Random access control, tunable coupling and enhanced lifetimes in superconducting circuits

I will describe recent experiments in the Schuster Lab at the University of Chicago that attempt to address various challenges in the realization of large scale superconducting quantum information processing. I will first describe efforts to improve qubit-connectivity, an issue in superconducting circuits which typically rely on nearest-neighbor interactions. We implement a random access superconducting quantum information processor using the eigenmodes of a linear array of coupled superconducting resonators, with a Josephson junction transmon circuit coupled to the edge of the array serving as the central processor.  We utilize stimulated vacuum Rabi oscillations induced by frequency modulation of the transmon to perform a universal set of quantum gates on 38 arbitrary pairs of eigenmodes and prepare multimode entangled states, using only two control lines. I will then describe experiments to control the interaction between superconducting circuit elements, while maintaining high coherence. We tune the coupling between a transmon circuit and a resonator using a parallel DC SQUID, while maintaining qubit coherence times exceeding 20 mus over a majority of the tuning range. We use the tunable coupler to implement a scheme to stabilize arbitrary states of the qubit, engineering the qubit bath using a combination of photon non conserving sideband transitions obtained by coupler flux modulation, as well as direct qubit Rabi drives. Lastly I will describe experiments on a capacitively-shunted Fluxonium circuit, in which suppressed decay matrix elements result in photon lifetimes as high as 8 ms. We overcome the commensurate decrease in direct single qubit gate speeds by using Raman transitions in a Lambda system formed with excited circuit energy levels.