M. Singh

Parametric multi-element coupling architecture for coherent and dissipative control of superconducting qubits

  1. G. B. P. Huber,
  2. F. A. Roy,
  3. L. Koch,
  4. I. Tsitsilin,
  5. J. Schirk,
  6. N. J. Glaser,
  7. N. Bruckmoser,
  8. C. Schweizer,
  9. J. Romeiro,
  10. G. Krylov,
  11. M. Singh,
  12. F. X. Haslbeck,
  13. M. Knudsen,
  14. A. Marx,
  15. F. Pfeiffer,
  16. C. Schneider,
  17. F. Wallner,
  18. D. Bunch,
  19. L. Richard,
  20. L. Södergren,
  21. K. Liegener,
  22. M. Werninghaus,
  23. and S. Filipp
As systems for quantum computing keep growing in size and number of qubits, challenges in scaling the control capabilities are becoming increasingly relevant. Efficient schemes to simultaneously
mediate coherent interactions between multiple quantum systems and to reduce decoherence errors can minimize the control overhead in next-generation quantum processors. Here, we present a superconducting qubit architecture based on tunable parametric interactions to perform two-qubit gates, reset, leakage recovery and to read out the qubits. In this architecture, parametrically driven multi-element couplers selectively couple qubits to resonators and neighbouring qubits, according to the frequency of the drive. We consider a system with two qubits and one readout resonator interacting via a single coupling circuit and experimentally demonstrate a controlled-Z gate with a fidelity of 98.30±0.23%, a reset operation that unconditionally prepares the qubit ground state with a fidelity of 99.80±0.02% and a leakage recovery operation with a 98.5±0.3% success probability. Furthermore, we implement a parametric readout with a single-shot assignment fidelity of 88.0±0.4%. These operations are all realized using a single tunable coupler, demonstrating the experimental feasibility of the proposed architecture and its potential for reducing the system complexity in scalable quantum processors.
arxiv pdf

Error Mitigation by Symmetry Verification on a Variational Quantum Eigensolver

  1. R. Sagastizabal,
  2. X. Bonet-Monroig,
  3. M. Singh,
  4. M. A. Rol,
  5. C. C. Bultink,
  6. X. Fu,
  7. C.H. Price,
  8. V.P. Ostroukh,
  9. N. Muthusubramanian,
  10. A. Bruno,
  11. M. Beekman,
  12. N. Haider,
  13. T. E. O'Brien,
  14. and L. DiCarlo
Variational quantum eigensolvers offer a small-scale testbed to demonstrate the performance of error mitigation techniques with low experimental overhead. We present successful error
mitigation by applying the recently proposed symmetry verification technique to the experimental estimation of the ground-state energy and ground state of the hydrogen molecule. A finely adjustable exchange interaction between two qubits in a circuit QED processor efficiently prepares variational ansatz states in the single-excitation subspace respecting the parity symmetry of the qubit-mapped Hamiltonian. Symmetry verification improves the energy and state estimates by mitigating the effects of qubit relaxation and residual qubit excitation, which violate the symmetry. A full-density-matrix simulation matching the experiment dissects the contribution of these mechanisms from other calibrated error sources. Enforcing positivity of the measured density matrix via scalable convex optimization correlates the energy and state estimate improvements when using symmetry verification, with interesting implications for determining system properties beyond the ground-state energy.
arxiv pdf