J. Kelly

Coherent Josephson qubit suitable for scalable quantum integrated circuits

  1. R. Barends,
  2. J. Kelly,
  3. A. Megrant,
  4. D. Sank,
  5. E. Jeffrey,
  6. Y. Chen,
  7. Y. Yin,
  8. B. Chiaro,
  9. J. Mutus,
  10. C. Neill,
  11. P. O'Malley,
  12. P. Roushan,
  13. J. Wenner,
  14. T. C. White,
  15. A. N. Cleland,
  16. and John M. Martinis
We demonstrate a planar, tunable superconducting qubit with energy relaxation times up to 44 microseconds. This is achieved by using a geometry designed to both minimize radiative loss
and reduce coupling to materials-related defects. At these levels of coherence, we find a fine structure in the qubit energy lifetime as a function of frequency, indicating the presence of a sparse population of incoherent, weakly coupled two-level defects. This is supported by a model analysis as well as experimental variations in the geometry. Our `Xmon‘ qubit combines facile fabrication, straightforward connectivity, fast control, and long coherence, opening a viable route to constructing a chip-based quantum computer.
arxiv pdf

Multiplexed dispersive readout of superconducting phase qubits

  1. Yu Chen,
  2. D. Sank,
  3. P. O'Malley,
  4. T. White,
  5. R. Barends,
  6. B. Chiaro,
  7. J. Kelly,
  8. E. Lucero,
  9. M. Mariantoni,
  10. A. Megrant,
  11. C. Neill,
  12. A. Vainsencher,
  13. J. Wenner,
  14. Yi Yin,
  15. A. N. Cleland,
  16. and John M. Martinis
We introduce a frequency-multiplexed readout scheme for superconducting phase qubits. Using a quantum circuit with four phase qubits, we couple each qubit to a separate lumped-element
superconducting readout resonator, with the readout resonators connected in parallel to a single measurement line. The readout resonators and control electronics are designed so that all four qubits can be read out simultaneously using frequency multiplexing on the one measurement line. This technology provides a highly efficient and compact means for reading out multiple qubits, a significant advantage for scaling up to larger numbers of qubits.
arxiv pdf

Excitation of superconducting qubits from hot non-equilibrium quasiparticles

  1. J. Wenner,
  2. Yi Yin,
  3. Erik Lucero,
  4. R. Barends,
  5. Yu Chen,
  6. B. Chiaro,
  7. J. Kelly,
  8. M. Lenander,
  9. Matteo Mariantoni,
  10. A. Megrant,
  11. C. Neill,
  12. P. J. J. O'Malley,
  13. D. Sank,
  14. A. Vainsencher,
  15. H. Wang,
  16. T. C. White,
  17. A. N. Cleland,
  18. and John M. Martinis
Superconducting qubits probe environmental defects such as non-equilibrium quasiparticles, an important source of decoherence. We show that „hot“ non-equilibrium quasiparticles,
with energies above the superconducting gap, affect qubits differently from quasiparticles at the gap, implying qubits can probe the dynamic quasiparticle energy distribution. For hot quasiparticles, we predict a non-neligable increase in the qubit excited state probability P_e. By injecting hot quasiparticles into a qubit, we experimentally measure an increase of P_e in semi-quantitative agreement with the model.
arxiv pdf

Controlled catch and release of microwave photon states

  1. Yi Yin,
  2. Yu Chen,
  3. Daniel Sank,
  4. P. J. J. O'Malley,
  5. T. C. White,
  6. R. Barends,
  7. J. Kelly,
  8. Erik Lucero,
  9. Matteo Mariantoni,
  10. A. Megrant,
  11. C. Neill,
  12. A. Vainsencher,
  13. J. Wenner,
  14. Alexander N. Korotkov,
  15. A. N. Cleland,
  16. and John M. Martinis
, in which the resonant cavity confines photons and promotes"]strong light-matter interactions. The cavity end-mirrors determine the performance of the coupled system, with higher mirror reflectivity yielding better quantum coherence, but higher mirror transparency giving improved measurement and control, forcing a compromise. An alternative is to control the mirror transparency, enabling switching between long photon lifetime during quantum interactions and large signal strength when performing measurements. Here we demonstrate the superconducting analogue, using a quantum system comprising a resonator and a qubit, with variable coupling to a measurement transmission line. The coupling can be adjusted through zero to a photon emission rate 1,000 times the intrinsic photon decay rate. We use this system to control photons in coherent states as well as in non-classical Fock states, and dynamically shape the waveform of released photons. This has direct applications to circuit quantum electrodynamics [3], and may enable high-fidelity quantum state transfer between distant qubits, for which precisely-controlled waveform shaping is a critical and non-trivial requirement [4, 5].
arxiv pdf