How much can you twist a ultrashort pulse?

If you have a ultrashort pulse, and you want to add angular momentum, you have limitations.

This is what Andrea Ornigotti, Claudio Conti, and Alexander Szameit, discovered and published in the Physical Review Letters. An extended and generalized analysis can be found in Physical Review A.

Angular momentum of light is nowadays largely studied because you can add information to a optical beam by twisting it, or you can rotate objects by lasers with angular momentum. But if you want to transmit information, the best thing to do is using light pulses and adding to any pulse a certain amount of orbital angular momentum (OAM).  For example, by using m levels of OAM, any single pulse can encode m symbols (2 symbols correspond to one bit). The shortest the pulse you use, the higher the number of symbols you can transmit in a second (the transmission rate). This approach can be used for new classical and quantum high-bit rate transmission systems in free space or in fiber. 

But Ornigotti and others find out that the number of OAM bits you can store in a single pulse is actually limited by the duration of the pulse and by its carrier frequency.

The following picture shows the link between OAM units m and the number of optical cycles in the pulse, these two quantized observables are actually strictly related.



These findings have important outcomes in the modern multilevel transmission systems, but also reveal a novel form of spatio-temporal coupling. The latter may lead to new kinds of entanglement, which may trigger applications in Quantum Optics.