  The idea of confinement states that in certain systems constituent particles
can be discerned only indirectly being bound by an interaction whose strength
increases with increasing particle separation. Though the most famous example
is the confinement of quarks to form baryons and mesons in (3+1)-dimensional
Quantum Chromodynamics, confinement can also be realized in the systems of
condensed matter physics such as spin-ladders which consist of two spin-1/2
antiferromagnetic chains coupled together by spin exchange interactions.
Excitations of individual chains (spinons) carrying spin S=1/2, are confined
even by an infinitesimal interchain coupling. The realizations studied so far
cannot illustrate this process due to the large strength of their interchain
coupling which leaves no energy window for the spinon excitations of individual
chains. Here we present neutron scattering experiments for a weakly-coupled
ladder material. At high energies the spectral function approaches that of
individual chains; at low energies it is dominated by spin 0,1 excitations of
strongly-coupled chains.
