We address the problem of the security of the continuous variable quantum key distribution with the coherent states in the presence of a trusted noise of different origins, e.g. preparation noise at the sender side and the detection noise at the remote receiver side. We show the essential difference between the impact of these types of noise under conditions of lossy quantum channel, available to an eavesdropper. While detection noise reduces the key rate without breaking the security, the preparation noise is destructive for the secure transmission. At the same time we show that by the state purification we can compensate for the destructive effect of the preparation noise, so that upon optimal purification its impact becomes similar to that of the detection noise. The effect is persistent also upon realistic error correction.
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