Abstract
Essential properties of superconductivity and Josephson tunneling are briefly reviewed. The superconducting flux qubit consists of three Josephson junctions—two of them with identical critical currents and the third with a smaller critical current—connected in series on a superconducting loop. We investigated the performance of flux qubits with an added thin film, shunt capacitor across the smallest junction. Since the shunt capacitance is substantially greater than any stray capacitance and the self-capacitance of the junction, virtually all the electric field energy is stored in the shunt capacitor, which has a much larger area then the qubit junctions. The electric fields at the edges of the junction and C-shunt are thereby substantially reduced, in turn reducing the loss in the nearby environment. As a result, T1 is increased, as is T2 at degeneracy. The new design allows us to reduce the junction critical current significantly, decreasing the qubit coupling to1/f flux noise. Further improvements are achieved by using high quality Al films deposited by molecular beam epitaxy on annealed sapphire substrates. Collectively, these improvements enabled us to achieve a maximum T1 at degeneracy of 55 ms. For a qubit with T1 = 44 ms at degeneracy, using a Carr-Purcell-Meiboom-Gill pulse echo sequence we achieved a decoherence time T2 of 85 ms. Interestingly, for a qubit with a splitting at degeneracy below 1 GHz, the value of T1 is limited by high frequency flux 1/f noise.