  We introduce a new class of nondiffracting optical pulses possessing orbital
angular momentum. By generalizing the X-waves solution of the Maxwell equation,
we discover the coupling between angular momentum and the temporal degrees of
freedom of ultra-short pulses. The spatial twist of propagation invariant light
pulse turns out to be directly related to the number of optical cycles. Our
results may trigger the development on novel multi-level classical and quantum
transmission channels free of dispersion and diffraction, may also find
application in the manipulation of nano-structured objects by ultra-short
pulses, and for novel approaches to the spatio-temporal measurements in
ultrafast photonics.
