G. Tancredi

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

Calibration of the cross-resonance two-qubit gate between directly-coupled transmons

  1. A. D. Patterson,
  2. J. Rahamim,
  3. T. Tsunoda,
  4. P. Spring,
  5. S. Jebari,
  6. K. Ratter,
  7. M. Mergenthaler,
  8. G. Tancredi,
  9. B. Vlastakis,
  10. M. Esposito,
  11. and P. J. Leek
Quantum computation requires the precise control of the evolution of a quantum system, typically through application of discrete quantum logic gates on a set of qubits. Here, we use
the cross-resonance interaction to implement a gate between two superconducting transmon qubits with a direct static dispersive coupling. We demonstrate a practical calibration procedure for the optimization of the gate, combining continuous and repeated-gate Hamiltonian tomography with step-wise reduction of dominant two-qubit coherent errors through mapping to microwave control parameters. We show experimentally that this procedure can enable a ZX^−π/2 gate with a fidelity F=97.0(7)%, measured with interleaved randomized benchmarking. We show this in a architecture with out-of-plane control and readout that is readily extensible to larger scale quantum circuits.
arxiv pdf

Double-sided coaxial circuit QED with out-of-plane wiring

  1. J. Rahamim,
  2. T. Behrle,
  3. M. J. Peterer,
  4. A. Patterson,
  5. P. Spring,
  6. T. Tsunoda,
  7. R. Manenti,
  8. G. Tancredi,
  9. and P. J. Leek
Superconducting circuits are well established as a strong candidate platform for the development of quantum computing. In order to advance to a practically useful level, architectures
are needed which combine arrays of many qubits with selective qubit control and readout, without compromising on coherence. Here we present a coaxial circuit QED architecture in which qubit and resonator are fabricated on opposing sides of a single chip, and control and readout wiring are provided by coaxial wiring running perpendicular to the chip plane. We present characterisation measurements of a fabricated device in good agreement with simulated parameters and demonstrating energy relaxation and dephasing times of T1=4.1μs and T2=5.7μs respectively. The architecture allows for scaling to large arrays of selectively controlled and measured qubits with the advantage of all wiring being out of the plane.
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