Optical secure communication system

Abstract

[Problem] To provide an optical secure communication system with higher safety by carrying a QNSC signal using a coherent Hermite-Gaussian pulse, multiplexing the QNSC signal using coherent Hermite-Gaussian pulses of different orders, and adding a masking effect in a time domain. [Solution] An optical secure communication system for a quantum noise stream cipher (QNSC) signal for masking the phase and / or the amplitude level of an optical signal using quantum noise, wherein carrier light in a transmission unit and locally oscillated light in a reception unit of the QNSC signal comprise a coherent Hermite-Gaussian pulse with an amplitude given by an Hermite-Gaussian function.

Description

Optical Secure Communication System

[0001] The present invention relates to an optical secure communication system.

[0002] In recent years, Internet-based businesses have rapidly expanded, and optical communication networks are increasingly being used for transmitting personal and confidential information. In this context, ensuring the security of information is becoming increasingly important as optical communication networks become faster and more powerful. Two well-known cryptographic techniques for secure optical communication are quantum key distribution (QKD) and quantum noise stream cipher (QNSC). The former is said to be able to unconditionally distribute a common key used in stream cipher transmissions securely by transmitting it using single photons or weak coherent light (see, for example, Non-Patent Documents 1 and 2). However, this requires the recipient to receive a disposable common key of the same length as the original message, which limits the speed of encrypted communication to the common key distribution speed (several hundred kbps). Furthermore, the transmission distance is short, at approximately 100 km.

[0003] In contrast, QNSC is expected to be a common-key optical cryptosystem that can be put to practical use on current optical networks and realize high-speed, long-distance communications at several tens of Gbit / s (see, for example, Non-Patent Documents 3, 4, 5, and Patent Documents 1, 2, and 3). QNSC uses pseudorandom numbers generated based on a common key to multilevel-modulate the phase, amplitude, or both of an optical signal, embedding data information in the optical phase or amplitude fluctuations (called quantum noise) to prevent eavesdroppers from accurately receiving the optical signal. This method increases the multilevel modulation level, making the interval between changes in the amplitude, phase, or both of the modulated optical signal sufficiently smaller than the quantum noise imparted during optical detection. This reduces the probability that an eavesdropper can correctly determine the signal level, thereby increasing security to a practically acceptable level.

[0004] The inventors of the present invention have developed a method for generating an optical pulse having a Hermite-Gaussian function as its pulse shape (hereinafter referred to as a Hermite-Gaussian pulse) (see Patent Application No. 2023-037273).

[0005] C. H. Bennett and G. Brassard, “Quantum cryptography: Public key distribution and coin tossing,” Proc. IEEE Int. Conf. Computers, Systems and Signal Processing, 1984, pp. 175-179; T. Hirano, H. Yamanaka, M. Ashikaga, T. Konishi, and R. Namiki, “Quantum cryptography using pulsed homodyne detection,” Phys. Rev., 2003, A 68, 042331; G. A. Barbosa, E. Corndorf, P. Kumar, H. P. Yuen, “Secure communication using mesoscopic coherent state,” Phys. Rev. Lett., 2003, vol. 90, 227901; O. Hirota, “Optical communication networks and quantum cryptography,” IEICE Transactions on Electronics, Information and Communication Engineers, Vol. B, 2004, vol. J87-B, pp. 478-486; M. Nakazawa, M. Yoshida, T. Hirooka, and K. Kasai, “QAM quantum stream cipher using digital coherent optical transmission”, Opt. Express, 2014, vol. 22, p. 4098-4107

[0006] JP 2006-303927 A JP 2014-093764 A JP 2017-050678 A

[0007] Figure 4(a) and (b) show block diagrams of the transmitter and receiver of a previously reported QNSC transmission system, respectively, in which one-dimensional amplitude (or intensity) modulation is used as the modulation signal.

[0008] In commonly used optical secure communication methods, the original binary data (source data) to be transmitted and a random number sequence generated by a random number generation circuit based on common key information are mathematically encrypted by performing an exclusive OR operation. The encrypted signal is then encoded into a predetermined format and transmitted via optical fiber. In this case, an eavesdropper could potentially decrypt the original data by, for example, inflicting bending loss on the optical fiber transmission line, detecting the 4-level amplitude modulation signal with high sensitivity from the light leaking into the cladding, and searching for the information in the random number sequence through a brute force method.

