Lindblad Tomography of a Superconducting Quantum Processor

  1. Gabriel O. Samach,
  2. Ami Greene,
  3. Johannes Borregaard,
  4. Matthias Christandl,
  5. David K. Kim,
  6. Christopher M. McNally,
  7. Alexander Melville,
  8. Bethany M. Niedzielski,
  9. Youngkyu Sung,
  10. Danna Rosenberg,
  11. Mollie E. Schwartz,
  12. Jonilyn L. Yoder,
  13. Terry P. Orlando,
  14. Joel I-Jan Wang,
  15. Simon Gustavsson,
  16. Morten Kjaergaard,
  17. and William D. Oliver
As progress is made towards the first generation of error-corrected quantum computers, careful characterization of a processor’s noise environment will be crucial to designing
tailored, low-overhead error correction protocols. While standard coherence metrics and characterization protocols such as T1 and T2, process tomography, and randomized benchmarking are now ubiquitous, these techniques provide only partial information about the dynamic multi-qubit loss channels responsible for processor errors, which can be described more fully by a Lindblad operator in the master equation formalism. Here, we introduce and experimentally demonstrate Lindblad Tomography, a hardware-agnostic characterization protocol for tomographically reconstructing the Hamiltonian and Lindblad operators of a quantum channel from an ensemble of time-domain measurements. Performing Lindblad Tomography on a small superconducting quantum processor, we show that this technique characterizes and accounts for state-preparation and measurement (SPAM) errors and allows one to place strong bounds on the degree of non-Markovianity in the channels of interest. Comparing the results of single- and two-qubit measurements on a superconducting quantum processor, we demonstrate that Lindblad Tomography can also be used to identify and quantify sources of crosstalk on quantum processors, such as the presence of always-on qubit-qubit interactions.

Realization of high-fidelity CZ and ZZ-free iSWAP gates with a tunable coupler

  1. Youngkyu Sung,
  2. Leon Ding,
  3. Jochen Braumüller,
  4. Antti Vepsäläinen,
  5. Bharath Kannan,
  6. Morten Kjaergaard,
  7. Ami Greene,
  8. Gabriel O. Samach,
  9. Chris McNally,
  10. David Kim,
  11. Alexander Melville,
  12. Bethany M. Niedzielski,
  13. Mollie E. Schwartz,
  14. Jonilyn L. Yoder,
  15. Terry P. Orlando,
  16. Simon Gustavsson,
  17. and William D. Oliver
High-fidelity two-qubit gates at scale are a key requirement to realize the full promise of quantum computation and simulation. The advent and use of coupler elements to tunably control
two-qubit interactions has improved operational fidelity in many-qubit systems by reducing parasitic coupling and frequency crowding issues. However, two-qubit gate errors still limit the capability of near-term quantum applications. In particular, the existing framework for tunable couplers based on the dispersive approximation does not fully incorporate three-body multi-level dynamics, which are essential for addressing coherent leakage to the coupler and parasitic longitudinal (ZZ) interactions during two-qubit gates. Here, we present a new systematic approach that goes beyond the dispersive approximation and outlines how to optimize the coupler-control and exploit the engineered level structure of the coupler. Using this approach, we experimentally demonstrate a CZ gate with 99.76 ± 0.10 % fidelity and a ZZ-free iSWAP gate with 99.86 ± 0.32 % fidelity, which are close to their T1 limits.

Coherent coupled qubits for quantum annealing

  1. Steven J. Weber,
  2. Gabriel O. Samach,
  3. David Hover,
  4. Simon Gustavsson,
  5. David K. Kim,
  6. Danna Rosenberg,
  7. Adam P. Sears,
  8. Fei Yan,
  9. Jonilyn L. Yoder,
  10. William D. Oliver,
  11. and Andrew J. Kerman
Quantum annealing is an optimization technique which potentially leverages quantum tunneling to enhance computational performance. Existing quantum annealers use superconducting flux
qubits with short coherence times, limited primarily by the use of large persistent currents Ip. Here, we examine an alternative approach, using qubits with smaller Ip and longer coherence times. We demonstrate tunable coupling, a basic building block for quantum annealing, between two flux qubits with small (∼50 nA) persistent currents. Furthermore, we characterize qubit coherence as a function of coupler setting and investigate the effect of flux noise in the coupler loop on qubit coherence. Our results provide insight into the available design space for next-generation quantum annealers with improved coherence.