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Classical users can share a secret key with a quantum user by using a semi-quantum key distribution (SQKD) protocol. Allowing two classical users to share a secret key is the objective of the mediated semi-quantum key distribution (MSQKD) protocol. However, the existing MSQKD protocols need a quantum user to assist two classical users in distributing the secret keys, and these protocols require that the classical users be equipped with a Trojan horse photon detector. This reduces the practicability of the MSQKD protocols. Therefore, in this study we propose a lightweight MSQKD, in which the two participants and third party are classical users. Due to the usage of the one-way transmission strategy, the proposed lightweight MSQKD protocol is free from quantum Trojan horse attack. The proposed MSQKD is more practical than the existing MSQKD protocols.

This paper puts forward a novel quantum dialogue (QD) protocol without information leakage based on single photons in both polarization and spatial-mode degrees of freedom. In this protocol, the receiver sends his private message to the sender in the way of quantum secure direct communication; and the receiver decodes out the private message of the sender with the help of auxiliary single photons in two degrees of freedom, double controlled-not (CNOT) operations and an auxiliary classical bit sequence sent from the sender. As a result, no information leakage takes place. Moreover, this protocol is proven to defeat the intercept-resend attack, the measure-resend attack, the entangle-measure attack and the Trojan horse attack from an outside eavesdropper. This protocol only employs single photons in two degrees of freedom as quantum resource and needs single-photon measurements. The qubit efficiency of this protocol is as high as 66.7%.

Most semi-quantum secret sharing protocols are suitable for the application with three participants. Based on the n-level states, a multiparty semi-quantum secret sharing protocol is proposed. Compared with similar protocols, it has the merits as follows. (1) The dealer only needs to prepare the single photon sequence without using any product state or entangle state. (2) To share a secret, the classical parties need not have the ability of measurement. They only need to reorder the photon sequence and send it to the receiver. (3) The dealer knows nothing about any agent’s secret shadow. Therefore, it is infeasible for the dealer to impersonate the classical participant to pool the secret shadow during the secret recovering phase. (4) Theoretically, the qubit efficiency of the proposed protocol is asymptotically 100%. The protocol is secure against famous attacks such as intercept-resend attack, measurement-resend attack and entangle-measure attack.

A quantum router can guide information from one quantum channel to another; thus, it has an increasingly important role in complex quantum networks. Here, we verify that the quantum router proposed by [L. Zhou et al., Phys. Rev. Lett111, 103604 (2013)] achieves quantum-state high fidelity. When the transition strengths match $g_a=g_b$, the fidelity of the atom decreases from the maximum value after a period of time and then rises to a maximum value of 1; if the transition strengths do not match, the fidelity will not be able to reach 1.

We propose a theoretical model for the single photon quantum router that consists of two plasmonic waveguides, one of which is infinite and the other is semi-infinite, coupled to two two-level quantum dots (QDs) with dipole–dipole interaction (DDI). The infinite waveguide, the main branch, couples with both QDs, and the semi-infinite, the side branch, couples with the main branch at the position of one of the QDs. In such a system the incident single photon can also be transferred to the side branch, not only be transmitted or reflected. The results show that the transfer rate mainly depends on the coupling strength between the QDs and the two waveguides, but there exists a certain limit in the increase of the transfer rate. The DDI between the QDs gives a remarkable change to the routing spectra and the routing properties are different according to the position of the side branch. The proposed system is indeed multi-channel single photon quantum router, which has more practical significance than the one-dimensional router and can be employed in multi-user quantum networking.

We present a protocol for concentrating an arbitrary four-photon entangled state into a maximally entangled state assisted with singled photons. The concentration protocol uses the linear optics and the cross-Kerr nonlinearity based on the post selection principle. Four parties called Alice, Bob, Charlie and Dan in different distant locations can obtain the cluster state from an arbitrary entangled four-photon state with a certain probability. Quantum non-demolition (QND) measurements are available in this protocol. Moreover, this scheme can be steady with a higher success possibility.

In this paper, we propose a new controlled quantum secure direct communication (CQSDC) protocol with single photons in both polarization and spatial-mode degrees of freedom. Based on the defined local collective unitary operations, the sender’s secret messages can be transmitted directly to the receiver through encoding secret messages on the particles. Only with the help of the third side, the receiver can reconstruct the secret messages. Each single photon in two degrees of freedom can carry two bits of information, so the cost of our protocol is less than others using entangled qubits. Moreover, the security of our QSDC network protocol is discussed comprehensively. It is shown that our new CQSDC protocol cannot only defend the outsider eavesdroppers’ several sorts of attacks but also the inside attacks. Besides, our protocol is feasible since the preparation and the measurement of single photon quantum states in both the polarization and the spatial-mode degrees of freedom are available with current quantum techniques.

Electron bombarded Active Pixel Sensor (EBAPS) is well known for its low noise in low-light level imaging, high mechanical integration, and a relatively low cost. It plays an important role in areas of the industrial process as well as the fundamental scientific research. However, the performance of EBAPS is intensively influenced by the structural parameters (i.e. the acceleration voltage between cathode and anode, thickness of the passivation layer, etc.). Due to the influence of these factors mentioned above, the performance of EBAPS is restricted to achieve its best condition. Herein, a model based on the optimized Monte Carlo method was proposed for effectively analyzing the scattering behavior of electrons within the electron multiplier layer. Unlike traditional simulation, which only deals with the electron scattering in longitudinal, in this paper, we simulate the electron scattering character not only in horizontal but also vertical among the multiplier layer, which would react to the influence induced by structural parameters more complete and more precise. Based on the proposed model, an experimental prototype of EBAPS is built and its detection sensitivity achieves $0.84\times 10^{-4}$ lux under spectral response of ultraviolet (UV) spectroscopy, which improved a lot from our former design. The proposed model can be used for analyzing the influence induced by structural parameters, which exhibit enormous potential for exploring the high-gain EBAPS.

Decision making is a vital function in the age of machine learning and artificial intelligence; however, its physical realization and theoretical fundamentals are not yet well understood. In our former study, we demonstrated that single photons can be used to make decisions in uncertain, dynamically changing environments. The two-armed bandit problem was successfully solved using the dual probabilistic and particle attributes of single photons. In this study, we present a category theoretic modeling and analysis of single-photon-based decision making, including a quantitative analysis that agrees well with the experimental results. The category theoretic model unveils complex interdependencies of the entities of the subject matter in the most simplified manner, including a dynamically changing environment. In particular, the octahedral structure and the braid structure in triangulated categories provide better understandings and quantitative metrics of the underlying mechanisms for the single-photon decision maker. This study provides insight and a foundation for analyzing more complex and uncertain problems for machine learning and artificial intelligence.

This paper reviews the single photon sources based on semiconductor quantum dots and their applications to quantum information systems. By optically pumping a system consisting of a semiconductor single quantum dot confined in a monolithic microcavity, it is possible to produce a single photon pulse stream at the Fourier transform limit with a negligible jitter. This single photon source is not only useful for BB84 quantum key distribution (QKD), but also find applications in other quantum information systems such as Ekert91/BBM92 QKD and quantum teleportation gate for linear optics quantum computers.