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24.11.2016 Mario Palma

24.11.2016 Mario Palma. Motivation. Quasiparticles (QPs) poisoning : Counting error in Superconducting Qubit Decrease the coherence of Superconducting Qubit Decoherence of Majorana Qubit Gap engineering Vortex trap Normal metal trap. J. Aumentado et al., PRL 92 ,066802 (2004).

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24.11.2016 Mario Palma

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  1. 24.11.2016 Mario Palma

  2. Motivation • Quasiparticles (QPs) poisoning : • Counting error in Superconducting Qubit • Decrease the coherence of Superconducting Qubit • Decoherence of MajoranaQubit • Gap engineering • Vortex trap • Normal metal trap J. Aumentadoet al., PRL92,066802 (2004). M. Taupinet al., Nat. Commun. 7,10977 (2016)

  3. Model NIS system electrons in N with energy ξn Tunneling Hamiltonian QPs in S with energy QPs tunneling from S into N Electrons escape from N into S BCS density of the states Energy independent

  4. Model relaxation rate of the electron in the normal metal due to electron-electron interaction or electron-phonon interaction There are many unoccupied state below ∆ in N

  5. Model Rate equations for the probability density in normal metal and in the superconductor Assuming steady-state distribution of the electrons in the normal metal Normalize QPs density Fast relaxation rate Slow relaxation Excitation in the normal metal fast relax at energy below the gap and cannot return in the super conductor

  6. Real case Assume that the diffusion time is bigger than 1/Γr The QPs density distribution can be describe through a diffusion equation: The trap component Pair breaking mechanisms The scale over which the density decay due to the trapping Source of QPs tinj =time that the source is on t = time after the source is switched off

  7. Device and experimental set up 3D transmonqubit Josephson junction Al/AlOx /Al Superconductive cavity Cu trap 20 µm<d<400 µm Antenna C. Wang et al., Nat.Commun. 5,5836 (2014)

  8. Decay rate w/o trap T1 = 19 µs 10 µs<T1 <22µs 22µm < d < 80 µm T1 is qualitative the same T1 =5 µs →d = 200 µm & T1 =7 µs →d = 400 µm T1 is reduce The time dependent part of the qubitsi directly proportional to the QPs density at the junction

  9. short & long trap Long trap Short trap the model predict: linear dependence of the characteristic time scale on the trap length Short trap Long trap saturation behavior Tfr =13 mKeff =2.42x105 s-1 Tfr =50 mKeff =3.74x105 s-1

  10. Conclusion • Γeff is energy dependent for time scale shorter than electron relaxation rate • Evacuation time depend linearly on the length of trap and saturated for long trap • The decay rate increase with length of the trap • For short trap Γeffincreases with temperature indicating back flow of QPs

  11. Gap engineering

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