Critical frequency control for arbitrarily slow decoherence of a qubit
Filippo Giraldi, Francesco Petruccione · Physical Review A · 2012
The decoherence of a qubit is studied for a general class of spectral densities with an arbitrary band gap. The exact dynamics reveals the existence of critical values of the qubit transition frequency, such that relaxations of coherence show discontinuities, jumping from decays with powers arbitrarily larger than unity to powers arbitrarily larger than 0. The critical frequency is the sum of the band-gap frequency and the first negative moment of the spectral density. In the superohmic case, coherence is not entirely lost and the ratio of the trapped population approaches unity as the second negative moment of the spectral density becomes negligible with respect to the scale frequency. Arbitrarily slow decoherence processes are obtained in the critical configuration by approaching the boundary between subohmic and superohmic regimes. The setup of critical configurations constitutes a different way to control qubit decoherence.