M.G. Vavilov

Measurement of a Superconducting Qubit with a Microwave Photon Counter

  1. A. Opremcak,
  2. I. V. Pechenezhskiy,
  3. C. Howington,
  4. B. G. Christensen,
  5. M. A. Beck,
  6. E. Leonard Jr.,
  7. J. Suttle,
  8. C. Wilen,
  9. K. N. Nesterov,
  10. G. J. Ribeill,
  11. T. Thorbeck,
  12. F. Schlenker,
  13. M.G. Vavilov,
  14. B. L. T. Plourde,
  15. and R. McDermott
Fast, high-fidelity measurement is a key ingredient for quantum error correction. Conventional approaches to the measurement of superconducting qubits, involving linear amplification
of a microwave probe tone followed by heterodyne detection at room temperature, do not scale well to large system sizes. Here we introduce an alternative approach to measurement based on a microwave photon counter. We demonstrate raw single-shot measurement fidelity of 92%. Moreover, we exploit the intrinsic damping of the counter to extract the energy released by the measurement process, allowing repeated high-fidelity quantum non-demolition measurements. Crucially, our scheme provides access to the classical outcome of projective quantum measurement at the millikelvin stage. In a future system, counter-based measurement could form the basis for a scalable quantum-to-classical interface.
arxiv pdf

Quantum–Classical Interface Based on Single Flux Quantum Digital Logic

  1. R. McDermott,
  2. M.G. Vavilov,
  3. B. L. T. Plourde,
  4. F.K. Wilhelm,
  5. P. J. Liebermann,
  6. O. A. Mukhanov,
  7. and T. A. Ohki
We describe an approach to the integrated control and measurement of a large-scale superconducting multiqubit circuit using a proximal coprocessor based on the Single Flux Quantum (SFQ)
digital logic family. Coherent control is realized by irradiating the qubits directly with classical bitstreams derived from optimal control theory. Qubit measurement is performed by a Josephson photon counter, which provides access to the classical result of projective quantum measurement at the millikelvin stage. We analyze the power budget and physical footprint of the SFQ coprocessor and discuss challenges and opportunities associated with this approach.
arxiv pdf

Phonon-Mediated Quasiparticle Poisoning of Superconducting Microwave Resonators

  1. U. Patel,
  2. Ivan V. Pechenezhskiy,
  3. B. L. T. Plourde,
  4. M.G. Vavilov,
  5. and R. McDermott
Nonequilibrium quasiparticles represent a significant source of decoherence in superconducting quantum circuits. Here we investigate the mechanism of quasiparticle poisoning in devices
subjected to local quasiparticle injection. We find that quasiparticle poisoning is dominated by the propagation of pair-breaking phonons across the chip. We characterize the energy dependence of the timescale for quasiparticle poisoning. Finally, we observe that incorporation of extensive normal metal quasiparticle traps leads to a more than order of magnitude reduction in quasiparticle loss for a given injected quasiparticle power.
arxiv pdf

Accurate Qubit Control with Single Flux Quantum Pulses

  1. R. McDermott,
  2. and M.G. Vavilov
We describe the coherent manipulation of harmonic oscillator and qubit modes using resonant trains of single flux quantum pulses in place of microwaves. We show that coherent rotations
are obtained for pulse-to-pulse spacing equal to the period of the oscillator. We consider a protocol for preparing bright and dark harmonic oscillator pointer states. Next we analyze rotations of a two-state qubit system. We calculate gate errors due to timing jitter of the single flux quantum pulses and due to weak anharmonicity of the qubit. We show that gate fidelities in excess of 99.9% are achievable for sequence lengths of order 20 ns.
arxiv pdf