  We present a protected superconducting qubit based on an effective circuit
element that only allows pairs of Cooper pairs to tunnel. These dynamics give
rise to a nearly degenerate ground state manifold indexed by the parity of
tunneled Cooper pairs. We show that, when the circuit element is shunted by a
large capacitance, this manifold can be used as a logical qubit that we expect
to be insensitive to multiple relaxation and dephasing mechanisms.
