Generation and application of squeezed light source has been the interest in the field of quantum optical experiments, especially with regards to the field of quantum key distribution. In this paper, a method using phase-squeezed state as the input state of the single photon quantum key distribution protocol is introduced, and the rate of the gained secure key of the proposed method after the post-processing techniques is analyzed. Compared to the conventional quantum key distribution protocol using weak coherent light source as the single photon source, it is shown that in the proposed method there are significant improvements in both the transmittable distance and the gained secure key rate.
Generation and application of squeezed light source has been the interest in the field of quantum optical experiments, especially with regards to the field of quantum key distribution. In this paper, a method using phase-squeezed state as the input state of the single photon quantum key distribution protocol is introduced, and the rate of the gained secure key of the proposed method after the post-processing techniques is analyzed. Compared to the conventional quantum key distribution protocol using weak coherent light source as the single photon source, it is shown that in the proposed method there are significant improvements in both the transmittable distance and the gained secure key rate.
Squeezing of the coherent state by optical parametric amplifier is shown to efficiently produce single-photon states with reduced multiphoton probabilities compared with the weak coherent light. It can be a better source for a longer-distance quantum key distribution and also for other quantum optical experiments. The necessary condition for a secure quantum key distribution given by Brassard et al. is analyzed as functions of the coherent-state amplitude and squeeze parameter. Similarly, the rate of the gained secure bits G after error correction and privacy amplification given by L\"utkenhaus is calculated. Compared with the weak coherent light, it is found that G is about ten times larger and its high level continues on about two times longer distance. By improvement of the detector efficiency it is shown that the distance extends further. Measurement of the intensity correlation function and the relation to photon antibunching are discussed for the experimental verification of the single-photon generation.
The security of quantum key distribution (QKD) is based on the basic principles of quantum mechanics, and therefore has unconditional security in theory. In existing quantum key distribution systems, weakly coherent sources (WCSs) are often used as light sources due to a high probability of vacuum pulses in these sources, resulting in limited transmission distances. Besides, there inevitably exist equipment defects in actual QKD systems, such as certain defects in phase modulators and intensity modulators, which lead to distinguishability of quantum states in higher dimensions and result in side-channel vulnerabilities. An eavesdropper can carry out corresponding attacks, thereby threatening the actual security of QKD systems. To overcome the above limitations, we propose an improved protocol on quantum key distribution based on monitoring heralded single-photon sources. Due to the simultaneity of parametric down-conversion photon pairs, we can accurately predict the arrival of one photon by measuring the arrival time of another photon. Through this way, we can greatly reduce the probability of vacuum states in the signal light, and increase the longest transmission distance of the QKD system. Moreover, a light source monitoring module is inserted into the sender’s side. By randomly selecting a certain period of time through the source monitoring module to measure the Hong-Ou-Mandel interference between the signal light and the idle light , we can estimate the side-channel information leakage of the source and then obtain the key generation rate. Compared with the QKD protocol based on monitoring weak coherent sources, our present protocol can give a better performance in either the transmission distance or the key generation rate, especially when the interference error is large. In addition, in principle, our present protocol can also be extended to other quantum key distribution protocols, such as the measurement-device-independent protocols, to further improve the security and practicability of QKD systems. Therefore, our present work can provide valuable references for realizing the large-scale application of quantum communication networks in the near future.
Increasing the bit rate and transmission distance of quantum key distribution (QKD) without quantum repeaters is a formidable challenge, which is limited by the secret-key capacity (SKC). Fortunately, phase-matching QKD (PM-QKD) can overcome this limitation by matching the phase of two coherent states and encoding the key information into the common phase. But the probability of the weak coherent source (WCS) sending a single photon is small, so studying the QKD of different signal sources is one of the effective ways to improve the performance of the QKD protocol. The single-photon-added coherent state (SPACS) has a high probability of sending single photons and is closer to an ideal single-photon source than WCS. We have done simulations combining the advantages of the source with the advantages of PM-QKD. Numerical simulations show that the SPACS-PM-QKD protocol also has the ability to break the SKC, and using the SPACS is better in terms of communication distance and key rate compared to WCS.
