
The investigation of two-level-state (TLS) loss in dielectric materials and interfaces remains at the forefront of materials research in superconducting quantum circuits. We demonstrate
a method of TLS loss extraction of a thin film dielectric by measuring a lumped element resonator fabricated from a superconductor-dielectric-superconductor trilayer. We extract the dielectric loss by formulating a circuit model for a lumped element resonator with TLS loss and then fitting to this model using measurements from a set of three resonator designs: a coplanar waveguide resonator, a lumped element resonator with an interdigitated capacitor, and a lumped element resonator with a parallel plate capacitor that includes the dielectric thin film of interest. Unlike other methods, this allows accurate measurement of materials with TLS loss lower than 10−6. We demonstrate this method by extracting a TLS loss of 1.02×10−3 for sputtered Al2O3 using a set of samples fabricated from an Al/Al2O3/Al trilayer. We observe a difference of 11% between extracted loss of the trilayer with and without the implementation of this method.

Overlap junctions for high coherence superconducting qubits

  1. X. Wu,
  2. J. L. Long,
  3. H. S. Ku,
  4. R.E. Lake,
  5. M. Bal,
  6. and D. P. Pappas
Fabrication of sub-micron Josephson junctions is demonstrated using standard processing techniques for high-coherence, superconducting qubits. These junctions are made in two separate
lithography steps with normal-angle evaporation. Most significantly, this work demonstrates that it is possible to achieve high coherence with junctions formed on aluminum surfaces cleaned in situ with Ar milling before the junction oxidation. This method eliminates the angle-dependent shadow masks typically used for small junctions. Therefore, this is conducive to the implementation of typical methods for improving margins and yield using conventional CMOS processing. The current method uses electron-beam lithography and an additive process to define the top and bottom electrodes. Extension of this work to optical lithography and subtractive processes is discussed.

Qubit gates using hyperbolic secant pulses

  1. H. S. Ku,
  2. J. L. Long,
  3. X. Wu,
  4. M. Bal,
  5. R.E. Lake,
  6. Edwin Barnes,
  7. Sophia E. Economou,
  8. and D. P. Pappas
It has been known since the early days of quantum mechanics that hyperbolic secant pulses possess the unique property that they can perform cyclic evolution on two-level quantum systems
independently of the pulse detuning. More recently, it was realized that they induce detuning- controlled phases without changing state populations. Here, we experimentally demonstrate the properties of hyperbolic secant pulses on superconducting transmon qubits and contrast them with the more commonly used Gaussian and square waves. We further show that these properties can be exploited to implement phase gates, nominally without exiting the computational subspace. This enables us to demonstrate the first microwave-driven Z-gates with a single control parameter, the detuning.

Improved superconducting qubit coherence using titanium nitride

  1. J. Chang,
  2. M. R. Vissers,
  3. A. D. Corcoles,
  4. M. Sandberg,
  5. J. Gao,
  6. David W. Abraham,
  7. Jerry M. Chow,
  8. Jay M. Gambetta,
  9. M. B. Rothwell,
  10. G. A. Keefe,
  11. Matthias Steffen,
  12. and D. P. Pappas
We demonstrate enhanced relaxation and dephasing times of transmon qubits, up to ~ 60 mu s by fabricating the interdigitated shunting capacitors using titanium nitride (TiN). Compared
to lift-off aluminum deposited simultaneously with the Josephson junction, this represents as much as a six-fold improvement and provides evidence that previous planar transmon coherence times are limited by surface losses from two-level system (TLS) defects residing at or near interfaces. Concurrently, we observe an anomalous temperature dependent frequency shift of TiN resonators which is inconsistent with the predicted TLS model.