  Coherence of superconducting qubits can be improved by implementing designs
that protect the parity of Cooper pairs on superconducting islands. Here, we
introduce a parity-protected qubit based on voltage-controlled semiconductor
nanowire Josephson junctions, taking advantage of the higher harmonic content
in the energy-phase relation of few-channel junctions. A symmetric
interferometer formed by two such junctions, gate-tuned into balance and
frustrated by a half-quantum of applied flux, yields a cos(2{\phi}) Josephson
element, reflecting coherent transport of pairs of Cooper pairs. We demonstrate
that relaxation of the qubit can be suppressed tenfold by tuning into the
protected regime.
