  Flaws in the process of modulation, or encoding of key bits in the
quadratures of the electromagnetic light field, can make continuous-variable
quantum key distribution systems susceptible to leakage of secret information.
Here, we report such a modulation leakage vulnerability in a system that uses
an optical in-phase and quadrature modulator to implement a single sideband
encoding scheme. The leakage arises from the limited suppression of a
quantum-information-carrying sideband during modulation. Based on the results
from a proof-of-concept experiment, we theoretically analyse the impact of this
vulnerability. Our results indicate that the leakage reduces the range over
which a positive secret key can be obtained, and can even lead to a security
breach if not properly taken into account. We also study the effectiveness of
additional trusted noise as a countermeasure to this vulnerability.