[0009] In contrast, to avoid this risk, the QNSC transmitter shown in FIG. 4( a) distributes common key information to a random number generation circuit 52 and a random number sequence generation circuit 53 via a key distribution circuit 51. The random number generation circuit 52 performs the aforementioned processing on a portion of the distributed common key information to generate random number sequence #1. The random number sequence generation circuit 53 generates random number sequence #2 based on the remaining distributed common key information. An exclusive-OR circuit 54 mathematically encrypts the original data and the random number sequence #1 by performing an exclusive-OR operation to obtain a 4-level amplitude modulation signal. This signal is then added to the random number sequence #2 by an adder circuit 55, and further converted (encrypted) into a high-level amplitude modulation format (16-level amplitude modulation in FIG. 4( a)) by an encoder 56. The encrypted signal is then converted into an analog signal by a D / A converter 57, and optically modulated by an optical modulator 59 using a laser from a CW light source 58. Furthermore, quantum noise emitted from a quantum noise source 60 is added to the modulated signal by an optical multiplexer 61 for masking, and the signal is transmitted as a QNSC signal. In other words, the security is enhanced by using not only a general mathematical encryption but also a physical encryption using optical noise. In other words, the use of this software (mathematical) encryption and hardware (physical) encryption in combination is a major difference from conventional methods.

[0010] On the other hand, the QNSC receiver (authorized receiver) shown in Figure 4(b) shares the aforementioned common key and the random number sequences #1 and #2 generated by the random number sequence generation circuits 52 and 53 with the transmitter in advance, and uses these to decrypt the original data. Specifically, the received QNSC signal is received by a photodetector 62 and converted to a digital signal by an A / D converter 63. The resulting 16-level amplitude modulated signal is then decoded to a 4-level amplitude modulated signal by a decoder 64 and a subtraction circuit 65, using the random number sequence #2 derived from the common key, performing the exact opposite operation of the transmitter. The original data is then restored by performing an exclusive OR operation in an exclusive OR circuit 66 using the random number sequence #1 derived from the common key. Since excessive optical noise added by the transmitter can cause errors in threshold determination of the original data, it is important to add optical noise within a range that does not cause such errors.

[0011] As shown in Figures 4(a) and (b), the QNSC method adjusts the level of optical noise used for masking so that the authorized receiver can demodulate the original data without error, but it is not possible to cover the entire transmitted QNSC signal. Therefore, there is a possibility that part of the information in random number sequence #2 contained in the QNSC signal will be leaked to an eavesdropper while remaining encrypted. Although this alone cannot be decrypted because there is no common key, there is a risk that the pattern of random number sequence #2 can be analyzed by analyzing correlations based on the leaked signal.

[0012] The present invention has been made in light of these problems, and aims to provide a more secure optical secure communications system by carrying a QNSC signal using a coherent Hermite-Gaussian pulse and multiplexing it with coherent Hermite-Gaussian pulses of different orders to add a masking effect in the time domain.

[0013] To achieve the above object, the present invention provides a secure optical communications system for a QNSC signal that uses quantum noise to mask at least one of the phase and amplitude level of the optical signal, characterized in that the carrier light at the transmitter and the local light at the receiver of the QNSC signal consist of coherent Hermite-Gaussian pulses whose amplitudes are given by a Hermite-Gaussian function.

[0014] In the optical secure communications system according to the present invention, the coherent Hermite-Gaussian pulse is preferably formed by generating N coherent Hermite-Gaussian pulses of orders 0 to (N-1) in a time slot for one symbol and multiplexing them.

