We introduce a superconducting qubit architecture that combines high-coherence qubits and tunable qubit-qubit coupling. With the ability to set the coupling to zero, we demonstrate that this architecture is protected from the frequency crowding probl
ems that arise from fixed coupling. More importantly, the coupling can be tuned dynamically with nanosecond resolution, making this architecture a versatile platform with applications ranging from quantum logic gates to quantum simulation. We illustrate the advantages of dynamic coupling by implementing a novel adiabatic controlled-Z gate, at a speed approaching that of single-qubit gates. Integrating coherence and scalable control, our gmon architecture is a promising path towards large-scale quantum computation and simulation.
We performed a search for the decay $K_L^0 rightarrow 3gamma$ with the E391a detector at KEK. In the data accumulated in 2005, no event was observed in the signal region. Based on the assumption of $K_L^0 rightarrow 3gamma$ proceeding via parity-viol
ation, we obtained the single event sensitivity to be $(3.23pm0.14)times10^{-8}$, and set an upper limit on the branching ratio to be $7.4times10^{-8}$ at the 90% confidence level. This is a factor of 3.2 improvement compared to the previous results. The results of $K_L^0 rightarrow 3gamma$ proceeding via parity-conservation were also presented in this paper.
We performed a search for a light pseudoscalar particle $X$ in the decay $K_L^0->pi0pi0X$, $X->gammagamma$ with the E391a detector at KEK. Such a particle with a mass of 214.3 MeV/$c^2$ was suggested by the HyperCP experiment. We found no evidence fo
r $X$ and set an upper limit on the product branching ratio for $K_L^0->pi0pi0X$, $X->gammagamma$ of $2.4 times 10^{-7}$ at the 90% confidence level. Upper limits on the branching ratios in the mass region of $X$ from 194.3 to 219.3 MeV/$c^2$ are also presented.
