Coherent quantum manipulation of superconducting qubits by fast flux pulses: decoherence, adiabaticity and speed issues

A double SQUID can be used as a solid state flux qubit. Its behavior is described by an effective potential the shape of which can be modified by two independent bias magnetic fluxes. In particular, it is possible to pass from a single-well to a double-well shape, and it is also possible to modify its symmetry/asymmetry. A rapid modification of the potential allows the manipulation of the quantum state in a very fast manner: we observed coherent oscillations with frequencies up to 70GHz, to be compared with the typical oscillations obtained by microwave manipulations on similar systems, below 1GHz. This is an interesting advantage from the point of view of quantum computing, since for similar decoherence times a greater speed means a large number of possible coherent operations. Moreover, the manipulation based on fast flux pulses allowed a deep insight in decoherence mechanism acting on the system. The drawback of this scheme concerns the fact that the manipulation must be in the same time non-adiabatic (for the two lower energy levels, the computational states), and adiabatic (for the upper ones). This is possible thanks to the energy level structure, but requires an accurate characterization of the system. We present principles and recent results, and discuss issues and perspectives about this scheme.