Fast high fidelity quantum non-demolition qubit readout via a non-perturbative cross-Kerr coupling

  1. R. Dassonneville,
  2. T. Ramos,
  3. V. Milchakov,
  4. L. Planat,
  5. É. Dumur,
  6. F. Foroughi,
  7. J. Puertas,
  8. S. Leger,
  9. K. Bharadwaj,
  10. J. Delaforce,
  11. K. Rafsanjani,
  12. C. Naud,
  13. W. Hasch-Guichard,
  14. J.J. García-Ripoll,
  15. N. Roch,
  16. and O. Buisson
Qubit readout is an indispensable element of any quantum information processor. In this work we propose an original coupling scheme between qubit and cavity mode based on a non-perturbative
cross-Kerr interaction. It leads to an alternative readout mechanism for superconducting qubits. This scheme, using the same experimental techniques as the perturbative cross-Kerr coupling (dispersive interaction), leads to an alternative readout mechanism for superconducting qubits. This new process, being non-perturbative, maximizes speed of qubit readout, single-shot fidelity and its quantum non-demolition (QND) behavior at the same time, while minimizing the effect of unwanted decay channels such as, for example, the Purcell effect. We observed 97.4 % single-shot readout fidelity for short 50 ns pulses. Using long measurement, we quantified the QND-ness to 99 %.

A V-shape superconducting artificial atom based on two inductively coupled transmons

  1. É. Dumur,
  2. B. Küng,
  3. A. K. Feofanov,
  4. T. Weissl,
  5. N. Roch,
  6. C. Naud,
  7. W. Guichard,
  8. and O. Buisson
Circuit quantum electrodynamics systems are typically built from resonators and two-level artificial atoms, but the use of multi-level artificial atoms instead can enable promising
applications in quantum technology. Here we present an implementation of a Josephson junction circuit dedicated to operate as a V-shape artificial atom. Based on a concept of two internal degrees of freedom, the device consists of two transmon qubits coupled by an inductance. The Josephson nonlinearity introduces a strong diagonal coupling between the two degrees of freedom that finds applications in quantum non-demolition readout schemes, and in the realization of microwave cross-Kerr media based on superconducting circuits.