Fabry-Peort (FP) cavity-based quantum key distribution has some advantages in the phase-coded quantum key distribution (QKD). Locking the frequency of the FP cavity transmission to the frequency of single photon carrying the key information determines the security of QKD. In this paper, we propose an FP cavity locked to a single photon (the mean photon number is about 0.1) frequency using single-photon modulation/demodulation method. The single photon detector working in the Geiger mode is used to detect modulated single photons then direactly demodulated by the lock-in amplifier. The discrete single photon response pulses accumulated fulfills the phase sensitive detection, and the signal-to-noise ratio of signal frequency discrimination is up to 112. The transmission frequency fluctuation of the locked FP is limited to 2 MHz.
Fabry-Peort (FP) cavity-based quantum key distribution has some advantages in the phase-coded quantum key distribution (QKD). Locking the frequency of the FP cavity transmission to the frequency of single photon carrying the key information determines the security of QKD. In this paper, we propose an FP cavity locked to a single photon (the mean photon number is about 0.1) frequency using single-photon modulation/demodulation method. The single photon detector working in the Geiger mode is used to detect modulated single photons then direactly demodulated by the lock-in amplifier. The discrete single photon response pulses accumulated fulfills the phase sensitive detection, and the signal-to-noise ratio of signal frequency discrimination is up to 112. The transmission frequency fluctuation of the locked FP is limited to 2 MHz.
Design principles of universal microwave photonics system (MPS) for quantum key distribution (QKD) with frequency coding are concerned. Its main modulation concept lies in single photon generation on sidebands of optical carrier and determination of photons ground state through its registration and the amplitude value of its carrier frequency as reference channel. So, it is necessary to solve problems of signal-to-carrier ratio of single photon detector (SPD) and aspects of photon number splitting (PNS) attack, nonlinear phase modulation (NPM) between carrier and sidebands in fiber, and finally, spectral selection of carrier in receiver. The technologies, based on the modulation conversion of an optical carrier, are widely used in microwave photonics. Due to the natural symmetry of modulated signals and the highest achievable ratio of the modulation conversions, amplitude-phase modulation with complete or partial suppression of the optical carrier has found a particularly wide application in MPS. The characteristics of advanced MPS for QKD with frequency coding and carrier suppression based on tandem amplitude modulation and phase commutation are presented. New systems can have classical symmetric or non-classical asymmetric structure for QKD based only on spectral selection of carrier and subcarriers without re-modulation.
The technology of frequency coding in channels of quantum key distribution allows to determine the ground state of photons through the value of the amplitude of its carrier, frequency modulated in phase (PM) or amplitude (AM) by a radio-frequency signal, and the resulting side components. Over the past twenty years, it has been substantially modified and improved. At the same time, in recent works, an expanded understanding of the frequency coding principle is used, in which each photon state is associated not with the phase of the modulating signal at a certain frequency, but with one or more sideband frequencies or the carrier frequency of the photon itself. In this paper, we present the results of constructing a frequency coding system of quantum key distribution (QKD) based on the serial electro-optical photon amplitude modulation and phase commutation. The possibility of reducing the probability of achieving a positive result in PNS attacks by Eva is shown by eliminating the carrier of the signal transmitted via the quantum channel from the structure of the key distribution. It was early noted, that the smallest value of QBER is achieved in schemes with a passive definition of one or two basic states of a photon, i.e. without the use of remodulation processes, that is realized in considering. Additionally it is not necessary to form a notch filter system, which is usually carried out using fiber Bragg gratings (FBG) or arrayed waveguide gratings (AWG) for discrimination the photon carrier and its sideband components.
The technology of frequency coding in channels of quantum key distribution allows to determine the ground state of photons through the value of the amplitude of its carrier, frequency modulated in phase (PM) or amplitude (AM) by a radio-frequency signal, and the resulting side components. Over the past twenty years, it has been substantially modified and improved. At the same time, in recent works, an expanded understanding of the frequency coding principle is used, in which each photon state is associated not with the phase of the modulating signal at a certain frequency, but with one or more sideband frequencies or the carrier frequency of the photon itself. The known schemes of electro-optical modulation and remodulation are described by the symmetric pairs PM-PM, AM-AM and AM-PM (PM-AM), where the first component determines the modulator type on the Alice side (legal subscriber - transmitter), and the second one on the side of Bob legal subscriber - receiver). It is noted, that the smallest value of QBER is achieved in schemes with a passive definition of one or two basic states of a photon, i.e. without the use of remodulation processes. The formation of a filter system is carried out using fiber Bragg gratings (FBG) or arrayed waveguide gratings (AWG), tuned to the photon carrier or it sideband components. In this paper, we present the results of constructing a universal system of quantum key distribution (QKD) based on the electro-optical AMPM-PMAM scheme, which makes it possible to realize all the above-mentioned schemes and additionally combined version. At the heart of the AMPMPMAM system of the QKD system is the modulation transformation of the photon carrier based on the Il'inMorozov's method and its one- and two-modulator realizations. For the mathematical description of the scheme, the principles of constructing a single-port modulation microwave photonic link of a sequential type and photon modeling of electro-optical modulators were used. The use of the universal scheme will allow using several levels of cryptographic protection, including switching schemes and protocols, with remodulation and only passive detection. In addition, it should be noted the high spectral efficiency of the Il'in-Morozov's modulation conversion, its spectral purity and the stability of the received characteristics of the output radiation when the conversion parameters deviate from the optimal ones. The possibility of reducing the probability of achieving a positive result in PNS attacks by Eva (illegal subscriber) is shown by eliminating the carrier of the signal transmitted via the quantum channel from the structure of the key distribution.
