Neural networks are becoming ubiquitous in various areas of physics as a successful machine learning (ML) technique for addressing different tasks. Based on ML technique, we propose and experimentally demonstrate an efficient method for state reconstruction of the widely used Sagnac polarization-entangled photon source. By properly modeling the target states, a multi-output fully connected neural network is well trained using only six of the sixteen measurement bases in standard tomography technique, and hence our method reduces the resource consumption without loss of accuracy. We demonstrate the ability of the neural network to predict state parameters with a high precision by using both simulated and experimental data. Explicitly, the mean absolute error for all the parameters is below 0.05 for the simulated data and a mean fidelity of 0.99 is achieved for experimentally generated states. Our method could be generalized to estimate other kinds of states, as well as other quantum information tasks.

Broadband photon pairs are highly desirable for quantum metrology, quantum sensing, and quantum communication. Such sources are usually designed through type-0 phase-matching spontaneous parametric down-conversion (SPDC) that makes the photon pairs hard to separate in the frequency-degenerate case and thus limits their applications. In this paper, we design a broadband frequency-degenerate telecom-band photon pair source via the type-II SPDC in a dispersion-engineered thin-film lithium niobate waveguide, where the polarization modes of photon pairs are orthogonal and thus are easily separated deterministically. With a 5-mm-long waveguide, our design can achieve a bandwidth of 5.56 THz (44.8 nm), which is 8.6 times larger than that of the bulk lithium niobate, and the central wavelength can be flexibly adjusted. Our design is a promising approach towards high-quality integrated photon sources and may have wide applications in photonic quantum technologies.

In principle, the asynchronous measurement-device-independent quantum key distribution (AMDI-QKD) can surpass the key rate capacity without phase tracking and phase locking. However, practical imperfections in sources or detections would dramatically depress its performance. Here, we present an improved model on AMDI-QKD to reduce the influence of these imperfections, including intensity fluctuation, the afterpulse effect, and the dead time of detectors. Furthermore, we carry out corresponding numerical simulations. Simulation results show that, by implementing our present work, it can have more than 100 km longer secure transmission distance and one order of magnitude enhancement in the key generation rate after 320 km compared with the standard method. Moreover, our model can still break the Pirandola-Laurenza-Ottaviani-Banchi (PLOB) bound even under realistic experimental conditions.

As the first stage of the quantum Internet, quantum key distribution (QKD) networks hold the promise of providing long-term security for diverse users. Most existing QKD networks have been constructed based on independent QKD protocols, and they commonly rely on the deployment of single-protocol trusted relay chains for long reach. Driven by the evolution of QKD protocols, large-scale QKD networking is expected to migrate from a single-protocol to a multi-protocol paradigm, during which some useful evolutionary elements for the later stages of the quantum Internet may be incorporated. In this work, we delve into a pivotal technique for large-scale QKD networking, namely, multi-protocol relay chaining. A multi-protocol relay chain is established by connecting a set of trusted/untrusted relays relying on multiple QKD protocols between a pair of QKD nodes. The structures of diverse multi-protocol relay chains are described, based on which the associated model is formulated and the policies are defined for the deployment of multi-protocol relay chains. Furthermore, we propose three multi-protocol relay chaining heuristics. Numerical simulations indicate that the designed heuristics can effectively reduce the number of trusted relays deployed and enhance the average security level versus the commonly used single-protocol trusted relay chaining methods on backbone network topologies.

Quantum digital signature (QDS) can guarantee the information-theoretical security of a signature with the fundamental laws of quantum physics. However, most current QDS protocols do not take source security into account, leading to an overestimation of the signature rate. In this paper, we propose to utilize Hong-Ou-Mandel interference to characterize the upper bound of the source imperfections, and further to quantify information leakage from potential side-channels. Additionally, we combine decoy-state methods and finite-size analysis in analyzing the signature rate. Simulation results demonstrate the performance and feasibility of our approach. Our current work can improve the practical security of QDS systems, thereby promoting their further networked applications.

Implementing quantum wireless multi-hop network communication is essential to improve the global quantum network system. In this paper, we employ eight-level GHZ states as quantum channels to realize multi-hop quantum communication, and utilize the logical relationship between the measurements of each node to derive the unitary operation performed by the end node. The hierarchical simultaneous entanglement switching (HSES) method is adopted, resulting in a significant reduction in the consumption of classical information compared to multi-hop quantum teleportation (QT) based on general simultaneous entanglement switching (SES). In addition, the proposed protocol is simulated on the IBM Quantum Experiment platform (IBM QE). Then, the data obtained from the experiment are analyzed using quantum state tomography, which verifies the protocol's good fidelity and accuracy. Finally, by calculating fidelity, we analyze the impact of four different types of noise (phase-damping, amplitude-damping, phase-flip and bit-flip) in this protocol.

