  We report on the microwave characterization of a novel one-dimensional
Josephson metamaterial composed of a chain of asymmetric superconducting
quantum interference devices (SQUIDs) with nearest-neighbor coupling through
common Josephson junctions. This metamaterial demonstrates a strong Kerr
nonlinearity, with a Kerr constant tunable over a wide range, from positive to
negative values, by a magnetic flux threading the SQUIDs. The experimental
results are in good agreement with the theory of nonlinear effects in Josephson
chains. The metamaterial is very promising as an active medium for Josephson
traveling-wave parametric amplifiers; its use facilitates phase matching in a
four-wave mixing process for efficient parametric gain.
