Apparatus and method for synchronization of quantum key distribution systems

An apparatus for synchronization by determining a clock offset between a current receiver clock time and a correct receiver clock time according to an embodiment is provided. The apparatus comprises an input interface (110) for receiving information on a received synchronization sequence comprising a sequence of symbols, wherein the information on the received synchronization sequence comprises information for each symbol of a plurality of symbols of the sequence of symbols that said symbol exhibits a first state out of two or more possible states being different from a second state of the two possible states. Moreover, the apparatus comprises an offset analyser (120) for determining the clock offset by determining an analysis result for each of a plurality of levels depending on the received synchronization sequence and depending on a level pattern being associated with said level, and by determining the clock offset using the analysis result of each of the plurality of levels. The level pattern for each of the plurality of levels comprises a sequence of symbols and is different from any other level pattern of the plurality of levels.