**2.3. Idea of physical safety**

This section mainly describes an example of optical intensity modulation that is generally used in optical communication. A semiconductor laser diode used in general optical communication uses high-output continuous light (coherent light). The generation probability of each photon that forms this coherent light in the phase and optical power directions is unstable during measurement due to quantum phenomenon, and the value of the probability is not uniquely determined. The distribution of this undefined value is handled as a part of shot noise that is generated during the photoelectric conversion in optical communication, which is an element that degrades the receiving sensitivity. However, effects of other classical noise (including thermal noise of receiving amplifier) generated in the receiver become dominant to the receiving sensitivity of optical communication using the normal optical intensity modulation system. Normal receivers of the devised optical modulation system in Y-00 can maintain little quantum noise effect as in the case of conventional optical communication system. Also Y-00 produces an effect of significantly degrading the receiving sensitivity (to an unreceivable level) for eavesdroppers. In other words, is established safety by great difference of Signal-to-Noiseratio (S/N) between at normal receiver and eavesdropper. In this case, safety will be improved by as close as possible to 0.5 the error rate of eavesdroppers. Measured value of quantum noise that affects optical communication varies at completely random against the phase value and power value of light as described in section 2.2. Therefore, the phase modulation system and intensity modulation system used in the existing optical communication can be applied to the Y-00 encryption system that uses this quantum noise. The following describes basic idea to establish safety.

1. Multi-value modulation

6 Optical Communication

complexity.

**Figure 1.** Image of the safety of Y-00

**2.3. Idea of physical safety** 

to get the initial key from acquired sample data like fast correlation attack that makes exhaustive search. In the exhaustive search, phenomena cannot be copied correctly due to effects of physical phenomena. It is disabling parallel processing [19]. Therefore, eavesdropper must be stacked in serial to find correlation of data. In addition, the sample data volume required for decryption is an order of 1E20~1E30 bytes or more by implementing the safety enhancing measures described later. That memory capacity to store the sample data are 1E20~1E30 bytes or more and the sample data acquisition time is several tens of millions to several hundred millions of years (at 10Gbps transmission) even if effects of physical phenomena cannot be diffused over the signal area as described above. This is limited and that is considered to be indecipherable safety. Because it does not depend on computer's performance unlike the present cryptography that pursues mathematical

This section mainly describes an example of optical intensity modulation that is generally used in optical communication. A semiconductor laser diode used in general optical communication uses high-output continuous light (coherent light). The generation probability of each photon that forms this coherent light in the phase and optical power directions is unstable during measurement due to quantum phenomenon, and the value of the probability is not uniquely determined. The distribution of this undefined value is handled as a part of shot noise that is generated during the photoelectric conversion in optical communication, which is an element that degrades the receiving sensitivity. However, effects of other classical noise (including thermal noise of receiving amplifier) generated in the receiver become dominant to the receiving sensitivity of optical The normal optical communication performs a multiple value transfer to increase transmission capacity. However in Y-00, only a single bit out of multiplexed values is used for transmitting information and other values are dummy information for eavesdroppers. It is important for multi-values to establish safety that the quantum noise distribution sufficiently overlaps between adjacent levels (both in optical intensity and phase). The number of values becomes several thousands or more depending on conditions.

2. Encryption and decryption

In the encryption by a transmitter, a combination of binary data (1-bit "1" or "0" level value) is selected for each bit of transmit data from multiple signal values created under conditions (1) using the initial key by the multi-value selection information. This selected binary combination is called base in the same way as phase modulation. The amplitude of this base (between two values) determines the receiving sensitivity of normal receivers in the case of intensity modulation. Therefore, 1/2 (180 degrees for phase modulation) of the maximum signal amplitude is the best value for normal receivers to obtain the optimum receiving sensitivity (Figure2). Decryption process of receiver is essential to distinguish the base that varies in each bit by the best threshold value at signal reception. The amplitude of "1" and "0" levels during reception is estimated based on the multi-value selection information generated by the initial key shared with the transmitter and the threshold value is momentarily moved to the best point to distinguish "1" level and "0" level of the signal. Synchronization of multi-level selection information is critical at this time between the transmitter and receiver. This information is changed at random in each bit between transmitter and receiver (Figure2) [17,18,20-22].

3. Tapping

People other than those who are engaged in optical communication believe in many cases that optical fiber does not be able to tapping unlike electric wires. The principle of optical fiber transmission is well known. Light travels in an optical fiber while repeating reflection using the refraction of light generated by junction of the core and clad in the optical fiber.

**Figure 2.** Signal basis and quantum noise

This reflectance varies by bending the optical fiber. If the optical fiber is bent at a sharp angle in particular, the refractive index of the core and clad extremely changes. Therefore optical signal not be able to total reflection. Part of the optical signal will leak out for that. Signal monitoring equipment that uses this principle has been commercialized and used as a measuring instrument. Tapping data from optical fibers has become relatively easy at present due to the technical advance (including high-speed, high-sensitivity detector and low-noise optical amplifier) in optical communication as shown in this example. Measures for improving safety to independently protect transmission paths have become imperative for background mentioned above (Figure3) [22,23].

**Figure 3.** Eavesdropping from optical fiber
