M. Neeley

Measurement-induced state transitions in a superconducting qubit: Beyond the rotating wave approximation

  1. Daniel Sank,
  2. Zijun Chen,
  3. Mostafa Khezri,
  4. J. Kelly,
  5. R. Barends,
  6. Y. Chen,
  7. A. Fowler,
  8. E. Jeffrey,
  9. E. Lucero,
  10. A. Megrant,
  11. J. Mutus,
  12. M. Neeley,
  13. P. Roushan,
  14. A. Vainsencher,
  15. T. White,
  16. B. Campbell,
  17. B. Chiaro,
  18. A. Dunsworth,
  19. C. Neill,
  20. P. J. J. O'Malley,
  21. C. Quintana,
  22. J. Wenner,
  23. Alexander N. Korotkov,
  24. and John M. Martinis
Many superconducting qubit systems use the dispersive interaction between the qubit and a coupled harmonic resonator to perform quantum state measurement. Previous works have found
that such measurements can induce state transitions in the qubit if the number of photons in the resonator is too high. We investigate these transitions and find that they can push the qubit out of the two-level subspace. Furthermore, these transitions show resonant behavior as a function of photon number. We develop a theory for these observations based on level crossings within the Jaynes-Cummings ladder, with transitions mediated by terms in the Hamiltonian which are typically ignored by the rotating wave approximation. We confirm the theory by measuring the photon occupation of the resonator when transitions occur while varying the detuning between the qubit and resonator.
arxiv pdf

Chiral groundstate currents of interacting photons in a synthetic magnetic field

  1. P. Roushan,
  2. C. Neill,
  3. A. Megrant,
  4. Y. Chen,
  5. R. Babbush,
  6. R. Barends,
  7. B. Campbell,
  8. Z. Chen,
  9. B. Chiaro,
  10. A. Dunsworth,
  11. A. Fowler,
  12. E. Jeffrey,
  13. J. Kelly,
  14. E. Lucero,
  15. J. Mutus,
  16. P. J. J. O'Malley,
  17. M. Neeley,
  18. C. Quintana,
  19. D. Sank,
  20. A. Vainsencher,
  21. J. Wenner,
  22. T. White,
  23. E. Kapit,
  24. and J. Martinis
The intriguing many-body phases of quantum matter arise from the interplay of particle interactions, spatial symmetries, and external fields. Generating these phases in an engineered
system could provide deeper insight into their nature and the potential for harnessing their unique properties. However, concurrently bringing together the main ingredients for realizing many-body phenomena in a single experimental platform is a major challenge. Using superconducting qubits, we simultaneously realize synthetic magnetic fields and strong particle interactions, which are among the essential elements for studying quantum magnetism and fractional quantum Hall (FQH) phenomena. The artificial magnetic fields are synthesized by sinusoidally modulating the qubit couplings. In a closed loop formed by the three qubits, we observe the directional circulation of photons, a signature of broken time-reversal symmetry. We demonstrate strong interactions via the creation of photon-vacancies, or „holes“, which circulate in the opposite direction. The combination of these key elements results in chiral groundstate currents, the first direct measurement of persistent currents in low-lying eigenstates of strongly interacting bosons. The observation of chiral currents at such a small scale is interesting and suggests that the rich many-body physics could survive to smaller scales. We also motivate the feasibility of creating FQH states with near future superconducting technologies. Our work introduces an experimental platform for engineering quantum phases of strongly interacting photons and highlight a path toward realization of bosonic FQH states.
arxiv pdf

Scalable in-situ qubit calibration during repetitive error detection

  1. J. Kelly,
  2. R. Barends,
  3. A. G. Fowler,
  4. A. Megrant,
  5. E. Jeffrey,
  6. T. C. White,
  7. D. Sank,
  8. J. Y. Mutus,
  9. B. Campbell,
  10. Yu Chen,
  11. Z. Chen,
  12. B. Chiaro,
  13. A. Dunsworth,
  14. E. Lucero,
  15. M. Neeley,
  16. C. Neill,
  17. P. J. J. O'Malley,
  18. C. Quintana,
  19. P. Roushan,
  20. A. Vainsencher,
  21. J. Wenner,
  22. and John M. Martinis
We present a method to optimize qubit control parameters during error detection which is compatible with large-scale qubit arrays. We demonstrate our method to optimize single or two-qubit
gates in parallel on a nine-qubit system. Additionally, we show how parameter drift can be compensated for during computation by inserting a frequency drift and using our method to remove it. We remove both drift on a single qubit and independent drifts on all qubits simultaneously. We believe this method will be useful in keeping error rates low on all physical qubits throughout the course of a computation. Our method is O(1) scalable to systems of arbitrary size, providing a path towards controlling the large numbers of qubits needed for a fault-tolerant quantum computer
arxiv pdf

Digitized adiabatic quantum computing with a superconducting circuit

  1. R. Barends,
  2. A. Shabani,
  3. L. Lamata,
  4. J. Kelly,
  5. A. Mezzacapo,
  6. U. Las Heras,
  7. R. Babbush,
  8. A. G. Fowler,
  9. B. Campbell,
  10. Yu Chen,
  11. Z. Chen,
  12. B. Chiaro,
  13. A. Dunsworth,
  14. E. Jeffrey,
  15. E. Lucero,
  16. A. Megrant,
  17. J. Y. Mutus,
  18. M. Neeley,
  19. C. Neill,
  20. P. J. J. O'Malley,
  21. C. Quintana,
  22. P. Roushan,
  23. D. Sank,
  24. A. Vainsencher,
  25. J. Wenner,
  26. T. C. White,
  27. E. Solano,
  28. H. Neven,
  29. and John M. Martinis
A major challenge in quantum computing is to solve general problems with limited physical hardware. Here, we implement digitized adiabatic quantum computing, combining the generality
of the adiabatic algorithm with the universality of the digital approach, using a superconducting circuit with nine qubits. We probe the adiabatic evolutions, and quantify the success of the algorithm for random spin problems. We find that the system can approximate the solutions to both frustrated Ising problems and problems with more complex interactions, with a performance that is comparable. The presented approach is compatible with small-scale systems as well as future error-corrected quantum computers.
arxiv pdf

Measuring and Suppressing Quantum State Leakage in a Superconducting Qubit

  1. Zijun Chen,
  2. Julian Kelly,
  3. Chis Quintana,
  4. R. Barends,
  5. B. Camppbell,
  6. Yu Chen,
  7. B. Chiaro,
  8. A. Dunsworth,
  9. A. Fowler,
  10. E. Lucero,
  11. E. Jeffrey,
  12. A. Megrant,
  13. J. Mutus,
  14. M. Neeley,
  15. C. Neill,
  16. P. J. J. O'malley,
  17. P. Roushan,
  18. D. Sank,
  19. A. Vainsencher,
  20. J. Wenner,
  21. T. C. White,
  22. A. N. Korotkov,
  23. and John M. Martinis
Leakage errors occur when a quantum system leaves the two-level qubit subspace. Reducing these errors is critically important for quantum error correction to be viable. To quantify
leakage errors, we use randomized benchmarking in conjunction with measurement of the leakage population. We characterize single qubit gates in a superconducting qubit, and by refining our use of Derivative Reduction by Adiabatic Gate (DRAG) pulse shaping along with detuning of the pulses, we obtain gate errors consistently below 10−3 and leakage rates at the 10−5 level. With the control optimized, we find that a significant portion of the remaining leakage is due to incoherent heating of the qubit.
arxiv pdf