I have gone on about alternating tread stairs for years; they take up so much less space than conventional stairs and are safer and more comfortable than they look. This one, by nC2 Architects of Brooklyn is particularly elegant and minimal.
The Architects describe it:
Two custom designed loft beds carefully integrated into the bedrooms of an apartment in a converted industrial building. The alternate tread stair was designed to be a perfect union of functionality, structure and form. With regard to functionality, the stair is comfortable, safe to climb, and spatially efficient; the open sides of the stair provide ample and well-placed grip locations. With regard to structure, the triangular geometry of the tread, riser and stringer allows for the tread and riser to be securely and elegantly fastened to a single, central, very minimal stringer.
Notwithstanding what the architect says and how beautiful it looks, these are still problematic in a lot of jurisdictions, which is a real shame. Even the manufacturer of them, Lapayre Stair, won't sell them to you for residential use and has a whole website page telling you why. Their disclaimer:
It is not possible to turn around on our stair. Nor can two feet be placed on the same level at the same time. It is difficult for children and the elderly to use our stair. In addition, the handrails do not meet the baluster (vertical rail) requirements for residential stairs. Children could easily fall through the rails to the ground below.