  Manipulating at will the propagation dynamics of high power laser pulses is a
long-standing dream whose accomplishment would lead to the control of a
plethora of fascinating physical phenomena emerging from laser-matter
interaction. The present work represents a significant step towards such an
ideal control by manipulating the nonlinear optical properties of the gas
medium at the quantum level. This is accomplished by engineering the intense
laser pulse experiencing filamentation at the subcycle level with a relatively
weak (about 1%) third-harmonic radiation. The control results from quantum
interferences between a single and a two-color (mixing the fundamental
frequency with its 3rd harmonic) ionization channel. This mechanism, which
depends on the relative phase between the two electric fields, is responsible
for wide refractive index modifications in relation with significant
enhancement or suppression of the ionization rate. As a first application, we
demonstrate the production and control of an axially modulated plasma channel
that could be used for quasi-phase matched laser wakefield acceleration.
