Leonid I. Glazman

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

Inelastic Microwave Photon Scattering off a Quantum Impurity in a Josephson-Junction Array

  1. Moshe Goldstein,
  2. Michel H. Devoret,
  3. Manuel Houzet,
  4. and Leonid I. Glazman
Quantum fluctuations in an anharmonic superconducting circuit enable frequency conversion of individual incoming photons. This effect, linear in the photon beam intensity, leads to
ramifications for the standard input-output circuit theory. We consider an extreme case of anharmonicity in which photons scatter off a small set of weak links within a Josephson junction array. We show that this quantum impurity displays Kondo physics and evaluate the elastic and inelastic photon scattering cross sections. These cross sections reveal many-body properties of the Kondo problem that are hard to access in its traditional fermionic version.
arxiv pdf