Decoherence and Relaxation in a Charge Qubit

Igor Lerner, Birmingham

The major contribution to decoherence of a quantum-dot or Josephson-junction charge qubit comes from the electrostatic coupling to charged impurities with a charge fluctuating due to hybridization with electrons in the reservoir. However, estimating the efficiency of such ``fluctuators'' for decoherence according to the previously developed theories shows that finding a sufficient number of effective fluctuators in a realistic experimental layout is rather improbable. We show that this paradox is resolved by allowing for a short-range Coulomb interaction of the fluctuators with the electrons in the reservoir dramatically enhances both their contribution to decoherence and the estimate of the number of effective fluctuators, resulting in the most dangerous decoherence mechanism for charge qubits.