For many implementations of quantum computing, $1/f$ and other types of broad-spectrum noise are important sources of decoherence. An important step forward would be the ability to extract the characteristics of this noise from qubit measurements to see if it leads to new physical effects. For certain types of qubits, the working point of the qubit can be varied. Using a new mathematical method that is suited to treating all working points, we present theoretical results that show how this degree of freedom can be used to extract noise parameters and predict a new effect: noise-induced looping on the Bloch sphere. We analyze data on superconducting qubits to show that they are very near the parameter regime where this looping should be observed.

• Received 2 July 2009

DOI:https://doi.org/10.1103/PhysRevA.81.010103