We show some numerical results of computer simulations of secret key reconciliation (SKR) protocol "Cascade" and clarify its properties. By using these properties, we propose to improve the protocol performance on the number of publicly exchanged bits which should be as few as possible.

In this paper, we analyze a photodetection process of new kind theoretically, which transforms a coherent state of light so as to lead to nonstandard property, namely, sub-Poissonian distribution of its output photoelectron during its photodetection process. The properties of the photoelectron distribution are studied used as preamplifiers of both direct-detection and homodyne detection cases.

In this paper, we propose to adopt the asymmetric error correcting code for photon communication systems. The asymmetric error correcting code is an binary code correcting only 10 type transition errors (asymmetric errors). We show the following advantages obtained by employing the asymmetric error correcting code:(i) the codeword error probability is smaller than that of the symmetric error correcting code. (ii) the information rate per photon is larger than that of the symmetric error correcting code. Moreover, for asymmetric error correcting codes, we obtain the lower bounds on the codeword error probability and the upper bounds on the information rate per photon. By using these bounds, we can show that some asymmetric error correcting codes are optimum for these criteria.

This letter clarifies properties of system using squeezed state signal in photon channel with energy loss. It is found that error rate of squeezed state system is always superior to that of coherent state, and that reduction of optimum squeezing parameter is allowed by optimization taking into account loss in comparison with optimization of transmitter squeezed state.

It is shown that error correcting code improves an essential perfomance limitation of photon communications with energy loss. The coded photon signals allow us the loss about 13 dB to keep the advantage of photon number state signals while uncoded one is about 7 dB. Furthermore the necessity of weight distribution control of code words is discussed.

This letter presents the theoretical analysis of error properties of coded modulation using several quantum state lights. The direct modulation of general code into light pulse has advantage in the case of the photon state light, while PPM is superior in the case of coherent state light.

For the quantum state control communication, one of the most serious problems is the effect of transmission loss which degrades the advantage of the quantum state controlled signal. In this paper, we investigate loss effect for the squeezed state signal, which is one of typical quantum state controlled signals. First, it is shown that when the squeezed state signal optimized regardless of the effect of loss is used as transmitter state in the communication system with energy loss, the signal to noise ratio of the received signal is higher than that of the coherent state signal with the same transmitted photon number when and only when the loss is less than 3 dB. Then this paper gives an optimum condition of the squeezed state signal for lossy channel. This optimum condition provides higher signal to noise ratio of the received signal than that of coherent state signal for any degree of loss. Furthermore, we compare the performance of the quantum coherent communication systems using the optimum squeezed state signal with that of the photon communication using the photon number state signal taking the effect of loss into account.

This paper clarifies properties of the quantum state control communication systems such as the quantum coherent communication systems (QCCS) and the photon communication systems (PCS). We compare properties of these two systems in the case of uncoded and coded schemes. In the former case, the energy-information efficiencies of both systems are given, taking into quantum state control account, and the Fano factor of PCS which corresponds to the same performance to the ideal QCCS is given. In the latter case, the reliability functions of both systems are considered. As a result, it is shown that effects of error correcting code in the PCS are much larger than that in the QCCS.

This letter clarifies properties of cutoff rate R0 of photon channels using coherent state and photon number state as quantum state of light. The relation between cutoff rate improvement and energy saving rate is found, when one uses the number state instead of coherent state.

In this paper, we analyze photon distributions of squeezed state from a viewpoint of quantum state control communications (QSCC). Especially, the effect of the channel loss on these properties are shown in transmitted quantum state control (TQSC). As a result, we show the general property of the photon distribution of squeezed state suffering the loss effect in channel with respect to the transparence efficiency. Although the photon distribution of squeezed state suffering the loss effect approaches the Poissonian one, the photon distribution on the way becomes a remarkable one in some region of a squeezing parameter. Then we show optimum conditions of squeezed state based on Signal to noise ratio (SNR) in the photon counting system. It is found that the degree of the squeezing to minimize error probability is much smaller than that for SNR. Moreover, we compare properties of the photon counting system using a squeezer as a preamplifier with those using a general linear optical amplifier.