Teresa Brecht

Materials loss measurements using superconducting microwave resonators

  1. Corey Rae Harrington McRae,
  2. Haozhi Wang,
  3. Jiansong Gao,
  4. Michael Vissers,
  5. Teresa Brecht,
  6. Andrew Dunsworth,
  7. David Pappas,
  8. and Josh Mutus
The performance of superconducting circuits for quantum computing is limited by materials losses. In particular, coherence times are typically bounded by two-level system (TLS) losses
at single photon powers and millikelvin temperatures. The identification of low loss fabrication techniques, materials, and thin film dielectrics is critical to achieving scalable architectures for superconducting quantum computing. Superconducting microwave resonators provide a convenient qubit proxy for assessing performance and studying TLS loss and other mechanisms relevant to superconducting circuits such as non-equilibrium quasiparticles and magnetic flux vortices. In this review article, we provide an overview of considerations for designing accurate resonator experiments to characterize loss, including applicable types of loss, cryogenic setup, device design, and methods for extracting material and interface losses, summarizing techniques that have been evolving for over two decades. Results from measurements of a wide variety of materials and processes are also summarized. Lastly, we present recommendations for the reporting of loss data from superconducting microwave resonators to facilitate materials comparisons across the field.
arxiv pdf

Surface participation and dielectric loss in superconducting qubits

  1. Chen Wang,
  2. Christopher Axline,
  3. Yvonne Y. Gao,
  4. Teresa Brecht,
  5. Luigi Frunzio,
  6. Michel H. Devoret,
  7. and Robert J. Schoelkopf
We study the energy relaxation times (T1) of superconducting transmon qubits in 3D cavities as a function of dielectric participation ratios of material surfaces. This surface participation
ratio, representing the fraction of electric field energy stored in a dissipative surface layer, is computed by a two-step finite-element simulation and experimentally varied by qubit geometry. With a clean electromagnetic environment and suppressed non-equilibrium quasiparticle density, we find an approximately proportional relation between the transmon relaxation rates and surface participation ratios. These results suggest dielectric dissipation arising from material interfaces is the major limiting factor for the T1 of transmons in 3D cQED architecture. Our analysis also supports the notion of spatial discreteness of surface dielectric dissipation.
arxiv pdf

Measurement and Control of Quasiparticle Dynamics in a Superconducting Qubit

  1. Chen Wang,
  2. Yvonne Y. Gao,
  3. Ioan M. Pop,
  4. Uri Vool,
  5. Chris Axline,
  6. Teresa Brecht,
  7. Reinier W. Heeres,
  8. Luigi Frunzio,
  9. Michel H. Devoret,
  10. Gianluigi Catelani,
  11. Leonid I. Glazman,
  12. and Robert J. Schoelkopf
Superconducting circuits have attracted growing interest in recent years as a promising candidate for fault-tolerant quantum information processing. Extensive efforts have always been
taken to completely shield these circuits from external magnetic field to protect the integrity of superconductivity. Surprisingly, here we show vortices can dramatically improve the performance of superconducting qubits by reducing the lifetimes of detrimental single-electron-like excitations known as quasiparticles. Using a contactless injection technique with unprecedented dynamic range, we directly demonstrate the power-law decay characteristics of the canonical quasiparticle recombination process, and show quantization of quasiparticle trapping rate due to individual vortices. Each vortex in our aluminium film shows a quasiparticle „trapping power“ of 0.067±0.005 cm2/s, enough to dominate over the vanishingly weak recombination in a modern transmon qubit. These results highlight the prominent role of quasiparticle trapping in future development of quantum circuits, and provide a powerful characterization tool along the way.
arxiv pdf

Non-Poissonian Quantum Jumps of a Fluxonium Qubit due to Quasiparticle Excitations

  1. Uri Vool,
  2. Ioan M. Pop,
  3. Katrina Sliwa,
  4. Baleegh Abdo,
  5. Chen Wang,
  6. Teresa Brecht,
  7. Yvonne Y. Gao,
  8. Shyam Shankar,
  9. Michael Hatridge,
  10. Gianluigi Catelani,
  11. Mazyar Mirrahimi,
  12. Luigi Frunzio,
  13. Robert J. Schoelkopf,
  14. Leonid I. Glazman,
  15. and Michel H. Devoret
As the energy relaxation time of superconducting qubits steadily improves, non-equilibrium quasiparticle excitations above the superconducting gap emerge as an increasingly relevant
limit for qubit coherence. We measure fluctuations in the number of quasiparticle excitations by continuously monitoring the spontaneous quantum jumps between the states of a fluxonium qubit, in conditions where relaxation is dominated by quasiparticle loss. Resolution on the scale of a single quasiparticle is obtained by performing quantum non-demolition projective measurements within a time interval much shorter than T1, using a quantum limited amplifier (Josephson Parametric Converter). The quantum jumps statistics switches between the expected Poisson distribution and a non-Poissonian one, indicating large relative fluctuations in the quasiparticle population, on time scales varying from seconds to hours. This dynamics can be modified controllably by injecting quasiparticles or by seeding quasiparticle-trapping vortices by cooling down in magnetic field.
arxiv pdf