Noise and Decoherence in Quantum Two-Level Systems

Abstract

Motivated by recent experiments with Josephson-junction circuits we\nreconsider decoherence effects in quantum two-level systems (TLS). On one hand,\nthe experiments demonstrate the importance of 1/f noise, on the other hand, by\noperating at symmetry points one can suppress noise effects in linear order.\nWe, therefore, analyze noise sources with a variety of power spectra, with\nlinear or quadratic coupling, which are longitudinal or transverse relative to\nthe eigenbasis of the unperturbed Hamiltonian. To evaluate the dephasing time\nfor transverse 1/f noise second-order contributions have to be taken into\naccount. Manipulations of the quantum state of the TLS define characteristic\ntime scales. We discuss the consequences for relaxation and dephasing\nprocesses.\n