[0015] In this case, the transmitter may carry the QNSC signal in one m-th order coherent Hermite-Gaussian pulse of the N coherent Hermite-Gaussian pulses, and carry a time series of dummy signals in the remaining N−1 coherent Hermite-Gaussian pulses, and the receiver may use the m-th order coherent Hermite-Gaussian pulse as the local light to perform homodyne detection of the signal light multiplexed with the N coherent Hermite-Gaussian pulses, and integrate the detected signal over a time slot for one symbol, thereby demultiplexing the m-th order coherent Hermite-Gaussian pulse from N coherent Hermite-Gaussian pulses multiplexed in the time domain by utilizing the orthogonality of the Hermite-Gaussian pulses, and demodulate the QNSC signal. Alternatively, the transmitter may use any n (n≦N) coherent Hermite-Gaussian pulses out of the N coherent Hermite-Gaussian pulses to carry QNSC signals for n channels, and use the remaining N−n coherent Hermite-Gaussian pulses to carry a time series of dummy signals, and the receiver may use the n coherent Hermite-Gaussian pulses as the local light, perform homodyne detection on the coherent Hermite-Gaussian pulse signal light carrying the QNSC signals for the n channels, using the local light, and integrate the detected n detection signals individually over a time slot for one symbol, thereby demultiplexing coherent Hermite-Gaussian pulses of an order equal to that of the local light and demodulating the QNSC signals for the n channels.

[0016] The optical secure communication system according to the present invention preferably uses any one of optical intensity modulation, optical phase modulation, and quadrature amplitude modulation.

[0017] Furthermore, the optical secure communications system according to the present invention may use a secret key securely distributed using quantum key distribution technology as the common key used for encryption, thereby further enhancing security.

[0018] In addition, in the optical secure communications system according to the present invention, the transmitter and receiver may each have a polarization multiplexing circuit and a polarization demultiplexing circuit, and may perform transmission using a polarization multiplexing transmission method, thereby doubling the transmission capacity.

[0019] In addition, in the optical secure communications system according to the present invention, the transmitter and receiver may each have a wavelength division multiplexing circuit and a wavelength division demultiplexing circuit, and may perform wavelength division multiplexing transmission using a multi-wavelength light source, thereby increasing the transmission capacity.

[0020] Furthermore, in the optical secure communications system according to the present invention, the transmitter and receiver may each have a space division multiplexing circuit and a space division demultiplexing circuit, and may perform space division multiplexing transmission using a multicore fiber or a multimode fiber as the optical transmission path, thereby enabling an increase in transmission capacity.

[0021] According to the present invention, it is possible to provide a more secure optical secure communications system by utilizing the time domain masking effect caused by multiplexing high-order coherent Hermite-Gaussian pulses.

[0022] 1 is an explanatory diagram showing a time domain masking effect and modulation / demodulation state using multiplexing of Hermite-Gaussian pulses of orders m = 0 to 3 in the optical secure communication system according to the present invention. FIG. 2 is a block diagram showing (a) a transmitter unit and (b) a receiver unit of a first embodiment of the optical secure communication system according to the present invention. FIG. 3 is a block diagram showing (a) a transmitter unit and (b) a receiver unit of a second embodiment of the optical secure communication system according to the present invention. FIG. 4 is a block diagram showing (a) a transmitter unit and (b) a receiver unit of a QNSC transmission system using a conventional amplitude modulation method.

[0023] Hereinafter, an embodiment of the present invention will be described with reference to the drawings, etc. The embodiment of the present invention is a secure optical communications system for QNSC signals that uses quantum noise to mask at least one of the phase and amplitude level of an optical signal, and the carrier light at the transmitter and the local light at the receiver of the QNSC signal are made of coherent Hermite-Gaussian pulses whose amplitudes are given by a Hermite-Gaussian function.

[0024] The Hermite-Gaussian function used in the optical secure communication system according to the embodiment of the present invention is given by the following equation (1).

[0025] Here, m is the order of the Hermite-Gaussian function, and T is a parameter representing the pulse width. m (t) is a function called a Hermite function, and is defined by the following formula (2) when m is an even order, and by the following formula (3) when m is an odd order.

[0026]

[0027] Specifically, when m = 0 to 3, it is defined by the following formulas (4-1) to (4-4).

[0028] The Hermite-Gaussian function of equation (1) is an orthogonal function, and has the property that the following equation (5) is satisfied between Hermite-Gaussian functions of different orders.