Hybrid entangled states are crucial in quantum physics, offering significant benefits for hybrid quantum communication and quantum computation, and then the conversion of hybrid entangled states is equally critical. This paper presents two novel schemes, that is, one converts the two-qubit hybrid Knill-Laflamme-Milburn (KLM) entangled state into Bell states and the other one transforms the three-qubit hybrid KLM state into Greenberger-Horne-Zeilinger (GHZ) states assisted by error-predicted and parity-discriminated devices. Importantly, the integration of single photon detectors into the parity-discriminated device enhances predictive capabilities, mitigates potential failures, and facilitates seamless interaction between the nitrogen-vacancy center and photons, so the two protocols operate in an error-predicted way, improving the experimental feasibility. Additionally, our schemes demonstrate robust fidelities (close to 1) and efficiencies, indicating their feasibility with existing technology.

In the process of quantum key distribution (QKD), the communicating parties need to randomly determine quantum states and measurement bases. To ensure the security of key distribution, we aim to use true random sequences generated by true random number generators as the source of randomness. In practical systems, due to the difficulty of obtaining true random numbers, pseudo-random number generators are used instead. Although the random numbers generated by pseudo-random number generators are statistically random, meeting the requirements of uniform distribution and independence, they rely on an initial seed to generate corresponding pseudo-random sequences. Attackers may predict future elements from the initial elements of the random sequence, posing a security risk to quantum key distribution. This paper analyzes the problems existing in current pseudo-random number generators and proposes corresponding attack methods and applicable scenarios based on the vulnerabilities in the pseudo-random sequence generation process. Under certain conditions, it is possible to obtain the keys of the communicating parties with very low error rates, thus effectively attacking the quantum key system. This paper presents new requirements for the use of random numbers in quantum key systems, which can effectively guide the security evaluation of quantum key distribution protocols.

Reference-frame-independent quantum key distribution (RFI-QKD) can avoid real-time calibration operation of reference frames and improve the efficiency of the communication process. However, due to imperfections of optical devices, there will inevitably exist intensity fluctuations in the source side of the QKD system, which will affect the final secure key rate. To reduce the influence of intensity fluctuations, an improved 3-intensity RFI-QKD scheme is proposed in this paper. After considering statistical fluctuations and implementing global parameter optimization, we conduct corresponding simulation analysis. The results show that our present work can present both higher key rate and a farther transmission distance than the standard method.

We propose a quantum-enhanced metrological scheme utilizing unbalanced entangled coherent states (ECSs) generated by passing a coherent state and a coherent state superposition through an unbalanced beam splitter (BS). We identify the optimal phase sensitivity of this scheme by maximizing the quantum Fisher information (QFI) with respect to the BS transmission ratio. Our scheme outperforms the conventional scheme with a balanced BS, particularly in the presence of single-mode photon loss. Notably, our scheme retains quantum advantage in phase sensitivity in the limit of high photon intensity, where the balanced scheme offers no advantage over the classical strategy.

Quantum secure direct communication (QSDC) is a communication method based on quantum mechanics and it is used to transmit secret messages. Unlike quantum key distribution, secret messages can be transmitted directly on a quantum channel with QSDC. Higher channel capacity and noise suppression capabilities are key to achieving long-distance quantum communication. Here, we report a continuous-variable QSDC scheme based on mask-coding and orbital angular momentum, in which the mask-coding is employed to protect the security of the transmitting messages and to suppress the influence of excess noise. The combination of orbital angular momentum and information block transmission effectively improves the secrecy capacity. In the $800$ information blocks $\times 1310$ bits length 10-km experiment, the results show a statistical average bit error rate of 0.38%, a system excess noise value of 0.0184 SNU, and a final secrecy capacity of 6.319$\times10^{6}$ bps. Therefore, this scheme reduces error bits while increasing secrecy capacity, providing a solution for long-distance large-scale quantum communication, which is capable of transmitting text, images and other information of reasonable size.

Mode-pairing quantum key distribution (MP-QKD) is an excellent scheme that can exceed the repeaterless rate-transmittance bound without complex phase locking. Nevertheless, MP-QKD usually needs to ensure that the communication distances of the two channels are equal. To address the problem, the asymmetric MP-QKD protocol is proposed. In this paper, we enhance the performance of the asymmetric MP-QKD protocol based on the advantage distillation (AD) method without modifying the quantum process. The simulation results show that the AD method can extend the communication distance by about 70 km in the case of asymmetry. And we observe that as the misalignment error increases, the AD method further increases the expandable communication distance. Our work can further enhance the robustness and promote the practical application of the asymmetric MP-QKD.

Free-space quantum key distribution (QKD) offers broader geographical coverage and more flexible system deployment than fiber-based systems. However, the free-space environment is highly complex, and various attenuation factors can significantly reduce the key distribution efficiency or even lead to encoding failures. This paper discusses and analyzes the impact of turbulence and fog in mountainous environments on free-space discrete-variable quantum key distribution. Through numerical simulation, this study examines the effects of altitude and visibility on transmittance and turbulence intensity, finding that turbulence intensity decreases with increasing altitude while transmittance increases; improvements in visibility also lead to increased transmittance. Beam wandering due to turbulence is also dominant. Combining these factors, the effects on the total transmittance and the secret key rate are taken into consideration. Our work could provide a reference for the deployment of practical QKD systems in actual mountainous environments.

