  We measure by inelastic neutron scattering the spin excitation spectra as a
function of applied magnetic field in the quantum spin-ladder material
(C5H12N)2CuBr4. Discrete magnon modes at low fields in the quantum disordered
phase and at high fields in the saturated phase contrast sharply with a spinon
continuum at intermediate fields characteristic of the Luttinger-liquid phase.
By tuning the magnetic field, we drive the fractionalization of magnons into
spinons and, in this deconfined regime, observe both commensurate and
incommensurate continua.