[0029]

[0030] Figure 1 illustrates optical modulation and demodulation using coherent Hermite-Gaussian pulses of orders m = 0 to 3. As an example, consider the case where true data (I1 + jQ1) is optically modulated onto a first-order coherent Hermite-Gaussian pulse and dummy data is optically modulated onto other orders of coherent Hermite-Gaussian pulses. When these are multiplexed in the same time slot, the time waveform of the multiplexed signal becomes trapezoidal, within which the first-order coherent Hermite-Gaussian pulse carrying the true data is hidden. By using such multiplexing of coherent Hermite-Gaussian pulses, the masking effect in the time domain makes it difficult for an eavesdropper to obtain any information regarding the true data. Meanwhile, the authorized receiver already shares the order information of the coherent Hermite-Gaussian pulse carrying the true data and can demodulate the true data by utilizing the orthogonality of the Hermite-Gaussian pulses shown in Equation (5). Specifically, the multiplexed signal of the coherent Hermite-Gaussian pulse is subjected to homodyne detection (corresponding to multiplication) with the first-order coherent Hermite-Gaussian pulse, and then the multiplexed signal is transmitted over a time slot (-T S / 2 < t < T S By integrating over the range of σ / 2, the true data is obtained as shown in equation (6) below.

[0031]

[0032] Note that the integration range of equation (6) is not -∞ < t < ∞ as shown in equation (5). However, as shown in the time waveform in Figure 1, S / 2 < t < T S When the bases of the Hermite-Gaussian pulses of each order are sufficiently attenuated within the range of t / 2, the integration range of equation (6) can be regarded as -∞< t<∞. Also, although an example of optically modulating true data onto a first-order coherent Hermite-Gaussian pulse has been shown here, true data may be optically modulated onto a coherent Hermite-Gaussian pulse of any order.

[0033] Two embodiments of the optical secure communication system utilizing the masking effect in the time domain due to the multiplexing of coherent Hermite-Gaussian pulses described above are shown below. Figure 2 shows block diagrams of (a) a transmitter and (b) a receiver of the optical secure communication system according to the first embodiment of the present invention.

[0034] As shown in FIG. 2( a), the transmission unit includes a key distribution circuit 2 that distributes a common key 1, an encryption circuit 5 that converts true data 3 into a random high-multilevel signal, N−1 encryption circuits 5′ that convert N−1 channels of dummy data 4 into random high-multilevel signals, a random number sequence generation circuit 6 that generates a random number sequence based on the common key 1 distributed from the key distribution circuit 2, a selector 7 that switches the paths of N signals based on the random number sequence output from the random number sequence generation circuit 6, N D / A converters 8 that convert the digital signals generated by the encryption circuits 5, 5′ into analog signals, N pulse light sources 9 and N optical modulators 10 for optically transporting the analog signals output from the D / A converter 8, a quantum noise source 11 that generates quantum noise to be imparted to the optically modulated optical signal, and an optical multiplexer 12 that multiplexes the N optical pulse signals and the noise signal.

[0035] The encryption circuits 5, 5' in Fig. 2(a) may be, for example, the conventional encryption circuit shown in Fig. 4(a). When encrypting true data 3, it is important to use a common key 1 shared with the authorized recipient, and a portion of the common key 1 distributed by the key distribution circuit 2 is used for encryption. On the other hand, when encrypting dummy data 4 on N-1 channels, no common key is required, and encryption can be performed using a separate key previously stored inside the N-1 encryption circuits 5'.

[0036] In the transmitting section, the order (m-th order) of the Hermite-Gaussian pulse for transmitting the true data 3 is determined from the random number sequence output from the random number sequence generation circuit 6 based on the common key 1, and the true data 3 is switched by the selector 7 to the signal path in which the m-th order Hermite-Gaussian pulse light source is arranged. Meanwhile, the dummy signals 4 of the N-1 channels may be arbitrarily distributed to the remaining signal paths.

[0037] The pulse light source 9 corresponds to a light source that generates coherent Hermite-Gaussian pulses of orders 0 to (N-1). Any method of generating the pulses may be used, such as a method of generating pulses from a mode-locked laser or a method of generating coherent Hermite-Gaussian pulses by shaping sidebands generated by combining a CW light source and an optical modulator.