The robustness of reference-frame-independent measurement-device-independent quantum key distribution (RFI-MDI-QKD) against detection system vulnerabilities and its tolerance to reference frame drifts make it an ideal choice for hybrid channels. However, the impact of atmospheric turbulence on transmittance fluctuations remains a significant challenge for enhancing the performance of RFI-MDI-QKD. In this paper, we apply prefixed-threshold real-time selection and advantage distillation techniques to RFI-MDI-QKD in a hybrid channels scenario. Then, we analytically derive formulas for secret key rate in hybrid channels. Simulation results show that our modified scheme has apparent advances in both maximum tolerant loss and secure key rate compared to the fiber-only channel. Specifically, the result demonstrates that the maximum transmission distance can be improved by 15 km and 28 km when $N=10^{12}$ and $10^{11}$. Our work not only provides a more robust key distribution protocol but also establishes a solid theoretical foundation for enhancing the performance of RFI-MDI-QKD in hybrid channels.

Quantum key distribution (QKD) is a method for secure communication that utilizes quantum mechanics principles to distribute cryptographic keys between parties. Integrated photonics offer benefits such as compactness, scalability, energy efficiency and the potential for extensive integration. We have achieved BB84 phase encoding and decoding, time-bin phase QKD, and the coherent one-way (COW) protocol on a planar lightwave circuit (PLC) platform. At the optimal temperature, our chip successfully prepared quantum states, performed decoding and calculated the secure key rate of the time-bin phase-decoding QKD to be 80.46 kbps over a 20 km transmission with a quantum bit error rate (QBER) of 4.23%. The secure key rate of the COW protocol was 18.18 kbps, with a phase error rate of 3.627% and a time error rate of 0.377%. The uniqueness of this technology lies in its combination of high integration and protocol flexibility, providing an innovative solution for the development of future quantum communication networks.

Previous bidirectional quantum private comparison (BQPC) protocols cannot meet the requirements in some special application scenarios, where only one party needs to obtain the comparison results without a third party (TP), such as scenarios for authority surveys or healthcare data sharing. In addition to this, the BQPC protocol has the potential of information leakage in multiple comparisons. Therefore, we design a new unidirectional quantum private comparison (UQPC) protocol based on quantum private query (QPQ) protocols with ideal database security and zero failure probability (IDS-ZF), for the reason that they have excellent unidirectionality and security. Concretely, we design a UQPC protocol based on Wei et al.'s work [IEEE Transactions on Computers 67 2 (2017)] and it includes an authentication process to increase the resistance to outside attacks. Moreover, we generalize the protocol and propose a general model that can transform a QPQ protocol with or without the IDS-ZF property into a secure UQPC protocol. Finally, our study shows that protocols using our model are secure, practical, and have the IDS-ZF property.

Quantum communication networks, such as quantum key distribution (QKD) networks, typically employ the measurement-resend mechanism between two users using quantum communication devices based on different quantum encoding types. To achieve direct communication between the devices with different quantum encoding types, in this paper, we propose encoding conversion schemes between the polarization bases (rectilinear, diagonal and circular bases) and the time-bin phase bases (two phase bases and time-bin basis) and design the quantum encoding converters. The theoretical analysis of the encoding conversion schemes is given in detail, and the basis correspondence of encoding conversion and the property of bit flip are revealed. The conversion relationship between polarization bases and time-bin phase bases can be easily selected by controlling a phase shifter. Since no optical switches are used in our scheme, the converter can be operated with high speed. The converters can also be modularized, which may be utilized to realize miniaturization in the future.

Using quantum discord (QD) and geometric quantum discord (GQD), quantum correlation dynamics is investigated for two coupled qubits within a multiqubit interacting system in the zero-temperature bosonic reservoir, under both weak and strong qubit-reservoir coupling regimes. The multiqubit system is connected with either a common bosonic reservoir (CBR) or multiple independent bosonic reservoirs (IBRs). In the CBR case, our findings indicate that both QD and GQD can be strengthened by increasing the number of qubits in the multiqubit system. Furthermore, we study the steady state QD and GQD in the strong coupling regime, and find that the stable value in the long-time limit is determined exclusively by the number of qubits. The evolution period of QD and GQD gets longer as the dipole-dipole interaction (DDI) strength increases, which helps prolong the correlation time and thus preserves the quantum correlation under the weak coupling regime. Further analysis reveals notable differences between the CBR and IBRs scenarios. In the IBRs case, the decay of QD and GQD becomes slower compared to the CBR case, with both measures tending to zero at a reduced rate. Moreover, GQD consistently exhibits lower values than QD in both scenarios. These findings provide valuable insights into the selection of appropriate correlation measurement techniques for quantifying quantum correlations.