eh?
So, why are 4.5 billion qubits needed for Shor-2048?
There's this bit in the paper:
"For a logical error rate pL ≤ 10^-18, d ≥ 32 is required [73]. Therefore, a logical cell requires V = (5d/4)^3 = 40^3 cluster cells. To perform a logical CNOT gate requires a cluster volume 2 × 2 in cross section and two logical cells in temporal depth."
40 ^ 3 = 64,000
64,000 x 2048 = 131,072,000
131,072,000 x 8 = 1,048,576,000
Maybe "d" actually equals 52? (((5*52)/4)^3)*2048*8 = 4,499,456,000
...and that's where I stop making stuff up.