Bulk and surface loss in superconducting transmon qubits

  1. Oliver Dial,
  2. Douglas T. McClure,
  3. Stefano Poletto,
  4. Jay M. Gambetta,
  5. David W. Abraham,
  6. Jerry M. Chow,
  7. and Matthias Steffen
Decoherence of superconducting transmon qubits is purported to be consistent with surface loss from two-level systems on the substrate surface. Here, we present a study of surface loss
in transmon devices, explicitly designed to have varying sensitivities to different surface loss contributors. Our experiments also encompass two particular different sapphire substrates, which reveal the onset of a yet unknown additional loss mechanism outside of surface loss for one of the substrates. Tests across different wafers and devices demonstrate substantial variation, and we emphasize the importance of testing large numbers of devices for disentangling di?erent sources of decoherence.

Characterizing a Four-Qubit Planar Lattice for Arbitrary Error Detection

  1. Jerry M. Chow,
  2. Srikanth J. Srinivasan,
  3. Easwar Magesan,
  4. A. D. Corcoles,
  5. David W. Abraham,
  6. Jay M. Gambetta,
  7. and Matthias Steffen
Quantum error correction will be a necessary component towards realizing scalable quantum computers with physical qubits. Theoretically, it is possible to perform arbitrarily long computations
if the error rate is below a threshold value. The two-dimensional surface code permits relatively high fault-tolerant thresholds at the ~1% level, and only requires a latticed network of qubits with nearest-neighbor interactions. Superconducting qubits have continued to steadily improve in coherence, gate, and readout fidelities, to become a leading candidate for implementation into larger quantum networks. Here we describe characterization experiments and calibration of a system of four superconducting qubits arranged in a planar lattice, amenable to the surface code. Insights into the particular qubit design and comparison between simulated parameters and experimentally determined parameters are given. Single- and two-qubit gate tune-up procedures are described and results for simultaneously benchmarking pairs of two-qubit gates are given. All controls are eventually used for an arbitrary error detection protocol described in separate work [Corcoles et al., Nature Communications, 6, 2015]

Blackbox Quantization of Superconducting Circuits using exact Impedance Synthesis

  1. Firat Solgun,
  2. David W. Abraham,
  3. and David P. DiVincenzo
We propose a new quantization method for superconducting electronic circuits involving a Josephson junction device coupled to a linear microwave environment. The method is based on
an exact impedance synthesis of the microwave environment considered as a blackbox with impedance function Z(s). The synthesized circuit captures dissipative dynamics of the system with resistors coupled to the reactive part of the circuit in a non-trivial way. We quantize the circuit and compute relaxation rates following previous formalisms for lumped element circuit quantization. Up to the errors in the fit our method gives an exact description of the system and its losses.

Implementing a strand of a scalable fault-tolerant quantum computing fabric

  1. Jerry M. Chow,
  2. Jay M. Gambetta,
  3. Easwar Magesan,
  4. Srikanth J. Srinivasan,
  5. Andrew W. Cross,
  6. David W. Abraham,
  7. Nicholas A. Masluk,
  8. B. R. Johnson,
  9. Colm A. Ryan,
  10. and M. Steffen
Quantum error correction (QEC) is an essential step towards realising scalable quantum computers. Theoretically, it is possible to achieve arbitrarily long protection of quantum information
from corruption due to decoherence or imperfect controls, so long as the error rate is below a threshold value. The two-dimensional surface code (SC) is a fault-tolerant error correction protocol} that has garnered considerable attention for actual physical implementations, due to relatively high error thresholds ~1%, and restriction to planar lattices with nearest-neighbour interactions. Here we show a necessary element for SC error correction: high-fidelity parity detection of two code qubits via measurement of a third syndrome qubit. The experiment is performed on a sub-section of the SC lattice with three superconducting transmon qubits, in which two independent outer code qubits are joined to a central syndrome qubit via two linking bus resonators. With all-microwave high-fidelity single- and two-qubit nearest-neighbour entangling gates, we demonstrate entanglement distributed across the entire sub-section by generating a three-qubit Greenberger-Horne-Zeilinger (GHZ) state with fidelity ~94%. Then, via high-fidelity measurement of the syndrome qubit, we deterministically entangle the otherwise un-coupled outer code qubits, in either an even or odd parity Bell state, conditioned on the syndrome state. Finally, to fully characterize this parity readout, we develop a new measurement tomography protocol to obtain a fidelity metric (90% and 91%). Our results reveal a straightforward path for expanding superconducting circuits towards larger networks for the SC and eventually a primitive logical qubit implementation.

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.