The key security of quantum key distribution (QKD) is guaranteed by the basic principle of quantum mechanics, which makes it possible to combine information theory security communication with one-time encryption. The key is usually encoded on the polarization dimension or phase dimension of a single-photon. It is considered that the birefringence effect of single-mode fiber leads to a random variation of polarization state, which would induce the bit error rate. So it is of great significance to keep the single-photon linear polarization state stable for both polarization encoding QKD system and phase encoding QKD system. By using the single-photon polarization modulation technology, the single-photon linear polarization state periodically varies with the external modulation signal. The flicker noise is suppressed effectively, and the signal-to-noise ratio (SNR) of single-photon counting is increased as indicated by the phase-sensitive detection with a lock-in amplifier (LIA). The error signal is generated by demodulating the modulated single photons and it is used to lock an arbitrary 1550 nm single-photon linear polarization state to the optical axis of in-line polarizer (ILP). The modulation frequency reaches up to 5 kHz, which can eliminate the influence of low frequency flicker noise. The LIA demodulates the single-photon pulses by using 78.1 Hz filter bandwidth, with a time constant of 1 ms and a filter slope of 24 dB. The SNR of error signal is 20. The zero-crossing point of error signal represents the single photon’s linear polarization state aligned to the optical axis of ILP. The linear slope around the zero-crossing point for the polarization state angle versus the error signal amplitude is 1.267 rad/V. When the negative feedback loop does not work, the polarization drift of single-photon pulses is 0.082 rad due to the random environmental noise. However, by using the single-photon polarization modulation technology, the polarization drift of stable single-photon pulses is limited to 0.0011 rad within 2000 s through the precise control with a polarization rotator, and the corresponding Allan deviation reaches the minimal value of 6.7×10–5 at an integration time of 128 ms. The advantages for the single-photon polarization modulation technology are as follows: i) the linear polarization state drift is compensated in real-time at the single-photon level; ii) single frequency polarization modulation can be extended to multiple frequency polarization modulation in order to achieve simultaneous locking of multiple linear polarization states of single-photon pulses; iii) these 1550 nm single-photon pulses with the 0.0011 rad linear polarization state stability can be directly used as the single-photon source in either polarization encoding or phase encoding QKD system.
The decoy-state method serves as a cornerstone for practical quantum key distribution (QKD) with inherent immunity to photon-number-splitting attacks. Leveraging optical injection locking, we propose a directly modulated light source with a scheme that simultaneously generates intensity- and phase-modulated signals without external modulators. Removing external modulators eliminates modulator-related side-channel vulnerabilities, fundamentally enhancing QKD security. Integrated into a high-speed phase-encoding BB84 system with optimized one-decoy protocol and finite-size analysis, our system achieves positive key rates up to 20 dB channel loss (100 km fiber) with commercial InGaAs single-photon detectors, reducing transmitter complexity while ensuring security for practical QKD applications.
It is the closest technology to practice in the quantum communication at present to apply single-photon scheme to implement the quantum key distribution (QKD), and may achieve the absolute security in principle. Because of the limit of the response speed of the present single-photon detector, the code rate is still too low to come into practical use. The paper has put up a new idea to design a quick single-photon detector with a multi-port optic-fiber splitter or an optical switch array and many avalanche photo diodes (APD). All of the ports with APD work on time division and cooperate with the logic discrimination and deciding unit by the clock signal, thus the quick detection can come true to high repetition-rate pulses with a single photon. The multiple of the detection rate is decided by the number of the port. The optoelectronic integration can be adopted for the reliable detector. The applying of this detector will largely raise the code rate of the QKD, and stimulate the commercial use.