[0038] The optical modulator 10 may be selected from a phase modulator, an amplitude (or intensity) modulator, and an IQ modulator depending on the format of the encrypted signal generated by the encryption circuits 5, 5' so as to mask at least one of the phase and amplitude level of the optical signal.

[0039] The quantum noise generated by the quantum noise source 11 can be spontaneous emission light output from an erbium-doped optical fiber amplifier or a Raman optical amplifier, or any random optical signal.

[0040] In the optical multiplexer 12, the N coherent Hermite-Gaussian pulse signals optically modulated by the optical modulator 11 are time-synchronized and multiplexed in the same time slot, so as to maintain orthogonality between the coherent Hermite-Gaussian pulses of each order. Furthermore, by adding optical noise from the quantum noise source 11 to the coherent Hermite-Gaussian pulse and outputting the optical signal to the optical transmission line in a state where the signal-to-noise ratio is intentionally degraded, it is desirable to ensure security by the masking effect of quantum noise even if an eavesdropper is able to read the signal immediately after the transmitter (without decrypting it).

[0041] The optical transmission line used to transmit the N-multiplexed Hermite-Gaussian pulse signals output from the transmitter to the receiver can be a conventional optical fiber transmission line or optical space propagation. In this case, it is also possible to extend the transmission distance by using optical amplification technologies such as erbium-doped optical fiber amplifiers and Raman amplifiers.

[0042] As shown in FIG. 2( b), the receiving unit has the same common key 1, key distribution circuit 2, random number sequence generation circuit 6, selector 7, and N pulse light sources 9 as the transmitting unit, and further includes a coherent receiving circuit 13 for homodyne detection of N-multiplexed coherent Hermite-Gaussian pulses in the time domain transmitted from the transmitting unit and the coherent Hermite-Gaussian pulse selected by the selector 7, an A / D converter 14 for converting an analog signal received by the coherent receiving circuit 13 into a digital signal, a time integration circuit 15 for demultiplexing a signal obtained by encrypting true data 3, and a decryption circuit 16 for restoring the original data from the demultiplexed encrypted signal.

[0043] Based on the information of the common key 1, the selector 7 in FIG. 2B selects the mth-order coherent Hermite-Gaussian pulse light source used to transmit the true data 3 in the transmitter from the random number sequence output from the random number sequence generation circuit 6. In the receiver, the mth-order coherent Hermite-Gaussian pulse is used as the local light, and the coherent Hermite-Gaussian pulse signal multiplexed in the time domain by N is homodyne detected using the coherent receiver circuit 13. The detected signal is then converted into a digital signal using the A / D converter 14, and integrated over a time slot equivalent to one symbol using the time integration circuit 15. As shown in Equation (6), only the encrypted signal transmitted by the mth-order coherent Hermite-Gaussian pulse (the signal obtained by encrypting the true data 3) is extracted (demultiplexed). Finally, the true data 3 is restored from the extracted encrypted signal using the decryption circuit 16.

[0044] In order to correctly detect the phase information of the encrypted signal by homodyne detection in the coherent receiving circuit 13, it is necessary to synchronize the optical phase of the m-th order coherent Hermite-Gaussian pulse light source 9, which is a local light source, with the optical phase of the transmitted N-multiplexed coherent Hermite-Gaussian pulse signal. Examples of phase synchronization techniques include a phase control method using an optical phase-locked loop circuit or an optical injection locking circuit. Alternatively, a method can be used in which homodyne detection is performed using an asynchronous local light source, the signal is converted into a digital signal by the A / D converter 14, and then phase fluctuations are compensated for by digital signal processing.

[0045] The decryption circuit 16 may be, for example, a conventional decryption circuit as shown in Fig. 4(b). During decryption, the same key as that used for encryption is used for decryption, using a common key 1 shared with the transmitter and a key distribution circuit 2.

[0046] Fig. 3 shows a block diagram of (a) a transmitter and (b) a receiver of an optical secure communications system according to a second embodiment of the present invention. As shown in Fig. 3(a), the transmitter differs from the first embodiment in that it simultaneously transmits n channels of true data 3. The differences from the first embodiment will be described below.

