E. Ginossar

Critical slowing down in the bistable regime of circuit quantum electrodynamics

  1. P. Brookes,
  2. G. Tancredi,
  3. A. D. Patterson,
  4. J. Rahamim,
  5. M. Esposito,
  6. P. J. Leek,
  7. E. Ginossar,
  8. and M. H. Szymanska
We investigate the dynamics of the bistable regime of the generalized Jaynes-Cummings Hamiltonian (GJC), realised by a circuit quantum electrodynamics (cQED) system consisting of a
transmon qubit coupled to a microwave cavity. In this regime we observe critical slowing down in the approach to the steady state. By measuring the response of the cavity to a step function drive pulse we characterize this slowing down as a function of driving frequency and power. We find that the critical slowing down saturates as the driving power is increased. We compare these results with the predictions of analytical and numerical calculations both with and without the Duffing approximation. We find that the Duffing approximation incorrectly predicts that the critical slowing down timescale increases exponentially with the drive, whereas the GJC model accurately predicts the saturation seen in our data, suggesting a different process of quantum activation.
arxiv pdf

Simultaneous bistability of qubit and resonator in circuit quantum electrodynamics

  1. Th. K. Mavrogordatos,
  2. G. Tancredi,
  3. M. Elliott,
  4. M. J. Peterer,
  5. A. Patterson,
  6. J. Rahamim,
  7. P. J. Leek,
  8. E. Ginossar,
  9. and M. H. Szymańska
We explore the joint activated dynamics exhibited by two quantum degrees of freedom: a cavity mode oscillator which is strongly coupled to a superconducting qubit in the strongly coherently
driven dispersive regime. Dynamical simulations and complementary measurements show a range of parameters where both the cavity and the qubit exhibit sudden simultaneous switching between two metastable states. This manifests in ensemble averaged amplitudes of both the cavity and qubit exhibiting a partial coherent cancellation. Transmission measurements of driven microwave cavities coupled to transmon qubits show detailed features which agree with the theory in the regime of simultaneous switching.
arxiv pdf

Quantum State Sensitivity of an Autoresonant Superconducting Circuit

  1. K. W. Murch,
  2. E. Ginossar,
  3. S. J. Weber,
  4. R. Vijay,
  5. S.M. Girvin,
  6. and I. Siddiqi
When a frequency chirped excitation is applied to a classical high-Q nonlinear oscillator, its motion becomes dynamically synchronized to the drive and large oscillation amplitude is
observed, provided the drive strength exceeds the critical threshold for autoresonance. We demonstrate that when such an oscillator is strongly coupled to a quantized superconducting qubit, both the effective nonlinearity and the threshold become a non-trivial function of the qubit-oscillator detuning. Moreover, the autoresonant threshold is sensitive to the quantum state of the qubit and may be used to realize a high fidelity, latching readout whose speed is not limited by the oscillator Q.
arxiv pdf