**4. Conclusion**

The implementation of tandem AMPM(C)-PM(C)AM schemes of symmetric and asymmetric types and analysis of their advantages and disadvantages will be considered in subsequent publications and are the goal of future work. In this chapter, we demonstrate only the opportunity of its creation, the theoretical justification of their bases and preliminary evaluation of its characteristics. We show that tandem AMPM(C)-PM(C)AM QKD system, based on microwave photonic principles transferred to photon level, can be used as universal frequency encoding system.

The application of such type QKD system will allow us to use multiple levels of cryptographic security, including modulation, commutation schemes and protocol choices, so and choice from re-modulation (re-commutation) and passive detection procedures. The two-time increase of electro-optic modulator number will undoubtedly increase the cost of the system. However, this increasing can be minimized by its universality, and therefore, the expanded functionality, in comparison with each of the known and described by us earlier systems with frequency encoding.

In addition, the high spectral purity and stability of photon tandem modulation based on Il'in-Morozov's method should be highlighted. Qualitatively, we presented the advantages of carrier excluding from quantum communication channel. First, it was shown that effects of nonlinear phase modulation are decreased. Second, in this case, the security condition is stricter to a level of single photon transmission. Third, the signal-to-noise ratio of the system is increased and leads to a significant decrease in the number of errors in the given channel.

For the first time, it was shown the possibility of constructing a nonclassic asymmetric structure using modulators only on the Alice's side and passive filters based on fiber Bragg or arrayed waveguide gratings on the Bob's side.

Therefore, the QKD technology with frequency coding, based on the modulation conversion of an optical carrier with its complete or partial suppressing in the case of tandem amplitude modulation and phase modulation/commutation, is the promising tool for designing perspective quantum communication channels.
