J. M. Gambetta

Experimental demonstration of a resonator-induced phase gate in a multi-qubit circuit QED system

  1. Hanhee Paik,
  2. A. Mezzacapo,
  3. Martin Sandberg,
  4. D. T. McClure,
  5. B. Abdo,
  6. A. D. Corcoles,
  7. O. Dial,
  8. D. F. Bogorin,
  9. B. L. T. Plourde,
  10. M. Steffen,
  11. A. W. Cross,
  12. J. M. Gambetta,
  13. and Jerry M. Chow
The resonator-induced phase (RIP) gate is a multi-qubit entangling gate that allows a high degree of flexibility in qubit frequencies, making it attractive for quantum operations in
large-scale architectures. We experimentally realize the RIP gate with four superconducting qubits in a three-dimensional (3D) circuit-quantum electrodynamics architecture, demonstrating high-fidelity controlled-Z (CZ) gates between all possible pairs of qubits from two different 4-qubit devices in pair subspaces. These qubits are arranged within a wide range of frequency detunings, up to as large as 1.8 GHz. We further show a dynamical multi-qubit refocusing scheme in order to isolate out 2-qubit interactions, and combine them to generate a four-qubit Greenberger-Horne-Zeilinger state.
arxiv pdf

Investigating surface loss effects in superconducting transmon qubits

  1. J. M. Gambetta,
  2. C. E. Murray,
  3. Y.-K.-K. Fung,
  4. D. T. McClure,
  5. O. Dial,
  6. W. Shanks,
  7. J. Sleight,
  8. and M. Steffen
Superconducting qubits are sensitive to a variety of loss mechanisms including dielectric loss from interfaces. By changing the physical footprint of the qubit it is possible to modulate
sensitivity to surface loss. Here we show a systematic study of planar superconducting transmons of differing physical footprints to optimize the qubit design for maximum coherence. We find that qubits with small footprints are limited by surface loss and that qubits with large footprints are limited by other loss mechanisms which are currently not understood.
arxiv pdf

Improved qubit bifurcation readout in the straddling regime of circuit QED

  1. Maxime Boissonneault,
  2. J. M. Gambetta,
  3. and A. Blais
We study bifurcation measurement of a multi-level superconducting qubit using a nonlinear resonator biased in the straddling regime, where the resonator frequency sits between two qubit
transition frequencies. We find that high-fidelity bifurcation measurements are possible because of the enhanced qubit-state-dependent pull of the resonator frequency, the behavior of qubit-induced nonlinearities and the reduced Purcell decay rate of the qubit that can be realized in this regime. Numerical simulations find up to a threefold improvement in qubit readout fidelity when operating in, rather than outside of, the straddling regime. High-fidelity measurements can be obtained at much smaller qubit-resonator couplings than current typical experimental realizations, reducing spectral crowding and potentially simplifying the implementation of multi-qubit devices.
arxiv pdf

Measurement-induced qubit state mixing in circuit QED from up-converted dephasing noise

  1. D. H. Slichter,
  2. R. Vijay,
  3. S. J. Weber,
  4. S. Boutin,
  5. M. Boissonneault,
  6. J. M. Gambetta,
  7. A. Blais,
  8. and I. Siddiqi
We observe measurement-induced qubit state mixing in a transmon qubit dispersively coupled to a planar readout cavity. Our results indicate that dephasing noise at the qubit-readout
detuning frequency is up-converted by readout photons to cause spurious qubit state transitions, thus limiting the nondemolition character of the readout. Furthermore, we use the qubit transition rate as a tool to extract an equivalent flux noise spectral density at f ~ 1 GHz and find agreement with values extrapolated from a $1/f^alpha$ fit to the measured flux noise spectral density below 1 Hz.
arxiv pdf