[0047] The n encryption circuits 5 used to encrypt the true data 3 operate based on a common key 1 shared with the authorized recipient. At this time, the individual keys distributed using the key distribution circuit 2 are assigned to the n encryption circuits 5. On the other hand, when encrypting dummy data 4 of Nn channels, no common key is required, and encryption can be performed based on a key previously stored inside the Nn encryption circuits 5'.

[0048] Based on the random number sequence output from the random number sequence generation circuit 6, the order of the n coherent Hermite-Gaussian pulses used to carry n channels of true data 3 is determined. Based on this selection, the selector 7 switches the n channels of true data 3 to a signal path in which a coherent Hermite-Gaussian pulse light source of the desired order is located. Meanwhile, the N-n channels of dummy data 4 can be arbitrarily distributed to the remaining signal paths. Other operations from the D / A converter 8 to the optical multiplexer 12 are the same as those in the first embodiment.

[0049] 3B, the receiver differs from the first embodiment in that it simultaneously decodes n channels of true data 3. The differences from the first embodiment will be described below.

[0050] The selector 7 selects n coherent Hermite-Gaussian pulse light sources used in the transmitter to carry n channels of true data 3. The receiver uses an optical splitter 17 to branch the transmitted N-multiplexed coherent Hermite-Gaussian pulse signal into n signals, and uses n coherent receiving circuits 13 to perform homodyne detection on the branched n N-multiplexed coherent Hermite-Gaussian pulse signals and the n coherent Hermite-Gaussian pulses (local light) selected by the selector 7. The n detected signals are then converted into digital signals by n A / D converters 14, and integrated over a time slot equivalent to one symbol by n time integration circuits 15, thereby demultiplexing the encrypted signals (signals obtained by encrypting the true data 3) carried by the n coherent Hermite-Gaussian pulses. Finally, n decryption circuits 16 are used to restore the n channels of true data 3 from the n encrypted signals.

[0051] When the receiver operates the selector 7, it uses the common key 1 shared with the transmitter, the key distribution circuit 2, and the random number sequence generation circuit 6 to correctly select the n coherent Hermite-Gaussian pulses used to convey the true data 3 in the transmitter.

[0052] In addition, when the receiving section operates the n decryption circuits 15, the receiving section uses the common key 1 shared with the transmitting section and the key distribution circuit 2, and uses the same key as that used for encryption.

[0053] In the optical secure communications system of the first or second embodiment, the modulation method used in the QNSC transmission system can be optical intensity modulation, optical phase modulation, or quadrature amplitude modulation.

[0054] In the optical secure communication system of the first or second embodiment, security can be further improved by using, as the common key 1, a private key distributed using quantum key distribution technology that uses single photons.

[0055] In the optical secure communications system of the first or second embodiment, the transmission capacity of the optical secure communications system can be doubled by providing a polarization multiplexing circuit and a polarization demultiplexing circuit in the transmitting unit and the receiving unit, respectively, and using a polarization multiplexing transmission method.

[0056] In the optical secure communications system of the first or second embodiment, the transmission capacity of the optical secure communications system can be increased by providing a time division multiplexing circuit and a time division demultiplexing circuit in the transmitting unit and the receiving unit, respectively, and utilizing a time division multiplexing transmission method.

[0057] In the optical secure communications system of the first or second embodiment, a wavelength division multiplexing circuit and a wavelength division demultiplexing circuit are provided in the transmitting unit and the receiving unit, respectively, and wavelength division multiplexing transmission is performed using a multi-wavelength light source, thereby increasing the transmission capacity of the optical secure communications system.

[0058] In the optical secure communications system of the first or second embodiment, the transmission capacity of the optical secure communications system can be increased by providing a space division multiplexing circuit and a space division demultiplexing circuit in the transmitting unit and the receiving unit, respectively, and performing space division multiplexing transmission using a multicore fiber or a multimode fiber as the optical transmission path.

[0059] As described above in detail, according to the present invention, in a QNSC transmission system that uses quantum noise to mask at least one of the phase or amplitude level of an optical signal, when a QNSC signal is carried using a coherent Hermite-Gaussian pulse, the signal is multiplexed with coherent Hermite-Gaussian pulses of different orders, thereby adding a masking effect in the time domain and providing an optical secure communications system that is more secure than conventional QNSC. Furthermore, by combining this with polarization multiplexing, time division multiplexing, wavelength division multiplexing, and / or spatial multiplexing transmission technology using multicore fiber or multimode fiber, an even larger capacity optical secure communications system can be provided.

