A quantum key distribution system based on the subcarrier wave modulation method has been demonstrated which employs the BB84 protocol with a strong reference to generate secure bits at a rate of 16.5 kbit/s with an error of 0.5% over an optical channel of 10 dB loss, and 18 bits/s with an error of 0.75% over 25 dB of channel loss. To the best of our knowledge, these results represent the highest channel loss reported for secure quantum key distribution using the subcarrier wave approach. A passive unidirectional scheme has been used to compensate for the polarization dependence of the phase modulators in the receiver module, which resulted in a high visibility of 98.8%. The system is thus fully insensitive to polarization fluctuations and robust to environmental changes, making the approach promising for use in optical telecommunication networks. Further improvements in secure key rate and transmission distance can be achieved by implementing the decoy states protocol or by optimizing the mean photon number used in line with experimental parameters.

We consider a subcarrier wave quantum key distribution (QKD) system, where quantum encoding is carried out at weak sidebands generated around a coherent optical beam as a result of electro-optical phase modulation. We study security of two protocols, B92 and BB84, against one of the most powerful attacks for this class of systems, the collective beam-splitting attack. Our analysis includes the case of high modulation index, where the sidebands are essentially multimode. We demonstrate numerically and experimentally that a subcarrier wave QKD system with realistic parameters is capable of distributing cryptographic keys over large distances in presence of collective attacks. We also show that BB84 protocol modification with discrimination of only one state in each basis performs not worse than the original BB84 protocol in this class of QKD systems, thus significantly simplifying the development of cryptographic networks using the considered QKD technique.

We theoretically study electro-optic light modulation based on a quantum model where the linear electrooptic effect and the externally applied microwave field result in the interaction between optical cavity modes. The model assumes that the number of interacting modes is finite and effects of the mode overlapping coefficient on the strength of the intermode interaction can be taken into account through dependence of the coupling coefficient on the mode characteristics. We show that, under certain conditions, the model is exactly solvable and can be analyzed using the technique of the Jordan mappings for the su(2) Lie algebra. Analytical results are applied to study effects of light modulation on the frequency dependence of the photon counting rate. In contrast to the limiting case of infinitely large number of interacting modes, when the number of interacting modes is finite, the sideband intensities reveal strongly nonmonotonic behavior supplemented with asymmetry of the intensity distribution provided the pumped mode is not central. We also analyze different regimes of two-modulator transmission and establish the conditions of validity of the semiclassical approximation by applying the methods of polynomially deformed Lie algebras for analysis of the model with quantized microwave field.

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In this paper we report a continuous-variable quantum key distribution protocol using multimode coherent states generated on subcarrier frequencies of the optical spectrum. We propose a coherent detection scheme where power from a carrier wave is used as a local oscillator. We compose a mathematical model of the proposed scheme and perform its security analysis in the finite-size regime using fully quantum asymptotic equipartition property technique. We calculate a lower bound on the secret key rate for the system under the assumption that the quantum channel noise is negligible compared to detector dark counts, and an eavesdropper is restricted to collective attacks. Our calculation shows that the current realistic system implementation would allow distributing secret keys over channels with losses up to 9 dB. Quantum key distribution (QKD) is a method of sharing symmetric cryptographic keys between two parties that is based on encoding information in the states of quantum objects and subsequent distillation of the key through a classic communication channel. The first quantum cryptography protocols exploited the quantum system with degrees of freedoms 1-3. A numerous amount of different techniques for security proofs for discrete variable QKD systems has already been presented 4-13. Experimental implementations of this family of QKD protocols rely on single-photon detectors for quantum state measurements. In turn, continuous-variable QKD (CV-QKD), which was proposed later, relies on methods of coherent detection, homodyne or heterodyne, for gaining information about the quantum states. In other words, single-photon detection is replaced by conventional optical communication methods. However, security proofs for CV-QKD protocols currently remain less advanced 14,15. There are two types of CV protocols that differ by signal modulation method: Gaussian 16,17 , where the complex amplitudes of coherent states are selected randomly from a normal distribution, and discrete modulation (DM) 18-22 with weak coherent phase-coded states. Other CV-QKD protocols are based on two-mode squeezed vacuum states transmission and measurement via homodyne or heterodyne detection 23. Security proofs for Gaussian CV-QKD protocols remain the most developed: they were presented against general attacks in the finite key regime using several different approaches 24. Security analysis for CV-QKD protocol with two-mode squeezed vacuum states was also performed 25,26. Discrete-variable CV-QKD protocols possess several important advantages; among those are relative implementation simplicity and a possibility to minimize the number of parameters that need to be monitored. Nevertheless, security proofs for discrete-modulation CV-QKD systems require special consideration. In the asymptotic limit, its security has been proven against collective attacks 27. Recently it was shown that security proof for CV-QKD with discrete modulation against general attacks is possible 27. Here we propose an implementation of CV-QKD protocol based on subcarrier wa...