is it possible that a single sword blow?

i really do not think it would be possible for a single blow to have enough energy to cut a body from head to butt, I thought about when chopping wood the effort needed to cut trees which might only be a few inches thick, very often the axe is stopped after a 1/2 inch, so if you think about bones being like wood, then i expect that no matter how strong the assailant and how sharp the sword, the energy needed to chop through flesh and bones over a length of about 18 inches would be beyond even the strongest of men.

No matter how sharp, a Japanese-style sword would lack the mass necessary to maintain momentum and complete the cut.

A huge and heavy beheading sword (a specialist form of broadsword) wielded by a very strong and expert swordsman would have the best chance, but would likely deflect and would not track right down the midline of the spine all the way to the pelvis.

That cut may be popular in comics and movies, but the fiction is not based upon fact.

With a light saber..... With steel? That is asking a lot. Japanese Katana swords will behead a man in one stroke however. There is too much bone & such to go lengthwise.

Nope.

Too much bones in the way.

Not from a human.

Why do you need to know that? :L