  In this work, the rate-distance limit of continuous variable quantum key
distribution is studied. We find that the excess noise generated on Bob's side
and the method for calculating the excess noise restrict the rate-distance
limit. Then, a realistic rate-distance limit is found. To break the realistic
limit, a method for calculating the secret key rate using pure excess noise is
proposed. The improvement in the rate-distance limit due to a higher
reconciliation efficiency is analyzed. It is found that this improvement is
dependent on the excess noise. From a finite-size analysis, the monotonicity of
the Holevo bound versus the transmission efficiency is studied, and a tighter
rate-distance limit is presented.
