B. L. T. Plourde

Copper waveguide cavities with reduced surface loss for coupling to superconducting qubits

  1. Daniela F. Bogorin,
  2. D. T. McClure,
  3. Matthew Ware,
  4. and B. L. T. Plourde
Significant improvements in superconducting qubit coherence times have been achieved recently with three-dimensional microwave waveguide cavities coupled to transmon qubits. While many
of the measurements in this direction have utilized superconducting aluminum cavities, other recent work has involved qubits coupled to copper cavities with coherence times approaching 0.1 ms. The copper provides a good path for thermalizing the cavity walls and qubit chip, although the substantial cavity loss makes conventional dispersive qubit measurements challenging. We are exploring various approaches for improving the quality factor of three-dimensional copper cavities, including electropolishing and coating with superconducting layers of tin. We have characterized these cavities on multiple cooldowns and found the tin-plating to be robust. In addition, we have performed coherence measurements on transmon qubits in these cavities and observed promising performance.
arxiv pdf

First-order sideband transitions with flux-driven asymmetric transmon qubits

  1. J. D. Strand,
  2. Matthew Ware,
  3. Félix Beaudoin,
  4. T. A. Ohki,
  5. B. R. Johnson,
  6. Alexandre Blais,
  7. and B. L. T. Plourde
We demonstrate rapid, first-order sideband transitions between a superconducting resonator and a frequency-modulated transmon qubit. The qubit contains a substantial asymmetry between
its Josephson junctions leading to a linear portion of the energy band near the resonator frequency. The sideband transitions are driven with a magnetic flux signal of a few hundred MHz coupled to the qubit. This modulates the qubit splitting at a frequency near the detuning between the dressed qubit and resonator frequencies, leading to rates up to 85 MHz for exchanging quanta between the qubit and resonator.
arxiv pdf

Process verification of two-qubit quantum gates by randomized benchmarking

  1. A. D. Córcoles,
  2. Jay M. Gambetta,
  3. Jerry M. Chow,
  4. John A. Smolin,
  5. Matthew Ware,
  6. J. D. Strand,
  7. B. L. T. Plourde,
  8. and M. Steffen
We implement a complete randomized benchmarking protocol on a system of two superconducting qubits. The protocol consists of randomizing over gates in the Clifford group, which experimentally
are generated via an improved two-qubit cross-resonance gate implementation and single-qubit unitaries. From this we extract an optimal average error per Clifford of 0.0936. We also perform an interleaved experiment, alternating our optimal two-qubit gate with random two-qubit Clifford gates, to obtain a two-qubit gate error of 0.0653. We compare these values with a two-qubit gate error of ~0.12 obtained from quantum process tomography, which is likely limited by state preparation and measurement errors.
arxiv pdf

Superconducting qubit in waveguide cavity with coherence time approaching 0.1ms

  1. Chad Rigetti,
  2. Stefano Poletto,
  3. Jay M. Gambetta,
  4. B. L. T. Plourde,
  5. Jerry M. Chow,
  6. A. D. Corcoles,
  7. John A. Smolin,
  8. Seth T. Merkel,
  9. J. R. Rozen,
  10. George A. Keefe,
  11. Mary B. Rothwell,
  12. Mark B. Ketchen,
  13. and M. Steffen
We report a superconducting artificial atom with an observed quantum coherence time of T2*=95us and energy relaxation time T1=70us. The system consists of a single Josephson junction
transmon qubit embedded in an otherwise empty copper waveguide cavity whose lowest eigenmode is dispersively coupled to the qubit transition. We attribute the factor of four increase in the coherence quality factor relative to previous reports to device modifications aimed at reducing qubit dephasing from residual cavity photons. This simple device holds great promise as a robust and easily produced artificial quantum system whose intrinsic coherence properties are sufficient to allow tests of quantum error correction.
arxiv pdf