  Soliton molecules, bound states composed of interacting fundamental solitons,
exhibit remarkable resemblance with chemical compounds and phenomena in quantum
mechanics. Whereas optical molecules composed of two or more temporally locked
solitons have been observed in a variety of platforms, soliton molecules formed
by bound solitons at different frequencies have only recently been
theoretically proposed. Here, we report the observation of polychromatic
soliton molecules within a mode-locked laser cavity, achieving the desired
dispersion by implementing a spectral pulse-shaper. This system supports two or
more coincident solitons with different frequencies, but common group
velocities. Our results open new directions of exploration in nonlinear
dynamics within systems with complex dispersion and offer a convenient platform
for optical analogies to mutual trapping and spectral tunneling in quantum
mechanics.
