Optical communication systems using quantum state control have received much attentions. For this quantum state control communication, however, one of the most serious problems is the effect of transmission loss which degrades the advantage of the quantum state controlled signal. In optical long transmission communications, in general, transmission loss is large so that the advantage using quantum state controlled signal is hardly lost and performance of quantum state control communications is almost equal to that of conventional communications. So it is necessaty for a new application of quantum state control to overcome the standard quantum limitation which is achieved by the conventional coherent communication system. In this paper, we propose a realization of a new optical communication system with received quantum state controller, so called Received Quantum State Control (RQSC) system, in order to cope with transmission loss problem. This system can provide the advantage of quantum state control regardless of amount of transmission loss. Particularly, it is shown that the system can overcome the standard quantum limitation which corresponds to the limitation achieved by conventional BPSK homodyne system using coherent state signal. This corresponds to Quantum Receiver" of the broad sense in Helstrom's receiver theory.

For optical digital communications with energy loss, it is shown that there is a state which achieves much superior bit error rate than the Glauber coherent state even if the signal to quantum noise ratio is the same. Moreover, the lower bound for bit error rate is obtained.

The problem of specifying the optimum receiver in quantum detection theory is considered for application to optical communications. The quantum minimax rule is formulated and a necessary and sufficient condition for it is given. This rule provides a powerful calculation method in obtaining the quantum optimum solution. Several examples are worked out to prove superiority of the minimax strategy in practical problems. For the case of ternary coherent signals, in particular, is obtained an explicit equation of error probability. Furthermore the minimax rule gives a good penetrating calculation method to obtain suboptimum receivers in case accompanied by an additive external noise.

In this letter, it is clarified that the quantum noise properties of the linear amplification and locking amplification in the injection locked laser process are different. The noise property of the locking amplification is newly given.

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.

The advantage of nonstandard quantum states such as two-photon coherent state (or squeezed states) and photon number state as transmitter state is strongly degraded by transmission loss in quantum communications. To cope with such a problem, a new application of these states is proposed, and it is shown that its system has infinite capacity.

The problems for the controllability of the quantum state are considered for application to optical communications. The new optimization problems arise in the quantum communication theory in addition to problems considered up to now. The concrete example of the controllability problems is Two-photon coherent state (TCS) generated by Two-photon laser. The optimum conditions for the parameters of TCS, which minimize the error probability, are given when TCS is employed as the transmission state in the quantum optimum receiver of the binary system, and its numerical behavior is shown. Furthermore the noise behavior associated with the TCS in the several optical receivers are shown. As the result, the ultimate noise limit hitherto can be reduced by using TCS in the optical communication systems. The saving power, when we employ the TCS as the transmission state in the binary communication system, is about 3 dB in the direct photo detector, and it is 5 dB in the optical homodyne receiver. Furthermore the saving power in the quantum optimum receiver is 4 dB. The SNR of the direct photo detector is improved by more than 6 dB. The information performance of the homo dyne is superior to the heterodyne when TCS is employed, while the heterodyne is superior to the homodyne in general when an ordinary coherent state is employed. Thus, it is suggested for reduction of the noise problems that the Two-photon laser which will be realized in the future will convey the significant benefits to the optical communications.

It is well known that energy loss brings a serious problem in the optical information transmision systems with the quantum state control at transmitter. This letter presents proposal of a new system for the application of the quantum state control, which copes with the energy attenuation problem, and shows the numerical properties of performances.

We show the general formula of the signal to noise ratio in chains with amplifiers and attenuators using nonclassical properties of light. From the results, we show that quantum noise can be effectively controlled. For the application we show that the optical fiber transmission system with conventional optical amplifiers can be designed as if the noiseless optical amplifiers are used.

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.

The noise properties of directly modulated laser diode influenced by reflected waves are given theoretically. Firstly, the quantum noise enhancement effect due to direct modulation is found theoretically in term of product between the quantum shot noise and the modulation current. Secondly, the high-frequency intensity noise due to reflected waves (often so-called self-coupling effect) is clarified to be due to resonance of the quantum shot noise to the external cavity. According to our theory, it is found that the noise of the narrow stripe laser diode with the stripe width comparable to the diffusion length of carrier is about 10 dB smaller than that of the wide stripe laser. And also, the reverse isolation loss of an optical isolator is estimated theoretically to be 30 dB to reduce the relative high-frequency intensity noise by 40 dB.

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.

In optical coherent communications with homodyne detection system, it is one of the most essential problems to synchronize the phase of local oscillator with that of received signal on the receiver. In general, the phase-locking performance is limited by frequency fluctuation and quantum noise of light source. So far some optical phase-locked loops have been proposed in order to optimize phase-locking performance. Their main purpose is to minimize phase error variance of systems with frequency fluctuation and quantum noise transformed into the electric region. To improve the phase-locking performance under the same situations, this paper proposes a new optical phase-locked loop with received quantum state controller, called a Squeezed-PLL, which can reduce the impact of quantum noise in the optical region. This system can eliminate the effect of the vacuum noise due to the beam splitting. Finally, the general signal to noise ratio of data-branch is shown, and phase-locking performance of the Squeezed-PLL is verified by computer simulation.

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.

This article briefly reviews the elementary concepts of the quantum communication theory. Specific topics on some of the optical communications by utilizing quantum nature of light wave will be discussed, especially for applications to information transmission of classes of squeezed state exceeding the present limitations, and for quantum coding theory relevant the photon communications.

A study on the limitation of optical communication systems has received much attention. A method to overcome the standard quantum limit is to apply non-standard quantum state, especially squeezed state. However, the advantage of the non-standard quantum state is degraded by the transmission energy loss. To cope with this problem, we have proposed a concept of the received quantum state control (RQSC), but the realization has some difficulties. In this paper, we propose a new system to realize the received quantum state control system, employing injection locked laser (ILL) system. Then we show that our new system can overcome the standard quantum limit.