A. Jayaraman

Quantum bath suppression in a superconducting circuit by immersion cooling

  1. M. Lucas,
  2. A. V. Danilov,
  3. L. V. Levitin,
  4. A. Jayaraman,
  5. A. J. Casey,
  6. L. Faoro,
  7. A. Ya. Tzalenchuk,
  8. S. E. Kubatkin,
  9. J. Saunders,
  10. and S. E. de Graaf
Quantum circuits interact with the environment via several temperature-dependent degrees of freedom. Yet, multiple experiments to-date have shown that most properties of superconducting
devices appear to plateau out at T≈50 mK — far above the refrigerator base temperature. This is for example reflected in the thermal state population of qubits, in excess numbers of quasiparticles, and polarisation of surface spins — factors contributing to reduced coherence. We demonstrate how to remove this thermal constraint by operating a circuit immersed in liquid 3He. This allows to efficiently cool the decohering environment of a superconducting resonator, and we see a continuous change in measured physical quantities down to previously unexplored sub-mK temperatures. The 3He acts as a heat sink which increases the energy relaxation rate of the quantum bath coupled to the circuit a thousand times, yet the suppressed bath does not introduce additional circuit losses or noise. Such quantum bath suppression can reduce decoherence in quantum circuits and opens a route for both thermal and coherence management in quantum processors.
arxiv pdf

The transition regime between traveling-wave and resonant parametric amplifier

  1. S. Kern,
  2. P. Neilinger,
  3. E. Il'ichev,
  4. A. Sultanov,
  5. M. Schmelz,
  6. S. Linzen,
  7. J. Kunert,
  8. G. Oelsner,
  9. R. Stolz,
  10. A. Danilov,
  11. S. Mahashabde,
  12. A. Jayaraman,
  13. V. Antonov,
  14. S. Kubatkin,
  15. and M. Grajcar
Traveling wave parametric amplifiers based on kinetic or Josephson nonlinear inductance are known to be microwave quantum limited amplifiers. Usually, a perfectly impedance-matched
model is used to describe their characteristics in terms of standard coupled mode theory. In practice, the amplifiers are unmatched nonlinear devices with finite length, exhibiting ripples in the transmission. Since commonly used models fail to describe the ripples of real parametric amplifiers, here we are introducing a theoretical approach with non-negligible reflections, which provides their gain and bandwidth properly for both 3-wave and 4-wave mixing. Predictions of the model are experimentally demonstrated on two types of TWPA, based on coplanar waveguides with a central wire consisting of i) high kinetic inductance superconductor, and ii) array of 2000 Josephson junctions.
arxiv pdf