[0060] 1 Common key 2 Key distribution circuit 3 True data 4 Dummy data 5, 5' Encryption circuit 6 Random number sequence generation circuit 7 Selector 8 D / A converter 9 Pulse light source 10 Optical modulator 11 Quantum noise source 12 Optical multiplexer 13 Coherent receiving circuit 14 A / D converter 15 Time integration circuit 16 Decryption circuit 17 Optical demultiplexer 51 Key distribution circuit 52, 53 Random number sequence generation circuit 54 Exclusive OR circuit 55 Adder circuit 56 Encoder 57 D / A converter 58 CW light source 59 Optical modulator 60 Quantum noise source 61 Optical multiplexer 62 Photodetector 63 A / D converter 64 Decoder 65 Subtraction circuit 66 Exclusive OR circuit

Claims

1. An optical secure communication system for a quantum noise stream cipher (QNSC) signal that masks at least one of the phase and amplitude levels of an optical signal using quantum noise, wherein the carrier light of the transmitter of the QNSC signal and the local light emission of the receiver are composed of coherent Hermite-Gaussian pulses whose amplitude is given by a Hermite-Gaussian function. An optical secure communication system characterized by that.

2. The optical secure communication system according to claim 1, wherein the coherent Hermite-Gaussian pulse is composed of a signal obtained by generating N coherent Hermite-Gaussian pulses of order 0 to N-1 in a time slot for one symbol and multiplexing them.

3. The transmitter conveys the QNSC signal to an m-th order coherent Hermite pulse among the N coherent Hermite-Gaussian pulses, and conveys a time series of dummy signals to the remaining N-1 coherent Hermite pulses. The receiver uses the m-th order coherent Hermite-Gaussian pulse as the local light emission, homodyne-detects the signal light in which the N coherent Hermite-Gaussian pulses are multiplexed, and integrates the detected detection signal over a time slot for one symbol. By using the orthogonality of the Hermite-Gaussian pulses, the m-th order coherent Hermite-Gaussian pulse is multiplexed and separated from the N-fold coherent Hermite-Gaussian pulses in the time domain, and the QNSC signal is demodulated. The optical secure communication system according to claim 2, characterized by that.

4. The transmitter conveys QNSC signals for n channels to any n (n≤N) of the N coherent Hermite-Gaussian pulses, and conveys a time series of dummy signals to the remaining N-n coherent Hermite-Gaussian pulses. The receiver uses the n coherent Hermite-Gaussian pulses as the local light emission, homodyne-detects individually the coherent Hermite-Gaussian pulse signal light that conveys the QNSC signals for the n channels by the local light emission, and individually integrates the n detected detection signals over a time slot for one symbol. By multiplexing and separating the coherent Hermite-Gaussian pulses of the same order as the local light emission, the QNSC signals for the n channels are demodulated. The optical secure communication system according to claim 2, characterized by that.

5. The optical secure communication system according to any one of claims 1 to 4, characterized in that any one of optical intensity modulation, optical phase modulation, and quadrature amplitude modulation is used as the modulation method.

6. The optical secure communication system according to any one of claims 1 to 4, characterized in that a secret key distributed using quantum key distribution technology is used as the common key for encryption.

7. The optical secure communication system according to any one of claims 1 to 4, characterized in that the transmission unit and the reception unit each have a polarization multiplexing circuit and a polarization separation circuit, and perform transmission using a polarization multiplexing transmission method.

8. The optical secure communication system according to any one of claims 1 to 4, characterized in that the transmission unit and the reception unit each have a wavelength division multiplexing circuit and a wavelength division separation circuit, and perform wavelength division multiplexing transmission using a multi-wavelength light source.

9. The optical secure communication system according to any one of claims 1 to 4, characterized in that the transmission unit and the reception unit each have a space division multiplexing circuit and a space division separation circuit, and perform space division multiplexing transmission using a multi-core fiber or a multi-mode fiber as the optical transmission line.

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