V. Negîrneac

Variational preparation of finite-temperature states on a quantum computer

  1. R. Sagastizabal,
  2. S. P. Premaratne,
  3. B. A. Klaver,
  4. M. A. Rol,
  5. V. Negîrneac,
  6. M. Moreira,
  7. X. Zou,
  8. S. Johri,
  9. N. Muthusubramanian,
  10. M. Beekman,
  11. C. Zachariadis,
  12. V.P. Ostroukh,
  13. N. Haider,
  14. A. Bruno,
  15. A. Y. Matsuura,
  16. and L. DiCarlo
The preparation of thermal equilibrium states is important for the simulation of condensed-matter and cosmology systems using a quantum computer. We present a method to prepare such
mixed states with unitary operators, and demonstrate this technique experimentally using a gate-based quantum processor. Our method targets the generation of thermofield double states using a hybrid quantum-classical variational approach motivated by quantum-approximate optimization algorithms, without prior calculation of optimal variational parameters by numerical simulation. The fidelity of generated states to the thermal-equilibrium state smoothly varies from 99 to 75% between infinite and near-zero simulated temperature, in quantitative agreement with numerical simulations of the noisy quantum processor with error parameters drawn from experiment.
arxiv pdf

High-fidelity controlled-Z gate with maximal intermediate leakage operating at the speed limit in a superconducting quantum processor

  1. V. Negîrneac,
  2. H. Ali,
  3. N. Muthusubramanian,
  4. F. Battistel,
  5. R. Sagastizabal,
  6. M. S. Moreira,
  7. J. F. Marques,
  8. W. Vlothuizen,
  9. M. Beekman,
  10. N. Haider,
  11. A. Bruno,
  12. and L. DiCarlo
We introduce the sudden variant (SNZ) of the Net Zero scheme realizing controlled-Z (CZ) gates by baseband flux control of transmon frequency. SNZ CZ gates operate at the speed limit
of transverse coupling between computational and non-computational states by maximizing intermediate leakage. The key advantage of SNZ is tuneup simplicity, owing to the regular structure of conditional phase and leakage as a function of two control parameters. We realize SNZ CZ gates in a multi-transmon processor, achieving 99.93±0.24% fidelity and 0.10±0.02% leakage. SNZ is compatible with scalable schemes for quantum error correction and adaptable to generalized conditional-phase gates useful in intermediate-scale applications.
arxiv pdf