Dipole to quadrapole interactions in a transmon qubit: Purcell protection and tunable coupling

In a recent experiment Houck et al [Phys. Rev. Lett. 101, 080502 (2008)] showed that the major source of decoherence in the transmon qubit was relaxation through the resonator. This relaxation is known as the Purcell effect and arises from the dipole interaction between the transmon qubit and the resonators electromagnetic field. In this talk I will present a modification of the transmon qubit that allows us to tune the transmon-resonator interaction from dipole to quadrapole, and hence turn the Purcell effect off with surprisingly little effect on the control and measurement.