Nonadditivity of decoherence rates in superconducting qubits

Abstract

We show that the relaxation and decoherence rates T1 -1 and T2 -1 of a qubit coupled to several noise sources are in general not additive, i.e., that the total rates are not the sums of the rates due to each individual noise source. To demonstrate this, we calculate the relaxation and pure dephasing rates T1 -1 and T -1 of a superconducting (SC) flux qubit in the Born-Markov approximation in the presence of several circuit impedances Zi using network graph theory and determine their deviation from additivity (the mixing term). We find that there is no mixing term in T -1 and that the mixing terms in T1 -1 and T2 -1 can be positive or negative, leading to reduced or enhanced relaxation and decoherence times T1 and T2. The mixing term due to the circuit inductance L at the qubit transition frequency ω01 is generally of second order in ω01 L Zi, but of third order if all impedances Zi are pure resistances. We calculate T1,2 for an example of a SC flux qubit coupled to two impedances. © 2005 The American Physical Society.