If the Bloch sphere is thought of as being embedded in R 3 with its center at the origin and with radius one, then the plane z = 0 (which intersects the Bloch sphere at a great circle; the sphere's equator, as it were) can be thought of as an Argand diagram. Plot point u in this plane — so that in R 3 it has coordinates ( u x , u y , 0 ) .