In this paper, we compare two solutions (well known and newly proposed) for subcarrier wave quantum key distribution systems that decrease the crucial impact of the errorless unambiguous state discrimination attack in order to ensure security in channels with higher losses. One of the well-known methods is based on adding more signal states and allows a decrease in the probability of successful discrimination. However, sifting fractions will decrease too, leading to lower key rates. Thus, we propose a technique that utilizes additional vacuum state (unmodulated carrier and vacuum states at the sidebands) with low a priori sending probability and that provides the same decrease (in the first order) of errorless unambiguous discrimination as for four signal states while maintaining the same sifting fraction as for two states.
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