Magnetic-field dependence of a Josephson traveling-wave parametric amplifier and integration into a high-field setup

  1. L. M. Janssen,
  2. G. Butseraen,
  3. J. Krause,
  4. A. Coissard,
  5. L. Planat,
  6. N. Roch,
  7. G. Catelani,
  8. Yoichi Ando,
  9. and C. Dickel
We investigate the effect of magnetic field on a photonic-crystal Josephson traveling-wave parametric amplifier (TWPA). We show that the observed change in photonic bandgap and plasma
frequency of the TWPA can be modeled by considering the suppression of the critical current in the Josephson junctions (JJs) of the TWPA due to the Fraunhofer effect and closing of the superconducting gap. Accounting for the JJ geometry is crucial for understanding the field dependence. In one in-plane direction, the TWPA bandgap can be shifted by 2 GHz using up to 60 mT of field, without losing gain or bandwidth, showing that TWPAs without SQUIDs can be field tunable. In the other in-plane direction, the magnetic field is perpendicular to the larger side of the Josephson junctions, so the Fraunhofer effect has a smaller period. This larger side of the JJs is modulated to create the bandgap. The field interacts more strongly with the larger junctions, and as a result, the TWPA bandgap closes and reopens as the field increases, causing the TWPA to become severely compromised already at 2 mT. A slightly higher operating limit of 5 mT is found in out-of-plane field, for which the TWPA’s response is hysteretic. These measurements reveal the requirements for magnetic shielding needed to use TWPAs in experiments where high fields at the sample are required; we show that with magnetic shields we can operate the TWPA while applying over 2 T to the sample.

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 %.

Quantum trajectories of superconducting qubits

  1. S. J. Weber,
  2. K. W. Murch,
  3. M. E. Schwartz,
  4. N. Roch,
  5. and I. Siddiqi
In this review, we discuss recent experiments that investigate how the quantum sate of a superconducting qubit evolves during measurement. We provide a pedagogical overview of the measurement
process, when the qubit is dispersively coupled to a microwave frequency cavity, and the qubit state is encoded in the phase of a microwave tone that probes the cavity. A continuous measurement record is used to reconstruct the individual quantum trajectories of the qubit state, and quantum state tomography is performed to verify that the state has been tracked accurately. Furthermore, we discuss ensembles of trajectories, time-symmetric evolution, two-qubit trajectories, and potential applications in measurement-based quantum error correction.

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.