Dekohärenz in Josephson-Quantenbits aufgrund von Ladungs- und Spin-Fluktuatoren

• Antragsteller: Professor Dr. Gerd Schön; Professor Dr. Alexander Shnirman
• Fachliche Zuordnung: Theoretische Physik der kondensierten Materie ; Statistische Physik, Nichtlineare Dynamik, Komplexe Systeme, Weiche und fluide Materie, Biologische Physik
• Förderung: Förderung von 2014 bis 2018
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 255576543

• Erstellungsjahr: 2019

Zusammenfassung der Projektergebnisse

The level of coherent control of quantum circuits in general and Josephson junctions in particular has advanced greatly in the last decade. Major sources of decoherence have been identified and suppressed by means of careful circuit engineering. As a consequence, the coherence times of Josephson quantum bits (qubits) reach routinely tens to even hundreds of microseconds. The remaining sources of noise are frequently related to defects in the materials used for the devices. The high-quality Josephson junction devices can be used to probe these properties. The theoretical investigation of some of these effects has been the main subject of the project. Specifically we investigated 6 subtopics. (i) We studied the properties of two-level systems (TLSs) in Josephson junctions and their signatures when probed by Josephson qubits. We collaborated with experimentalists who used Josephson qubits as probes of the defects. We provided a theoretical explanation of the observations based on the standard tunneling model, and identified the interaction with incoherent low-frequency (thermal) TLSs as the major mechanism of the pure dephasing of coherent high-frequency TLS. (ii) For the same experimental systems we studied the influence of non-equilibrium quasiparticles on the decoherence of the Josephson qubit and the TLSs. (iii) In a numerical simulation we studied the effects of paramagnetic spins on surfaces and interfaces. We found them to lead to 1/fα noise of the magnetic flux with an exponent slightly smaller than 1 and a magnitude as observed in SQUIDs and flux qubits. (iv) We developed a theoretical formalism for calculating the fluctuations of linear response coefficients, such as the inductance of SQUID loops, which was observed in experiments. (v) We analyzed the limitations on digital quantum simulations which arise due to nonperfect gates, as expressed by a gate-fidelity lower than 100%. (vi) We have further developed the diagrammatic technique based on Majorana representation of spin operators. (vii) For a specific setup serving as Majorana qubit, and the read out of the state of the qubit by a current measurement we investigated the dynamics of the quantum measurement process and the induced decoherence.

Projektbezogene Publikationen (Auswahl)

• Flux 1 /fα noise in 2D Heisenberg spin glasses: effects of weak anisotropic interactions. Phys. Rev. B 90, 014206 (2014)
  J. Atalaya, J. Clarke, G. Schön and A. Shnirman
  
• Nonequilibrium Spin Noise and Noise of Susceptibility. Phys. Rev. B 90, 205419 (2014)
  P. Schad, B.N. Narozhny, G. Schön and A. Shnirman
  
• Majorana representation for dissipative spin systems, Annals of Physics 361, 401 (2015)
  P. Schad, Yu. Makhlin, B.N. Narozhny, G. Schön, A. Shnirman
  
• Qubit dephasing due to quasiparticle tunneling. Phys. Rev. B 91, 174504 (2015)
  S. Zanker and M. Marthaler
  
• Decoherence of a quantum two-level system by spectral diffusion. Phys. Rev. B 93, 134208 (2016)
  S. Matityahu, A. Shnirman, G. Schön and M. Schechter
  
• Decoherence spectroscopy with individual two-level tunneling defects. Sci. Rep. 6, 23786 (2016)
  J. Lisenfeld, A. Bilmes, S. Matityahu, S. Zanker, M. Marthaler, M. Schechter, G. Schön, A. Shnirman, G. Weiss and A.V. Ustinov
  
• Electronic Decoherence of Two-Level Systems in a Josephson Junction. Phys. Rev. B 96, 064504 (2017)
  A. Bilmes, S. Zanker, A. Heimes, M. Marthaler, G. Schön, G. Weiss, A.V. Ustinov and J. Lisenfeld
  
• Rabi noise spectroscopy of individual two-level tunneling defect, Phys. Rev. B 95, 241409 (2017)
  S. Matityahu, J. Lisenfeld, A. Bilmes, A. Shnirman, G. Weiss, A. V. Ustinov, and M. Schechter
  
• Effects of gate errors in digital quantum simulations of fermionic systems. Quantum Science and Technology 3, 045008 (2018)
  J.M. Reiner, S. Zanker, I. Schwenk, J. Leppäkangas, F.K. Wilhelm, G. Schön and M. Marthaler
  
• Transport Signatures of a Majorana Qubit and Read-out-induced Dephasing. New J. Phys.
  L. Qin, X.Q. Li, A. Shnirman and G. Schön