Figure 4. Protocol where Alice and Bob each use n MUBs.

The key rate is plotted for various n∈{2, 3, 4, 5, 6} and for d_A=d_B=5. This is an unstructured protocol, since for intermediate values of n the symmetry group and hence the key rate is unknown. However, our numerics provides the dependence of key rate on error rate for any n, as shown. The inset shows the error tolerance—the smallest error rate that makes the key rate vanish—as a function of n. Note that the largest jump in the error tolerance occurs from n=2 to n=3.