Experiments on a Superconducting Qubit Manipulated by Fast Flux Pulses: The Issue of Non-adiabaticity

The double SQUID qubit is a superconducting interferometer (SQUID) made of two Josephson junctions and two superconducting loops. Its energy potential can be greatly modified in shape and symmetry by using two magnetic control fluxes that can change the potential from a double well to an almost harmonic single well: This feature is exploited for manipulating the qubit without resorting to the usual NMR-like techniques with microwave irradiation. The qubit machinery relies on these operations being performed non-adiabatically, realizing a transition between the two lowest-lying energy levels, at the same time avoiding excitation of upper levels, a condition that can be satisfied by using control pulses with proper risetime. We show experimental results referring to manipulation of the qubit at different rates and make a qualitative comparison with theoretical expectations.