Bad Qubits — Learn Quantum by Doing

Multiple choice

The Holevo bound states that for every measurement Bob can make on a quantum ensemble {p_x, rho_x}, the accessible information I(X:Y) is bounded by the Holevo quantity chi. What is chi?

chi = -sum_x p_x log2(p_x), the Shannon entropy of the label distribution
chi = S(sum_x p_x rho_x) + sum_x p_x S(rho_x), the sum of the two entropy terms
chi = sum_x p_x S(rho_x) - S(sum_x p_x rho_x), the average entropy minus the entropy of the average
chi = S(sum_x p_x rho_x) - sum_x p_x S(rho_x), the entropy of the average state minus the average of the entropies