We consider the security of practical continuous-variable quantum keydistribution implementation with the local oscillator (LO) fluctuating in time,which opens a loophole for Eve to intercept the secret key. We show that Evecan simulate this fluctuation to hide her Gaussian collective attack byreducing the intensity of the LO. Numerical simulations demonstrate that, ifBob does not monitor the LO intensity and does not scale his measurements withthe instantaneous intensity values of LO, the secret key rate will becompromised severely.
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