In practical quantum key distribution systems, there inevitably exist errors in the quantum state preparation process due to imperfections in realistic equipment and devices. Those errors would lead to some security loopholes in the quantum key distribution systems. According to the work of Tamaki et al. ( Phys. Rev. A 90 052314 ), here in this work we propose a state preparation error tolerant quantum key distribution protocol through using heralded single-photon sources. In this protocol, we characterize the size of the error in the preparation state of Alice and bring it into the security analysis, thereby avoiding possible security loopholes and improving the security of the system. Moreover, we take the three-intensity decoy-state method for example to introduce the method of constructing the model and estimating the parameters, and carry out corresponding numerical simulations. We make a comparison between the loss tolerant protocol with weak coherent source (WCS) and our present protocol using heralded single-photon source (HSPS). Simulation results show that under the same state preparation error, the key generation rate of the protocol based on WCS is higher than that of protocol based on HSPS at short transmission distances (e.g. less than 150 km). The main reason is that the detection efficiency of the local detector used in the latter scheme is low. However, in the case of long transmission distances (e.g. greater than 200 km), the key generation rate of scheme with WCS drops deeply, while the decline of the key generation rate of the present scheme is much flatter. As a result, the former can no longer generate keys after 211 km, while the latter can transmit a maximum distance of 228 km. Moreover, we also make a comparison between the present scheme and the GLLP protocol with HSPS. The simulation results show that the GLLP protocol with HSPS is very sensitive to the state preparation error and its key generation rate will rapidly decrease with the increase of the state preparation error. On the contrary, our present protocol shows almost no performance degradation under practical state preparation errors. It thus verify the robustness against the state preparation errors of our present work. In addition, in principle, the method can also be combined with the measurement-device-independent quantum key distribution protocol and the twin-field quantum key distribution protocol to further increase the secure communication transmission distance that the present system can reach. Therefore, this work may provide an important reference value for the practical application of long-distance quantum secure communication in the near future.
Quantum key distribution (QKD) offers a secure means of communication based on the laws of quantum physics. Despite its remarkable advancements, the current reliance of QKD on attenuated coherent (laser) light sources has imposed a fundamental limit on the secure key rate (SKR) per channel use. This constraint stems from the scarcity of single-photon components within coherent light, inherently bounded by a maximum of 1/e. Here, we report comprehensive demonstrations of single-photon-source-based high-rate QKD, surpassing the fundamental SKR limit imposed by the weak coherent light. By employing an on-demand, bright single-photon source with an efficiency of 0.71(2), coupled with narrow-bandwidth filtering and random polarization modulation, we demonstrated a field QKD trial over a 14.6(1.1) dB-loss free-space urban channel, achieving a SKR of 1.08×10^{-3} bits per pulse. This SKR surpasses the practical limit of weak coherent-light-based QKD by 79%. These findings unequivocally demonstrate the superior performance of single-photon sources over weak coherent light for QKD applications, marking a pivotal stride towards realizing a global quantum internet.
Most qubit-based quantum key distribution (QKD) protocols extract the secure key merely from single-photon component of the attenuated lasers. However, with the Scarani-Acin-Ribordy-Gisin 2004 (SARG04) QKD protocol, the unconditionally secure key can be extracted from the two-photon component by modifying the classical post-processing procedure in the BB84 protocol. Employing the merits of SARG04 QKD protocol and six-state preparation, one can extract secure key from the components of single photon up to four photons. In this paper, we provide the exact relations between the secure key rate and the bit error rate in a six-state SARG04 protocol with single-photon, two-photon, three-photon, and four-photon sources. By restricting the mutual information between the phase error and bit error, we obtain a higher secure bit error rate threshold of the multiphoton components than previous works. Besides, we compare the performances of the six-state SARG04 with other prepare-and-measure QKD protocols using decoy states.