  We follow the evolution of the elementary excitations of the quantum
antiferromagnet TlCuCl3 through the pressure-induced quantum critical point,
which separates a dimer-based quantum disordered phase from a phase of
long-ranged magnetic order. We demonstrate by neutron spectroscopy the
continuous emergence in the weakly ordered state of a low-lying but massive
excitation corresponding to longitudinal fluctuations of the magnetic moment.
This mode is not present in a classical description of ordered magnets, but is
a direct consequence of the quantum critical point.
