  Quantum Key Distribution (QKD) with classical Bob has recently been suggested
and proven robust. Following this work, QKD with classical Alice was also
suggested and proven robust. The above protocols are ideal in the sense that
they make use of qubits. However, in the past, well-known QKD protocols that
were proven robust and even proven unconditionally secure, when qubits are
used, were found to be totally insecure when photons are used. This is due to
sensitivity to photon losses (e.g., Bennett's two-state protocol) or
sensitivity to losses combined with multi-photon states (e.g., the
photon-number-splitting attack on the weak-pulse Bennett-Brassard protocol,
BB84). Here we prove that QKD with classical Alice is still robust when photon
losses and even multi-photon states are taken into account.

Quantum Physics