  We study eavesdropping in quantum key distribution with the six state
protocol,when the signal states are mixed with white noise. This situation may
arise either when Alice deliberately adds noise to the signal states before
they leave her lab, or in a realistic scenario where Eve cannot replace the
noisy quantum channel by a noiseless one. We find Eve's optimal mutual
information with Alice, for individual attacks, as a function of the qubit
error rate. Our result is that added quantum noise can make quantum key
distribution more robust against eavesdropping.
